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Notes of Ch 4 Climate| Class 11th Geography

Difference Between Weather And Climate

Weather	Climate
Weather is the momentary state of the atmosphere	Climate refers to the average of the weather condition over a longer period of time.
Weather changes quickly, may be within a day or week	Climate changes imperceptibly and may be noted after 50 years or even more

Unity And Diversity In The Monsoon Climate

Unity of The Climate

• The monsoon regime emphasizes the unity of India with the rest of southeast Asian region.

• This view of broad unity of the monsoon type of climate should not, however, lead one to ignore its regional variations which differentiate the weather and climate of different regions of India.

• The climate of Kerala and Tamil Nadu in the south are so different from that of Uttar Pradesh and Bihar in the north, and yet all of these have a monsoon type of climate.

Diversity of Climate

• The climate of India has many regional variations expressed in the pattern of:
→ Winds,
→ Temperature
→ Rainfall,
→ Rhythm of seasons
→ Degree of wetness or dryness.

Temperature Variation

• While in the summer the mercury occasionally touches 55°C in the western Rajasthan, it drops down to as low as minus 45°C in winter around Leh.

• Churu in Rajasthan may record a temperature of 50°C or more on a June day while the mercury hardly touches 19°C in Tawang (Arunachal Pradesh) on the same day.

• On a December night, temperature in Drass (Jammu and Kashmir) may drop down to minus 45°C while Thiruvananthapuram or Chennai on the same night records 20°C or 22°C.

• In Kerala and in the Andaman Islands, the difference between day and night temperatures may be hardly seven or eight degree Celsius. But in the Thar desert, if the day temperature is around 50°C, at night, it may drop down considerably up to 15°-20°C.

The Regional Variations In Precipitation

• While snowfall occurs in the Himalayas, it only rains over the rest of the country.

• While Cherrapunji and Mawsynramin the Khasi Hills of Meghalaya receive rainfall over 1,080 cm in a year, Jaisalmer in Rajasthan rarely gets more than 9 cm of rainfall during the same period.

• Tura situated in the Garo Hills of Meghalaya may receive an amount of rainfall in a single day which is equal to 10 years of rainfall at Jaisalmer.

• While the annual precipitation is less than 10 cm in the northwest Himalayas and the western deserts, it exceeds 400 cm in Meghalaya.

• The Ganga delta and the coastal plains of Odisha are hit by strong rain-bearing storms almost every third or fifth day in July and August while the Coromandal coast, a thousand km to the south, goes generally dry during these months.

• Most parts of the country get rainfall during June-September, but on the coastal areas of Tamil Nadu, it rains in the beginning of the winter season.

Factors Determining The Climate Of India

• It can be broadly divided into two groups factors:
→ Factors related to location and relief
→ Factors related to air pressure and winds

Factors related to Location and Relief

• Latitude: Northern part of the India lies in sub-tropical and temperate zone and the part lying south of the Tropic of Cancer falls in the tropical zone. The tropical zone being nearer to the equator, experiences high temperatures throughout the year with small daily and annual range. Area north of the Tropic of Cancer being away from the equator experiences extreme climate with high daily and annual range of temperature.

• The Himalayan Mountains: The lofty Himalayas in the north along with its extensions act as an effective climatic divide. The towering mountain chain provides an invincible shield to protect the
subcontinent from the cold northern winds. The Himalayas also trap the monsoon winds, forcing them to shed air pressure causes reversal in the direction of monsoon winds.

• Distance from the Sea: With a long coastline, large coastal areas have an equable climate. Areas in the interior of India are far away from the moderating influence of the sea. Such areas have extremes of climate their moisture within the subcontinent.

• Distribution of Land and Water: India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north. As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent.

• Difference in Altitude: Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains.

• Relief: The physiography or relief of India also affects the temperature, air pressure, direction and speed of wind and the amount and distribution of rainfall. The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.

Factors Related to Air Pressure and Wind

• Distribution of air pressure and winds on the surface of the earth.

• Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams.

• Inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India, creating weather conditions favourable to rainfall.

Mechanism of Weather in the Winter Season

Surface Pressure and Winds

• In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia.

• A high pressure centre in the region lying to the north of the Himalayas develops during winter. This centre of high pressure gives rise to the flow of air at the low level from the north towards the Indian subcontinent, south of the mountain range.

• The surface winds blowing out of the high pressure centre over Central Asia reach India in the form of a dry continental air mass. These continental winds come in contact with trade winds over northwestern India.

Jet Stream and Upper Air Circulation

• Higher up in the lower troposphere, about three km above the surface of the earth, a different pattern of air circulation is observed.

• The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation.

• All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east.

• These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. These are known as jet streams. branch blows in an eastward direction, south of the Himalayas.

• It has its mean position at 25°N in February at 200-300 mb level. It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India.

Direction of Winds in India in Winter at the Height of 9-13 km

Western Cyclonic Disturbance and Tropical Cyclones

• The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months, originate over the Mediterranean Sea and are brought into India by the westerly jet stream.

• An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances.

• Tropical cyclones originate over the Bay of Bengal and the Indian ocean.

• These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast.

• Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies.

Mechanism of Weather in the Summer Season

Surface Pressure and Winds

• As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels.

• By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, By this time, the westerly jet stream withdraws from the Indian region.

• It is generally believed that there is a cause and effect relationship between the ictz and jet stream.

• The ITCZ being a zone of low pressure, attracts inflow of winds from different directions.

• The maritime tropical airmass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general southwesterly direction. It is this moist air current which is popularly known as the southwest monsoon.

Jet Streams and Upper Air Circulation

• An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour.

• In August, it is confined to 15oN latitude, and in September up to 22o N latitudes. The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.

Easterly Jet Stream and Tropical Cyclones

• The easterly jet stream steers the tropical depressions into India.

• These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.

• The tracks of these depressions are the areas of highest rainfall in India.

• The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.

The Inter Tropical Convergence Zone (ITCZ)

• A low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend.

• In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough.

• This monsoon trough encourages the development of thermal low over north and northwest India.

• Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes southwest monsoon.

• In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called northeast monsoons.

The Nature Of Indian Monsoon

(i) The onset of the monsoon.

(ii) Rain-bearing systems (e.g. tropical cyclones) and the relationship between their

frequency and distribution of monsoon rainfall.

(iii) Break in the monsoon.

Onset of the Monsoon

• During April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian ocean gets intensely heated. This causes the formation of an intense low pressure in the northwestern part of the subcontinent.

• Since the pressure in the Indian Ocean in the south of the landmass is high as water gets heated slowly, the low pressure cell attracts the southeast trades across the Equator.

• These conditions help in the northward shift in the position of the ITCZ. The southwest monsoon may thus, be seen as a continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equator. These winds cross the Equator between 40°E and 60°E longitudes.

Entry of Monsoon into India

• The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. By mid July, southwest monsoon engulfs the entire subcontinent.

Rain-bearing Systems and Rainfall Distribution

• Bay of Bengal Branch
• Arabian sea branch

The rainfall distribution is based on two factors:

(i) The offshore meteorological conditions.

(ii) The position of the equatorial jet stream along the eastern coast of Africa.

EI-Nino and the Indian Monsoon

• EI-Nino is a complex weather system that appears once every three to seven years, bringing drought, floods and other weather extremes to different parts of the world.

• The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India.

• EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humbolt current (locate these currents in your atlas). This current increases the temperature of water on the Peruvian coast by 10°C. This results in:
(i) the distortion of equatorial atmospheric circulation;
(ii) irregularities in the evaporation of sea water;
(iii) reduction in the amount of planktons which further reduces the number of fish in the sea.

• The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere).

• EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.

Break in the Monsoon

• During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon.

• These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:

(i) In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
(ii) Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

The Rhythm Of Seasons

The climatic conditions of India can best be described in terms of an annual cycle of seasons. Four seasons are:
(i) The cold weather season
(ii) The hot weather season

(iii) The southwest monsoon season

(iv) The retreating monsoon season

The Cold Weather Season

• Found from November to February

• December and January are the coldest months in the northern plain.

• The mean daily temperature remains below 21°C, over most parts of northern India.

• The night temperature may be quite low, sometimes going below freezing point in Punjab and Rajasthan.

There are three main reasons for the excessive cold in north India during this season:
(i) States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.

(ii) The snowfall in the nearby Himalayan ranges creates cold wave situation; and

(iii) Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India.

Pressure and Winds:

• By the end of December (22nd December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere.

• The weather in this season is characterized by feeble high pressure conditions over the northern plain. In south India, the air pressure is slightly lower respectively.

• As a result, winds start blowing from northwestern high pressure zone to the low air pressure zone over the Indian Ocean in the south. Due to low pressure gradient, the light winds with a low velocity of about 3-5 km per hour begin to blow outwards.

• By and large, the topography of the region influences the wind direction. They are westerly or northwesterly down the Ganga Valley. They become northerly in the Ganga-Brahmaputra delta. Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal.

• During the winters, the weather in India is pleasant.

• The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India.

Rainfall:

• Winter monsoons do not cause rainfall as they move from land to the sea.

• It is because firstly, they have little humidity;

• Secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces. So, most parts of India do not have rainfall in the winter season.

However, there are some exceptions to it:

• In northwestern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although the amount is meagre, it is highly beneficial for Rabi crops. The precipitation is in the form of snowfall in the lower Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months. The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains.

• Central parts of India and northern parts of southern Peninsula also get winter rainfall occasionally.

• Arunachal Pradesh and Assam in the northeastern parts of India also have rains between 25 mm and 50 mm during these winter months.

• During October and November, northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and south east Kerala.

The Hot Weather Season

• With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures start rising in north India.

• April, May and June are the months of summer in north India. In most parts of India, temperatures recorded are between 30°-32°C.

• In March, the highest day temperature of about 38°C occurs in the Deccan Plateau while in April, temperature ranging between 38°C and 43°C are found in Gujarat and Madhya Pradesh.

• In May, the heat belt moves further north, and in the north-western part of India, temperatures around 48°C are not uncommon.

• Temperatures remain between 26°C and 32°C. Due to altitude, the temperatures in the hills of Western Ghats remain below 25°C.

Pressure and Winds:

• The summer months are a period of excessive heat and falling air pressure in the northern half of the country.

• Roughly, this elongated low pressure monsoon trough extends over the Thar desert in the
north​west to Patna and Chotanagpur plateau in the east-southeast.

• The location of the ITCZ attracts a surface circulation of the winds which are southwesterly on the west coast as well as along the coast of West Bengal and Bangladesh.

• They are easterly or southeasterly over north Bengal and Bihar.

• In the heart of the ITCZ in the northwest, the dry and hot winds known as ‘Loo’, blow in the afternoon, and very often, they continue to well into midnight.

• Dust storms in the evening are very common during May in Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh.

• These temporary storms bring a welcome respite from the oppressing heat since they bring with them light rains and a pleasant cool breeze.

• Occasionally, the moisture-laden winds are attracted towards the periphery of the trough.

• A sudden contact between dry and moist air masses gives rise to local storms of great intensity.

• These local storms are associated with violent winds, torrential rains and even hailstorms.

Some Famous Local Storms of Hot Weather Season

• Mango Shower: Towards the end of summer, there are pre-monsoon showers which are a common phenomena in Kerala and coastal areas of Karnataka. Locally, they are known as mango showers since they help in the early ripening of mangoes.

• Blossom Shower: With this shower, coffee flowers blossom in Kerala and nearby areas.

• Norwesters: These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature can be understood from the local nomenclature of ‘Kalbaisakhi’, a calamity of the month of Baisakh.

• These showers are useful for tea, jute and rice cultivation. In Assam, these storms are known as “Bardoli Chheerha”.

• Loo: Hot, dry and oppressing winds blowing in the Northern plains from Punjab to Bihar with higher intensity between Delhi and Patna.

The Southwest Monsoon Season

• Due to high temperature there is lo to attract the trade winds of Southern Hemisphere coming from the Indian Ocean.

• These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea,.

• After crossing the equator, they follow a southwesterly direction. That is why they are known as southwest monsoons.

• The rain in the southwest monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially.

• This sudden onset of the moisture-laden winds associated with violent thunder and lightning, is often termed as the “break” or “burst” of the monsoons.

The monsoon approaches the landmass in two branches:
(i) The Arabian Sea branch
(ii) The Bay of Bengal branch

Monsoon Winds of the Arabian Sea

The monsoon winds originating over the Arabian Sea further split into three branches:

(i) Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900-1200 m. Soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up.
(ii) Another branch of the Arabian sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.

(iii) A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.

Monsoon Winds of the Bay of Bengal

• The Bay of Bengal branch strikes the coast of Myanmar and part of south east Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent.

• The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India.

• Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.

• Here it is important to know why the Tamil Nadu coast remains dry during this season.

There are two factors responsible for it:
(i) The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon.
(ii) It lies in the rain shadow area of the Arabian Sea branch of the south-west monsoon.

Characteristics of Monsoonal Rainfall

• Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.

• Monsoonal rainfall is largely governed by relief or topography. For instance the windward side of the Western Ghats register a rainfall of over 250 cm. Again, the heavy rainfall in the north -eastern states can be attributed to their hill ranges and the Eastern Himalayas.

• The monsoon rainfall has a declining trend with increasing distance from the sea. Kolkata receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.

• The monsoon rains occur in wet spells of few days duration at a time. The wet spells are interspersed with rainless, interval known as ‘breaks’. These breaks in rainfall are related to the cyclonic depressions mainly formed at the head of the Bay of Bengal, and their crossing into the mainland. Besides the frequency and intensity of these depressions, the passage followed by them determines the spatial distribution of rainfall.

• The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.

• Monsoons play a pivotal role in the agrarian economy of India because over three-fourths of the total rain in the country is received during the southwest monsoon season.

• Its spatial distribution is also uneven which ranges from 12 cm to more than 250 cm.

• The beginning of the rains sometimes is considerably delayed over the whole or a part of the country.

• The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Season of Retreating Monsoon

• October and November are known for retreating monsoons.

• By the end of September, the southwest monsoon becomes weak as the low pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun.

• The monsoon retreats from the western Rajasthan by the first week of September.

• It withdraws from Rajasthan, Gujarat, Western Ganga plain and the Central Highlands by the end of the month.

• By the beginning of October, the low pressure covers northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu.

• By the middle of December, the centre of low pressure is completely removed from the Peninsula.

• The retreating southwest monsoon season is marked by clear skies and rise in temperature.

• The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’. Here, October and November are the rainiest months of the year.

• The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula.

Traditional Indian Seasons

Seasons	Months (According to the Indian Calendar)	Months (According to the Indian Calendar)
Vasanta	Chaitra-Vaisakha	March-April
Grishma	Jyaistha-Asadha	May-June
Varsha	Sravana-Bhadra	July-August
Sharada	Asvina-Kartika	September-October
Hemanta	Margashirsa-Pausa	November-December
Shishira	Magha-phalguna	January-February

Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but it has great spatial variations.

• Areas of High Rainfall : The highest rainfall occurs along the west coast, on the Western Ghats, as well as in the sub-Himalayan areas is the northeast and the hills of Meghalaya. Here the rainfall exceeds 200 cm. In some parts of Khasi and Jaintia hills, the rainfall exceeds 1,000 cm. In the Brahmaputra valley and the adjoining hills, the rainfall is less then 200 cm.

• Areas of Medium Rainfall : Rainfall between 100-200 cm is received in the southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula covering Odisha, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-Himalayas and the Cachar Valley and Manipur.

• Areas of Low Rainfall : Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50-100 cm.

• Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm.Snowfall is restricted to the Himalayan region.

Climatic Regions of India

Major climatic types of India based on Koeppen‘s scheme: Koeppen based his scheme of Climatic classification on monthly values of temperature and precipitation. He identified five major climatic types, namely:

(i) Tropical climates, where mean monthly temperature throughout the year is over 18°C.

(ii) Dry climates, where precipitation is very low in comparison to temperature, and hence, dry. If dryness is less, it is semi arid (S); if it is more, the climate is arid(W).

(iii) Warm temperate climates, where mean temperature of the coldest month is between 18°C and minus 3°C.

(iv) Cool temperate climates, where mean temperature of the warmest month is over 10°C, and mean temperature of the coldest month is under minus 3°C.

(v) Ice climates, where mean temperature of the warmest month is under 10°C.

Class 11 Geography Notes Chapter 3 Drainage System

On the basis of discharge of water (orientations to the sea), it may be grouped into:

• The Arabian Sea drainage; and
• The Bay of Bengal drainage.

Nearly 77 per cent of the drainage area consisting of the Ganga, the Brahmaputra, the Mahanadi, the Krishna, etc. are oriented towards the Bay of Bengal while 23 per cent comprising the Indus, the Narmada, the Tapi, the Mahi and the Periyar systems discharge their waters in the Arabian Sea.

On the basis of the size of the watershed, the drainage basins of India are grouped into three categories:

• Major river basins with more than 20,000 sq. km of catchment area. It includes 14 drainage basins such as the Ganga, the Brahmaputra, the Krishna, the Tapi, the Narmada, the Mahi, the Penner, the Sabarmati, the Barak, etc.
• Medium river basins with catchment area between 2,000-20,000 sq. km incorporating 44 river basins such as the Kalindi, the Periyar, the Meghna, etc.

Minor river basins with catchment area of less than 2,000 sq. km include fairly good number of rivers flowing in the area of low rainfall.

The Indus System is one of the largest river basins of the world, covering an area of 11,65,000 sq. km (in India it is 321, 289 sq. km) and a total length of 2,880 km and in India its length is 1,114 km.

The Jhelum, an important tributary of the Indus, rises from a spring at Verinag situated at the foot of the Pir Panjal in the south-eastern part of the valley of Kashmir. It flows through Srinagar and the Wular lake before entering Pakistan through a deep narrow gorge. It joins the Chenab near Jhang in Pakistan.

The Chenab is the largest tributary of the Indus. It is formed by two streams, the Chandra and the Bhaga, which join at Tandi near Keylong in Himachal Pradesh. Hence, it is also known as Chandrabhaga.

The Ravi is another important tributary of the Indus. It rises west of the Rohtang pass in the Kullu hills of Himachal Pradesh and flows through the Chamba valley of the state.

The Beas is another important tributary of the Indus, originating from the Beas Kund near the Rohtang Pass at an elevation of 4,000 m above the mean sea level. The river flows through the Kullu valley and forms gorges at Kati and Largi in the Dhaoladhar range.

Satluj river flows almost parallel to the Indus for about 400 km before entering India, and comes out of a gorge at Rupar. It passes through the Shipki La on the Himalayan ranges and enters the Punjab plains. It is an antecedent river. It is a very important tributary as it feeds the canal system of the Bhakra Nangal project.

The Ganga System rises in the Gangotri glacier near Gaumukh (3,900 m) in the Uttarkashi district of Uttarakhand. Here, it is known as the Bhagirathi. It cuts through the Central and the Lesser Himalayas in narrow gorges. At Devprayag, the Bhagirathi meets the Alaknanda; hereafter, it is known as the Ganga. The Alaknanda has its source in the Satopanth glacier above Badrinath.

The Alaknanda consists of the Dhauli and the Vishnu Ganga which meet at Joshimath or Vishnu Prayag. The other tributaries of Alaknanda such as the Pindar joins it at Kama Prayag while Mandakini or Kali Ganga meets it at Rudra Prayag.

The Ganga river has a length of 2,525 km. It is shared by Uttarakhand (110 km) and Uttar Pradesh (1,450 km), Bihar (445 km) and West Bengal (520 km).

The Ganga basin covers about 8.6 lakh sq. km area in India alone. The Ganga river system is the largest in India having a number of perennial and non-perennial rivers originating in the Himalayas in the north and the Peninsula in the south, respectively. The Son is its major right bank tributary. The important left bank tributaries are the Ramganga, the Gomati, the Ghaghara, the Gandak, the Kosi and the Mahananda.

The Yamuna is the western most and the longest tributary of the Ganga. It has its source in the Yamunotri glacier on the western slopes of Banderpunch range (6,316 km). It joins the Ganga at Prayag in Allahabad. It is joined by the Chambal, the Sind, the Betwa and the Ken on its right bank which originates from the Peninsular plateau while the Hindan, the Rind, the Sengar, the Varuna, etc. join it on its left bank.



The Chambal rises near Mhow in the Malwa plateau of Madhya Pradesh and flows northwards through a gorge up wards of Kota in Rajasthan, where the Gandhisagar dam has been constructed. From Kota, it traverses down to Bundi, Sawai Madhopur and Dholpur, and finally joins the Yamuna.

The Gandak comprises two streams, namely Kaligandak and Trishulganga. It rises in the Nepal Himalayas between the Dhaulagiri and Mount Everest and drains the central part of Nepal.

The Brahmaputra is one of the largest rivers of the world. It has its origin in the Chemayungdung glacier of the Kailash range near the Mansarovar lake.

The Brahmaputra receives numerous tributaries in its 750 km long journey through the Assam valley. Its major left bank tributaries are the Burhi Dihing and Dhansari (South) whereas the important right bank tributaries are the Subansiri, Kameng, Manas and Sankosh.. The Subansiri which has its origin in Tibet, is an antecedent river.

The Peninsular drainage system is older than the Himalayan one. This is evident from the broad, largely-graded shallow valleys, and the maturity of the rivers.

Most of the major Peninsular rivers except Narmada and Tapi flow from west to east. The Chambal, the Sind, the Betwa, the Ken, the Son, originating in the northern part of the Peninsula belong to the Ganga river system. The other major river systems of the Peninsular drainage are – the Mahanadi, the Godavari, the Krishna and the Kaveri. Peninsular rivers are characterised by fixed course, absence of meanders and non- perennial flow of water.

Three major geological events in the distant past have shaped the present drainage systems of Peninsular India:

• Subsidence of the western flank of the Peninsula leading to its submergence below the sea during the early tertiary period.
• Upheaval of the Himalayas when the northern flank of the Peninsular block was subjected to subsidence and the consequent trough faulting.
• Slight tilting of the Peninsular block from north-west to the south-eastern direction gave orientation to the entire drainage system towards the Bay of Bengal during the same period.

There are some problems in river water usage. Some of these are:

• No availability in sufficient quantity.
• River water pollution.
• Load of silt in the river water.
• Uneven seasonal flow of water.
• River water disputes between states.
• Shrinking of channels due to the extension of settlements towards the thalweg.

Class 11 Geography Notes Chapter 3 Important Terms:

• Drainage: The flow of water through well-defined channels is known as ‘drainage.’
• Drainage System: The network of drainage channels is called a ‘drainage system’.
• Dendritic Drainage System: The drainage pattern resembling the branches of a tree is known as “dendritic” the examples of which are the rivers of northern plain.
• Radial Drainage System: When the rivers originate from a hill and flow in all directions, the drainage pattern is known as ‘radial’. The rivers originating from the Amarkantak range present a good example of it.
• Trellis: When the primary tributaries of rivers flow parallel to each other and secondary tributaries join them at right angles, the pattern is known as ‘trellis’.
• Centripetal Drainage System: When the rivers discharge their waters from all directions in a lake or depression, the pattern is known as ‘centripetal’.
• Catchment area: A river drains the water collected from a specific area, which is called its ‘catchment area’.
• Drainage Basin: An area drained by a river and its tributaries is called a drainage basin.
• Watershed: The boundary line separating one drainage basin from the other is known as the watershed.
• Sorrow of Bengal: River Damodar is also known as the ‘Sorrow of Bengal’ as it changes its course very frequently and causes floods in Bihar.
• Sorrow of Bihar: River Kosi is called Sorrow of Bihar.
• River System: A river or a river system is a body of water flowing in a channel through the surface of the earth. It consists of four important parts: river course, river source, tributaries and river mouth.
• River Source: A place at which begins or originates. This is usually found in mountainous areas. The source may be melting snow from the top of a mountain on a lake with stream flowing out of it. A river flow downhill from its source due to the force of gravity
• River Course: The path on which the river flows along.
• Singi Khamban or Lion’s mouth: Kailash Mountain range is known as ‘Singi Khamban; or Lion’s mouth in Tibet.
• Regime: The pattern of flow of water in a river channel over a year is known as its regime.
• Cusecs: It means cubic feet per second.
• Cumecs: It stands for cubic metres per second.

Class 11 Geography Notes Chapter 2 Structure and Physiography

These geological regions broadly follow the physical features:

• The Peninsular Block
• The Himalayas and other Peninsular Mountains
• Indo-Ganga-Brahmaputra Plain.

The northern boundary of the Peninsular Block may be taken as an irregular line running from Kachchh along the western flank of the Aravali Range near Delhi and then roughly parallel to the Yamuna and the Ganga as far as the Rajmahai Hills and the Ganga delta. Apart from these, the Karbi Anglong and the Meghalaya Plateau in the north-east and Rajasthan in the west are also extensions of this block.

The Peninsula is formed essentially by a great complex of very ancient gneisses and granites, which constitutes a major part of it. The rift valleys of the Narmada, the Tapi and the Mahanadi and the Satpura block mountains are some examples of it. The Peninsula mostly consists of relict and residual mountains like the Aravali hills, the Nallamala hills, the Javadi hills, the Veliconda hills, the Palkonda range and the Mahendragiri hills, etc.

Most of the east flowing rivers form deltas before entering into the Bay of Bengal. The deltas formed by the Mahanadi, the Krishna, the Kaveri and the Godavari are important examples.

The Himalayas along with other Peninsular mountains are young, weak and flexible in their geological structure unlike the rigid and stable Peninsular Block. Consequently, they are still subjected to the interplay of exogenic and endogenic forces, resulting in the development of faults, folds and thrust plains.

The third geological division of India comprises the plains formed by the river Indus, the Ganga and the Brahmaputra. Originally, it was a geo-synclinal depression which attained its maximum development during the third phase of the Himalayan mountain formation approximately about 64 million years ago. Since then, it has been gradually filled by the sediments brought by the Himalayan and Peninsular rivers. Average depth of alluvial deposits in these plains ranges from 1,000-2,000 m.

India can be divided into the following physio-graphic divisions:

• The Northern and North-eastern Mountains
• The Northern Plain
• The Peninsular Plateau
• The Indian Desert
• The Coastal Plains
• The Islands.

The North and North-eastern Mountains consist of the Himalayas and the North-eastern hills. The Himalayas consist of a series of parallel mountain ranges. Some of the important ranges are the Greater Himalayan range, which includes the Great Himalayas and the Trans-Himalayan range, the Middle Himalayas and the Shiwalik.

The approximate length of the Great Himalayan range, also known as the central axial range, is 2,500 km from east to west, and their width varies between 160-400 km from north to south.

On the basis of relief, alignment of ranges and other geomorphological features, the Himalayas can be divided into the following sub-divisions:

• Kashmir or North-western Himalayas.
• Himachal and Uttaranchal Himalayas.
• Darjeeling and Sikkim Himalayas.
• Arunachal Himalayas.
• Eastern Hills and Mountains.

Kashmir or North-western Himalayas comprise a series of ranges such as the Karakoram, Ladakh, Zaskar and Pir Panjal. The north-eastern part of the Kashmir Himalayas is a cold desert, which lies between the Greater Himalayas and the Karakoram ranges.

The word shiwalik has its origin in the geological formation found in and around a place called Sivawala near Dehra Dun which was once a headquarter of the Imperial Survey and which subsequently established its permanent headquarters at Dehra Dun.

The Himachal and Uttarakhand Himalayas lies approximately between the Ravi in the west and the Kali (a tributary of Ghaghara) in the east. In this section of Lesser Himalayas, the altitude between 1,000-2,000 m specially attracted to the British colonial administration, and subsequently, some of the important hill stations such as Dharamshala, Mussoorie, Shimla, Kaosani and the cantonment towns and health resorts such as Shimla, Mussoorie, Kasauli, Almora, Lansdowne and Ranikhet, etc. were developed in this region.

The two distinguishing features of this region from the point of view of physiography are the ‘Shiwalik’ and ‘Dun formations’, Some important duns located in this region are the Chandigarh – Kalka Dun, Nalagarh Dun, Dehra Dun, Harike Dun and the Kota Dun.

In the Great Himalayan range, the valleys are mostly inhabited by the Bhotia’s. These are nomadic groups who migrate to ‘Bugyals’ (the summer glass lands in the higher reaches) during summer months and return to the valleys during winters. The famous ‘Valley of flowers’ is also situated in this region. The places of pilgrimage such as the Gangotri, Yamunotri, Kedarnath, Badrinath and Hemkund Sahib are also situated in this part.

Sikkim and Darjeeling Himalayas are also known for their scenic beauty and rich flora and fauna, particularly various types of orchids.

An important aspect of the Arunachal Himalayas is the numerous ethnic tribal community inhabiting in these areas. Some of the prominent ones from west to east are the Monpa, Daffla, Abor, Mishmi, Nishi and the Nagas. Most of these communities practise Jhumming. It is also known as shifting or slash and burn cultivation. This region is rich in biodiversity which has been preserved by the indigenous communities. Due to rugged topography, the inter-valley transportation linkages are nominal. Hence, most of the interactions are carried through the duar region along the Arunachal-Assam border.

The Eastern Hills and Mountains are having their general alignment from the north to the south direction. They are known by different local names. In the north, they are known as Patkai Bum, Naga hills, the Manipur hills and in the south as Mizo or Lushai hills. These are low hills, inhabited by numerous tribal groups practising Jhum cultivation.

The northern plains are formed by the alluvial deposits broiight by the rivers – the Indus, the Ganga and the Brahmaputra. These plains extend approximately 3,200 km from the east to the west. The average width of these plains varies between 150-300 km. The maximum depth of alluvium deposits varies between 1,000-2,000 m. From the north to the south, these can be divided into three major zones: the Bhabar, the Tarai and the alluvial plains. The alluvial plains can be further divided into the Khadar and the Bhangar.

Bhabar is a narrow belt ranging between 8-10 km parallel to the Shiwalik foothills at the break-up of the slope. As a result of this, the streams and rivers coming from the mountains deposit heavy materials of rocks and boulders, and at times, disappear in this zone. South of the Bhabar is the Tarai belt, with an approximate width of 10-20 km where most of the streams and rivers re-emerge without having any properly demarcated channel, thereby, creating marshy and swampy conditions known as the Tarai.

Northern Plains is a featureless plain with a general elevation of 50-150 m above the mean sea level. The states of Haryana and Delhi form a water divide between the Indus and the Ganga river systems.

Rising from the height of 150 m above the river plains up to an elevation of 600-900 m is the irregular triangle known as the Peninsular plateau. Delhi ridge in the northwest, (extension of Aravalis), the Rajmahal hills in the east, Gir range in the west and the Cardamom hills in the south constitute the outer extent of the Peninsular plateau. However, an extension of this is also seen in the northeast, in the form of Shillong and Karbi-Anglong plateau. The Peninsular India is made up of a series of patland plateaus such as the Hazaribagh plateau, the Palamu plateau, the Ranchi plateau, the Malwa plateau, the Coimbatore plateau and the Karnataka plateau, etc.

On the basis of the prominent relief features, the Peninsular plateau can be divided into three broad groups:

• The Deccan Plateau
• The Central Highlands
• The North-eastern Plateau.

The Deccan Plateau is bordered by the Western Ghats in the west, Eastern Ghats in the east and the Satpura, Maikal range and Mahadeo hills in the north. Western Ghats are locally known by different names such as Sahyadri in Maharashtra, Nilgiri hills in Karnataka and Tamil Nadu and Anaimalai hills and Cardamom hills in Kerala. Western Ghats are comparatively higher in elevation and more continuous than the Eastern Ghats. Their average elevation is about 1,500 m with the height increasing from north to south. ‘Anaimudi (2,695 m), the highest peak of Peninsular plateau is located on the Anaimalai hills of the Western Ghats followed by Dodabetta (2,637 m) on the Nilgiri hills.

Some of the important ranges in Eastern Ghats include the Javadi hills, the Palconda . range, the Nallamala hills, the Mahendragiri hills, etc.

The Meghalaya plateau is further sub-divided into three:

• The Garo Hills
• The Khasi Hills
• The Jaintia Hills

These are named after the tribal groups inhabiting this region. An extension of this is also seen in the Karbi Anglong hills of Assam.

To the north-west of the Aravali hills lies the Great Indian Desert. It is a land of undulating topography dotted with longitudinal dunes and barchans. This region receives low rainfall below 150 mm per year; hence, it has arid climate with low vegetation cover.

On the basis of the location and active geomorphological processes, it can be broadly divided into two:

• The western coastal plains
• The eastern coastal plains.

There are two major island groups in India – one in the Bay of Bengal and the other in the Arabian Sea. The Bay of Bengal island groups consist of about 572 islands/islets. These are situated roughly between 6°N-14°N and 92°E -94°E. The two principal groups of islets include the Ritchie’s archipelago and the Labrynth island.

Some important mountain peaks in Andaman and Nicobar islands are Saddle peak (North Andaman – 738 m), Mount Diavolo (Middle Andaman – 515 m), Mount Koyob (South Andaman – 460 m) and Mount Thuiller (Great Nicobar – 642 m). These islands are located at a distance of 280 km-480 km off the Kerala coast. The entire island group.is built of coral deposits. There are approximately 36 islands of which 11 are inhabited. Minicoy is the largest island with an area of 453 sq. km.

Class 11 Geography Notes Chapter 2 Important Terms:

• Physiography: ‘Physiography’ of an area is the outcome of structure, process and the stage of development.
• Central axial range: The approximate length of the Great Himalayan range is known as the central axial range. It is 2,500 km from east to west
• Indo-Ganga-Brahmaputra Plain: It is a geological division of India that comprises the plains formed by the river Indus, the Ganga and the Brahmaputra.
• Bhabar: Bhabar is a narrow belt ranging between 8-10 km parallel to the Shiwalik foothills at the break-up of the slope. As a result of this, the streams and rivers coming from the mountains deposit heavy materials of rocks and boulders, and at times, disappear in this zone.
• Kashmir or North-western Himalayas: It comprise a series of ranges such as the Karakoram, Ladakh, Zaskar and Pir Panjal. The north-eastern part of the Kashmir Himalayas is a cold desert, which lies between the Greater Himalayas and the Karakoram ranges.
• Duns: The southernmost part of this region consists of longitudinal valleys. These are known as ‘duns’. Jammu dun and Pathankot dun are important examples.
• Tarai: South of the Bhabar is the Tarai belt, with an approximate width of 10-20 km where most of the streams and rivers re-emerge without having any properly demarcated channel, thereby, creating marshy and swampy conditions known as the Tarai.
• Bhangar: The south of Tarai is a belt consisting of old alluvial deposits which is known as the Bhangar.
• Khadar: The south of Tarai is a belt consisting of new alluvial deposits is known as Khadar.
• Dhaoladhar: The Lesser Himalayas is locally known as Dhaoladhar in Himachal Pradesh.
• Nagtibha: The Lesser Himalayas are called Nagtibha in Uttarakhand.
• Ten Degree Channel: The Andaman in the north and the Nicobar in the south are separated by a water body. It is called the Ten degree channel.
• The Peninsular Plateau: Rising from the height of 150 m above the river plains upto an elevation of 600-900 m is the irregular triangle known as the Peninsular plateau.
• The Central Highlands: They are bounded to the west by the Aravali range.
• Satpura Range: The Satpura range is formed by a series of scarped plateaus on the south, generally at an elevation varying between 600-900 m above the mean sea level.
• Barchans: The extension of the Peninsular plateau can be seen as far as Jaisalmer in the West, where it has been covered by the longitudinal sand ridges and crescent-shaped sand dunes. These are called barchans.
• Loktak: The physiography of Manipur is unique by the presence of a large lake known as ‘Loktak’.
• Molassis Basin: Mizoram is also known as the ‘Molassis basin’ which is made up of soft unconsolidated deposits.
• Kayals: Boatwaters are called kayals in Kerala.
• Karewas: Karewas are the thick deposits of glacial clay and other materials embedded with moraines.
• Dhaya: Bangal is called dhaya in Punjab.
• Bate: Khadar is called bate in Punjab.

Notes of Ch 1 India-Location| Class 11th Geography

India – Location

• India extends from Kashmir in the north to Kanniyakumari in the south and Arunachal Pradesh in the east to Gujarat in the west.

• India’s territorial limit further extends towards the sea up to 12 nautical miles (about 21.9 km) from the coast.

• The southern boundary extends up to 6°45′ N latitude in the Bay of Bengal.

• The latitudinal and longitudinal extent of India, they are roughly about 30 degrees, whereas the actual distance measured from north to south extremity is 3,214 km, and that from east to west is only 2,933 km.

• The distance between two longitudes decreases towards the poles whereas the distance between two latitudes remains the same everywhere.

• The southern part of the country lies within the tropics and the northern part lies in the sub-tropical zone or the warm temperate zone. This location is responsible for large variations in landforms, climate, soil types and natural vegetation in the country.

• From the values of longitude, it is quite visible that there is a variation of nearly 30 degrees, which causes a time difference of nearly two hours between the easternmost and the westernmost parts of our country

• A general understanding among the countries of the world to select the standard meridian in multiples of 7°30′ of longitude. That is why 82°30′ E has been selected as the ‘standard meridian’ of India. Indian Standard Time is ahead of Greenwich Mean Time by 5 hours and 30 minutes.

• There are some countries where there is more than one standard meridian due to their vast east-to-west extent. For example, the USA has seven time zones.

• India with its area of 3.28 million sq. km accounts for 2.4 percent of the world’s land surface area and stands as the seventh-largest country in the world.

Size

Indian subcontinent

• Indian subcontinent includes the countries — Pakistan, Nepal, Bhutan, Bangladesh, and India.

• The Himalayas, together with other ranges, have acted as a formidable physical barrier in the past.

• Except for a few mountain passes such as the Khyber, the Bolan, the Shipkila, the Nathula, the Bomdila, etc. it was difficult to cross it. These passes contributed towards the evolving of a unique regional identity of the Indian subcontinent.

• Peninsular part of India extends towards the Indian Ocean. This has provided the country with a coastline of 6,100 km in the mainland and 7,517 km in the entire geographical coast of the mainland plus the island groups.

• Andaman and Nicobar located in the Bay of Bengal and the Lakshadweep in the Arabian Sea.

India and its neighbours

• India is located in the south-central part of the continent of Asia, bordering the Indian ocean and its two arms extending in the form of the Bay of Bengal and the Arabian Sea.

• This maritime location of Peninsular India has provided links to its neighboring regions through the sea and air routes.

• Sri Lanka and the Maldives are the two island countries located in the Indian Ocean, which are our neighbors.

• Sri Lanka is separated from India by the Gulf of Mannar and Palk Strait.

Notes of Ch 16 Biodiversity and Conservation| Class 11th Geography

Introduction

• The basic cause for such weathering variations and resultant biodiversity is the input of solar energy and water.

• Biodiversity is a system in constant evolution, from a view point of species, as well as from view point of an individual organism

• Biodiversity is not found evenly on the earth.

• Biodiversity itself is a combination of two words, Bio (life) and diversity (variety). In simple terms, biodiversity is the number and variety of organisms found within a specified geographic region.

• It refers to the varieties of plants, animals and micro-organisms, the genes they contain and the ecosystems they form. It relates to the variability among living organisms on the earth, including the variability within and between the species and that within and between the ecosystems.

• Biodiversity can be discussed at three levels:
(i) Genetic diversity;
(ii) Species diversity;
(iii) Ecosystem diversity

Genetic Diversity

• Genes are the basic building blocks of various life forms. Genetic biodiversity refers to the variation of genes within species.

• Groups of individual organisms having certain similarities in their physical characteristics are called species.

• Human beings genetically belong to the homo sapiens group and also differ in their characteristics such as height, colour, physical appearance, etc., considerably. This is due to genetic diversity.

• This genetic diversity is essential for a healthy breeding of population of species.

Species Diversity

• This refers to the variety of species. It relates to the number of species in a defined area.

• The diversity of species can be measured through its richness, abundance and types. Some areas are more rich in species than others.

• Areas rich in species diversity are called hotspots of diversity.

Ecosystem Diversity

• The broad differences between ecosystem types and the diversity of habitats and ecological processes occurring within each ecosystem type constitute the ecosystem diversity.

• The ‘boundaries’ of communities and ecosystems are not very rigidly defined. Thus, the demarcation of ecosystem boundaries is difficult and complex.

Importance of Biodiversity

• Biodiversity has contributed in many ways to the development of human culture and, in turn, human communities have played a major role in shaping the diversity of nature at the genetic, species and ecological levels.

• Biodiversity plays the following roles: ecological, economic and scientific.

Ecological Role of Biodiversity

• Every organism, besides extracting its needs, also contributes something of useful to other organisms.

• Species capture and store energy, produce and decompose organic materials, help to cycle water and nutrients throughout the ecosystem, fix atmospheric gases and help regulate the climate. These functions are important for ecosystem function and human survival.

• The more diverse an ecosystem, better are the chances for the species to survive through adversities and attacks, and consequently, is more productive.

Economic Role of Biodiversity

• For all humans, biodiversity is an important resource in their day-to-day life.

• One important part of biodiversity is ‘crop diversity’, which is also called agro-biodiversity.

• Biodiversity is seen as a reservoir of resources to be drawn upon for the manufacture of food, pharmaceutical, and cosmetic products.

• This concept of biological resources is responsible for the deterioration of biodiversity.

• Some of the important economic commodities that biodiversity supplies to humankind are: food crops, livestock, forests, fish, medicinal resources, etc.

Scientific Role of Biodiversity

• Biodiversity is important because each species can give us some clue as to how life evolved and will continue to evolve.

• Biodiversity also helps in understanding how life functions and the role of each species in sustaining ecosystems of which we are also a species.

• The level of biodiversity is a good indicator of the state of our relationships with other living species. In fact, the concept of biodiversity is an integral part of many human cultures.

Loss of biodiversity

• Tropical regions which occupy only about one-fourth of the total area of the world, contain about three- fourth of the world human population. Over- exploitation of resources and deforestation have become rampant to fulfill the needs of large population.

• As these tropical rain forests contain 50 per cent of the species on the earth, destruction of natural habitats have proved disastrous for the entire biosphere.

• Natural calamities such as earthquakes, floods, volcanic eruptions, forest fires, droughts, etc. cause damage to the flora and fauna of the earth, bringing change the biodiversity of respective affected regions.

• Pesticides and other pollutants such as hydrocarbons and toxic heavy metals destroy the weak and sensitive species.

• Species which are not the natural inhabitants of the local habitat but are introduced into the system, are called exotic species.

• The International Union of Conservation of Nature and Natural Resources (IUCN) has classified the threatened species of plants and animals into three categories for the purpose of their conservation.

Endangered Species

• It includes those species which are in danger of extinction. The IUCN publishes information about endangered species world-wide as the Red List of threatened species.

• Red Panda — an endangered species

• Zenkeria Sebastinei — a critically endangered grass in Agasthiyamalai peak (India)

Vulnerable Species

• This includes the species which are likely to be in danger of extinction in near future if the factors threatening to their extinction continue.

• Survival of these species is not assured as their population has reduced greatly

Rare Species

• Population of these species is very small in the world; they are confined to limited areas or thinly scattered over a wider area.

Conservation of biodiversity

• Biodiversity is important for human existence. All forms of life are so closely interlinked that disturbance in one gives rise to imbalance in the others.

• There is an urgent need to educate people to adopt environment-friendly practices and reorient their activities in such a way that our development is harmonious with other life forms and is sustainable.

• There is an increasing consciousness of the fact that such conservation with sustainable use is possible only with the involvement and cooperation of local communities and individuals. For this, the development of institutional structures at local levels is necessary.

• The critical problem is not merely the conservation of species nor the habitat but the continuation of process of conservation.

• The Government of India along with 155 other nations have signed the Convention of Biodiversity at the Earth Summit held at Rio de Janeiro, Brazil in June 1992.

• The world conservation strategy has suggested the following steps for biodiversity conservation:

→ Efforts should be made to preserve the species that are endangered.
→ Prevention of extinction requires proper planning and management.
→ Varieties of food crops, forage plants, timber trees, livestock, animals and their wild relatives should be preserved;
→ Each country should identify habitats of wild relatives and ensure their protection.
→ Habitats where species feed, breed, rest and nurse their young should be safeguarded and protected.
→ International trade in wild plants and animals be regulated.

• To protect, preserve and propagate the variety of species within natural boundaries, the Government of India passed the Wild Life (Protection) Act, 1972, under which national parks and sanctuaries were established and biosphere reserves declared.

• There are some countries which are situated in the tropical region, they possess a large number of the world’s species diversity. They are called mega diversity centres.

• There are 12 such countries, namely Mexico, Columbia, Ecuador, Peru, Brazil, emocratic Republic of Congo, Madagascar, China, India, Malaysia, Indonesia and Australia in which these centres are located.

• Hotspots are defined according to their vegetation. Plants are important because these determine the primary productivity of an ecosystem.

• Most, but not all, of the hotspots rely on species-rich ecosystems for food, firewood, cropland, and income from timber. In Madagascar, for example, about 85 per cent of the plants and animals are found nowhere else in the world.

• Other hotspots in wealthy countries are facing different types of pressures. The islands of Hawaii have many unique plants and animals that are threatened by introduced species and land development.

Notes of Ch 15 Life on the Earth| Class 11th Geography

Introduction

• The biosphere includes all the living components of the earth. It consists of all plants and animals, including all the micro-organisms that live on the planet earth and their interactions with the surrounding environment.

• Most of the organisms exist on the lithosphere and/or the hydrosphere as well as in the atmosphere. There are also many organisms that move freely from one realm to the other.

• Life on the earth is found almost everywhere. Living organisms are found from the poles to the equator, from the bottom of the sea to several km in the air, from freezing waters to dry valleys, from under the sea to underground water lying below the earth’s surface.

Ecology

• Ecology is the study of the earth as a ‘household’, of plants, human beings, animals and micro-organisms. They all live together as interdependent components.

• Ecological Systems: The interactions of a particular group of organisms with abiotic factors within a particular habitat resulting in clearly defined energy flows and material cycles on land, water and air.

Ecological adaptation

• Different types of ecosystems exist with varying ranges of environmental conditions where various plants and animal species have got adapted through evolution.

Types of Ecosystems

• Terrestrial ecosystem- can be further be classified into ‘biomes’.

• A biome can be defined as the total assemblage of plant and animal species interacting within specific conditions. These include rainfall, temperature, humidity and soil conditions. Some of the major biomes of the world are: forest, grassland, desert and tundra biomes.

• Aquatic ecosystems- can be classed as marine and freshwater ecosystems. Marine ecosystem includes the oceans, estuaries and coral reefs. Freshwater ecosystem includes lakes, ponds, streams, marshes and bogs

Structure and Functions of Ecosystems

• The structure of an ecosystem involves a description of the available plant and animal species.

• From a structural point of view, all ecosystems consist of:
→ Abiotic
→ Biotic factors.

• Abiotic factors include rainfall, temperature, sunlight, atmospheric humidity, soil conditions, inorganic substances (carbon dioxide, water, nitrogen, calcium, phosphorus, potassium, etc.).

• Biotic factors include the producers, the consumers (primary, secondary, tertiary) and the decomposers.

• Producers include all the green plants, which manufacture their own food through photosynthesis.

• The primary consumers include herbivorous animals like deer, goats, mice and all plant-eating animals. The carnivores include all the flesh-eating animals like snakes, tigers and lions.

• Certain carnivores that feed also on carnivores are known as top carnivores like hawks and mongooses.

• Decomposers are those that feed on dead organisms (for example, scavengers like vultures and crows), and further breaking down of the dead matter by other decomposing agents like bacteria and various micro- organisms.

Two types of food-chains

• Grazing food-chain, the first level starts with plants as producers and ends with carnivores as consumers at the last level, with the herbivores being at the intermediate level. There is a loss of energy at each level which may be through respiration, excretion or decomposition. The levels involved in a food- chain range between three to five and energy is lost at each level.

• A detritus food-chain is based on autotrophs energy capture initiated by grazing animals and involves the decomposition or breaking down of organic wastes and dead matter derived from the grazing food-chain.

Types of Biomes

• There are five major biomes
(i) Forest
(ii) Desert
(iii) Grassland
(iv) Aquatic
(v) Altitudinal biomes

Forests

Subtypes:
A. Tropical
1. Equatorial
2. Deciduous
B. Temperate
C. Boreal

Regions:
A. 1. 10° N-S A
2. 10° – 25° N-S
B. Eastern North America, N.E. Asia, Western and Central Europe
C. Broad belt of Eurasia and North America (parts of Siberia, Alaska, Canada and Scandinavia)

Climatic Characteristics:
A. 1. Temp. 20- 25C, evenly distributed
2. Temp. 25- 30°C, Rainfall, ave. ann. 1,000mm, seasonal
B. Temp. 20-30°
C. Rainfall evenly distributed 7501,500mm, Welldefined seasons and distinct winter.
D. Short moist moderately warm summers and long cold dry winter; very low temperatures. Precipitation mostly snowfall 400 -1,000 mm

Soil:
A. 1. Acidic, poor in nutrients
2. Rich in Nutrients
B. Fertile, enriched with decaying litter
C. Acidic and poor in nutrients, thin soil cover

Flora and Fauna:
A. Multi-layered canopy tall and large trees.
A2. Less dense, trees of medium height;many varieties coexist. Insects,bats, birds and mammals are common species in both.
B. Moderately dense broad leaved trees. With less diversity of plant species. Oak, Beach, Maple etc. are some common species. Squirrels, rabbits, skunks, birds, black bears, mountain lions etc.

C. Evergreen conifers like pine, fur and spruce etc. Wood peckers, hawks, bears, wolves, deer, hares and bats are common animals.

Desert

Subtypes:

A. Hot and Dry desert

B. Semi arid desert

C. Coastal desert

D. Cold desert

Regions:

A. Sahara, Kalahari, Marusthali, Rub-el-Khali

B. Marginal areas of hot deserts

C. Atacama

D. Tundra climatic regions

Climatic Characteristics:

A. Temp. 20 – 45°C.

B. 21 – 38°C.

C. 15 – 35°C.

D. 2 – 25°C A-D Rainfall is less than 50 mm

Soil:
Rich in nutrients with little or no organic matter

Flora and Fauna:
A-C. Scanty vegetation; few large mammals, insects, reptiles and birds
D. Rabbits, rats, Antelopes and ground squirrels

Grassland

Subtypes:
A. Tropical Savannah
B. Temperate Steppe

Regions:
A. Large areas of Africa, Australia, South America and India
B. Parts of Eurasia and North America

Climatic Characteristics:
A. Warm hot climates, Rainfall 500-1,250 mm
B. Hot summers and cold winter. Rainfall 500 900 mm

Soil:
A. Porous with thin layer of humus.
B. Thin flocculated soil, rich in bases

Flora and Fauna:
A. Grasses; trees and large shrubs absent; giraffes zebras, buffalos, leopards, hyenas, elephants, mice, moles, snakes and worms etc., are common animals
B. Grasses; occasional trees such as cotton​woods, oaks and willows; gazelles, zebras, rhin-oceros, wild horses, lions, varieties of birds, worms, snakes etc., are common animal

Aquatic

Subtypes:
A. Freshwater
B. Marine

Regions:
A. Lakes, streams, rivers and wetlands
B. Oceans, coral reefs, lagoons and estuaries

Climatic Characteristics:
A-B Temperatures vary widely with cooler air temperatures and high humidity

Soil:
A. Water, swamps and marshes
B. Water, tidal swamps and marshes

Flora and Fauna: Algal and other aquatic and marine plant communities with varieties of water dwelling animals.

Altitudinal

Regions:
Slopes of high mountain ranges like the Himalayas, the Andes and the Rockies

Climatic Characteristics:
Temperature and precipitation vary depending upon latitudinal zone

Soil:
Regolith over slopes

Flora and Fauna:
Deciduous to tundra vegetation varying according to altitude

Biogeochemical Cycles

• The sun is the basic source of energy on which all life depends. This energy initiates life processes in the biosphere through photosynthesis, the main source of food and energy for green plants.

• Out of the total solar insolation that reaches the earth’s surface, only a very small fraction (0.1 per cent) is fixed in photosynthesis.

• Life on earth consists of a great variety of living organisms. These living organisms exist and survive in a diversity of associations. Such survival involves the presence of systemic flows such as flows of energy, water and nutrients.

• Balance of the chemical elements is maintained by a cyclic passage through the tissues of plants and animals. The cycle starts by absorbing the chemical elements by the organism and is returned to the air, water and soil through decomposition.

• These cycles are largely energised by solar insolation. These cyclic movements of chemical elements of the biosphere between the organism and the environment are referred to as biogeochemical cycles.

• There are two types of biogeochemical cycles : the gaseous and the sedimentary cycle. In the gaseous cycle, the main reservoir of nutrients is the atmosphere and the ocean.

• In the sedimentary cycle, the main reservoir is the soil and the sedimentary and other rocks of the earth’s crust.

The Water Cycle

• All living organisms, the atmosphere and the lithosphere maintain between them a circulation of water in solid, liquid or gaseous form referred to as the water or hydrologic cycle.

The Carbon Cycle

• Carbon basic elements of all living organisms.

• The carbon cycle is mainly the conversion of carbon dioxide. This conversion is initiated by the fixation of carbon dioxide from the atmosphere through photosynthesis.

• Such conversion results in the production of carbohydrate, glucose that may be converted to other organic compounds such as sucrose, starch, cellulose, etc. Here, some of the carbohydrates are utilised directly by the plant itself.

• During this process, more carbon dioxide is generated and is released through its leaves or roots during the day. The remaining carbohydrates not being utilised by the plant become part of the plant tissue. Plant tissues are either being eaten by the herbivorous animals or get decomposed by the micro- organisms.

• The herbivores convert some of the consumed carbohydrates into carbon dioxide for release into the air through respiration.

• The micro-organisms decompose the remaining carbohydrates after the animal dies. The carbohydrates that are decomposed by the micro-organisms then get oxidised into carbon.

The Oxygen Cycle

• Oxygen is the main by-product of photosynthesis. It is involved in the oxidation of carbohydrates with the release of energy, carbon dioxide and water.

• The cycling of oxygen is a highly complex process. Oxygen occurs in a number of chemical forms and combinations. It combines with nitrogen to form nitrates and with many other minerals and elements to form various oxides such as the iron oxide, aluminium oxide and others.

• Much of oxygen is produced from the decomposition of water molecules by sunlight during photosynthesis and is released in the atmosphere through transpiration and respiration processes of plants.

The Nitrogen Cycle

• Nitrogen is a major constituent of the atmosphere comprising about seventy-nine per cent of the atmospheric gases.

• It is also an essential constituent of different organic compounds such as the amino acids, nucleic acids, proteins, vitamins and pigments.

• Only a few types of organisms like certain species of soil bacteria and blue green algae are capable of utilising it directly in its gaseous form.

• Generally, nitrogen is usable only after it is fixed. Ninety per cent of fixed nitrogen is biological.

• The principal source of free nitrogen is the action of soil micro-organisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil.

• Nitrogen can also be fixed in the atmosphere by lightning and cosmic radiation. In the oceans, some marine animals can fix it.

• After atmospheric nitrogen has been fixed into an available form, green plants can assimilate it.

• Herbivorous animals feeding on plants, in turn, consume some of it.

• Dead plants and animals, excretion of nitrogenous wastes are converted into nitrites by the action of bacteria present in the soil.

• Some bacteria can even convert nitrites into nitrates that can be used again by green plants. There are still other types of bacteria capable of converting nitrates into free nitrogen, a process known as denitrification.

Other Mineral Cycles

Phosphorus, sulphur, calcium and potassium

• They usually occur as salts dissolved in soil water or lakes, streams and seas.

• Mineral salts come directly from the earth’s crust by weathering where the soluble salts enter the water cycle, eventually reaching the sea.

• Other salts are returned to the earth’s surface through sedimentation, and after weathering, they again enter the cycle.

• All living organisms fulfill their mineral requirements from mineral solutions in their environments. Other animals receive their mineral needs from the plants and animals they consume.

• After the death of living organisms, the minerals are returned to the soil and water through decomposition and flow.

Ecological Balance

• Ecological balance is a state of dynamic equilibrium within a community of organisms in a habitat or ecosystem. It can happen when the diversity of the living organisms remains relatively stable.

• Gradual changes do take place but that happens only through natural succession. It can also be explained as a stable balance in the numbers of each species in an ecosystem. This occurs through competition and cooperation between different organisms where population remains stable.

• This balance is brought about by the fact that certain species compete with one another determined by the environment in which they grow. This balance is also attained by the fact that some species depend on others for their food and sustenance.

• Such accounts are encountered in vast grasslands where the herbivorous animals (deer, zebras, buffaloes, etc.) are found in plenty. On the other hand, the carnivorous animals (tigers, lions, etc.) that are not usually in large numbers, hunt and feed on the herbivores, thereby controlling their population.

• In the plants, any disturbance in the native forests such as clearing the forest for shifting cultivation usually brings about a change in the species distribution.

• This change is due to competition where the secondary forest species such as grasses, bamboos or pines overtakes the native species changing the original forest structure. This is called succession.

Notes of Ch 14 Movements of Ocean Water| Class 11th Geography

Introduction

• The horizontal motion refers to the ocean currents and waves. The vertical motion refers to tides.

Ocean currents are the continuous flow of huge amount of water in a definite direction while the waves are the horizontal motion of water.

• Water moves ahead from one place to another through ocean currents while the water in the waves does not move, but the wave trains move ahead.

• The vertical motion refers to the rise and fall of water in the oceans and seas. Due to attraction of the sun and the moon, the ocean water is raised up and falls down twice a day. The upwelling of cold water from subsurface and the sinking of surface water are also forms of vertical motion of ocean water.

Waves

• Waves are actually the energy, not the water as such, which moves across the ocean surface.

• Water particles only travel in a small circle as a wave passes. Wind provides energy to the waves.

• Wind causes waves to travel in the ocean and the energy is released on shorelines.

• The motion of the surface water seldom affects the stagnant deep bottom water of the oceans. As a wave approaches the beach, it slows down. This is due to the friction occurring between the dynamic water and the sea floor.

• When the depth of water is less than half the wavelength of the wave, the wave breaks.

• The largest waves are found in the open oceans. Waves continue to grow larger as they move and absorb energy from the wind.

• When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until white caps appear in the breaking waves.

• A wave’s size and shape reveal its origin. Steep waves are fairly young ones and are probably formed by local wind.

• Slow and steady waves originate from far away places, possibly from another hemisphere.

• The maximum wave height is determined by the strength of the wind, i.e. how long it blows and the area over which it blows in a single direction.

• Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward. The falling water pushes the former troughs upward, and the wave moves to a new position

Characteristics of Waves

• Wave crest and trough: The highest and lowest points of a wave are called the crest and trough respectively.

• Wave height: It is the vertical distance from the bottom of a trough to the top of a crest of a wave.

• Wave amplitude: It is one-half of the wave height.

• Wave period: It is merely the time interval between two successive wave crests or troughs as they pass a fixed point.

• Wavelength: It is the horizontal distance between two successive crests.

• Wave speed: It is the rate at which the wave moves through the water, and is measured in knots.

Tides

• The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.

• Movement of water caused by meteorological effects (winds and atmospheric pressure changes)are called surges. Surges are not regular like tides.

• The moon’s gravitational pull to a great extent and to a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides.

• Another factor is centrifugal force, which is the force that acts to counter balance the gravity.

• Together, the gravitational pull and the centrifugal force are responsible for creating the two major tidal bulges on the earth. On the side of the earth facing the moon, a tidal bulge occurs while on the opposite side though the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes tidal bulge on the other side.

• The tidal bulges on wide continental shelves, have greater height. When tidal bulges hit the mid-oceanic islands they become low. The shape of bays and estuaries along a coastline can also magnify the intensity of tides. Funnel-shaped bays greatly change tidal magnitudes.

• When the tide is channelled between islands or into bays and estuaries they are called tidal currents.
Tides of Bay of Fundy, Canada The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal bulge is 15 – 16 m

Types of Tides

Tides based on Frequency

• Semi-diurnal tide : The most common tidal pattern, featuring two high tides and two low tides each day. The successive high or low tides are approximately of the same height.

• Diurnal tide : There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height.

• Mixed tide : Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.

Tides based on the Sun, Moon and the Earth Positions

• The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth. Spring tides and neap tides come under this category:

• Spring tides: The position of both the sun and the moon in relation to the earth has direct bearing on tide height.

• When the sun, the moon and the earth are in a straight line, the height of the tide will be higher.

• These are called spring tides and they occur twice a month, one on full moon period and another during new moon period.

• Neap tides: Normally, there is a seven day interval between the spring tides and neap tides.

• At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another.

• The Moon’s attraction, though more than twice as strong as the sun’s, is diminished by the counteracting force of the sun’s gravitational pull.

• Once in a month, when the moon’s orbit is closest to the earth (perigee), unusually high and low tides occur. During this time the tidal range is greater than normal. Two weeks later, when the moon is farthest from earth (apogee), the moon’s gravitational force is limited and the tidal ranges are less than their average height.

• The time between the high tide and low tide, when the water level is falling, is called the ebb. The time between the low tide and high tide, when the tide is rising, is called the flow or flood.

Importance of Tides

• Since tides are caused by the earth-moon-sun positions which are known accurately, the tides can be predicted well in advance.

• This helps the navigators and fishermen plan their activities. Tidal flows are of great importance in navigation.

• Tidal heights are very important, especially harbours near rivers and within estuaries having shallow ‘bars’ at the entrance, which prevent ships and boats from entering into the harbour.

• Tides are also helpful indesilting the sediments and in removing polluted water from river estuaries.

• Tides are used to generate electrical power (in Canada, France, Russia, and China).

• A 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is under way.

Ocean Currents

• Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction.

• Ocean currents are influenced by two types of forces namely:
(i) primary forces that initiate the movement of water;
(ii) secondary forces that influence the currents to flow.

• The primary forces that influence the currents are:
(i) heating by solar energy;
(ii) wind;
(iii) gravity;
(iv) Coriolis force.

• Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope.

• Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course.

• Gravity tends to pull the water down the pile and create gradient variation.

• The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere. These large accumulations of water and the flow around them are called Gyres.

• Differences in water density affect vertical mobility of ocean currents

Characteristics of Ocean Currents

• Currents are referred to by their “drift”.

• Usually, the currents are strongest near the surface and may attain speeds over five knots. At depths, currents are generally slow with speeds less than 0.5 knots.

• The speed of a current as its “drift.” Drift is measured in terms of knots. The strength of a current refers to the speed of the current.

Types of Ocean Currents

• The ocean currents may be classified based on their depth as:

(i) Surface currents

(ii) Deep water currents

• Surface currents constitute about 10 per cent of all the water in the ocean, these waters are the upper 400 m of the ocean; deep water currents make up the other 90 per cent of the ocean water.

• These waters move around the ocean basins due to variations in the density and gravity.

• Ocean currents can also be classified based on temperature as:

(i) Cold currents

(ii) Warm currents

• Cold currents bring cold water into warm water areas. These currents are usually found on the west coast of the continents in the low and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere.

• Warm currents bring warm water into cold water areas and are usually observed on the east coast of continents in the low and middle latitudes (true in both hemispheres). In the northern hemisphere they are found on the west coasts of continents in high latitudes.

Major Ocean Currents

• Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and Coriolis force.

• The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern.

• The air circulation over the oceans in the middle latitudes is mainly anticyclonic (more pronounced in the southern hemisphere than in the northern hemisphere).

• In regions of pronounced monsoonal flow, the monsoon winds influence the current movements.

• Due to the coriolis force, the warm currents from low latitudes tend to move to the right in the northern hemisphere and to their left in the southern hemisphere.

• The oceanic circulation transports heat from one latitude belt to another in a manner similar to the heat transported by the general circulation of the atmosphere.

• The cold waters of the Arctic and Antarctic circles move towards warmer water in tropical and equatorial regions, while the warm waters of the lower latitudes move pole wards.

Effects of Ocean Currents

• Ocean currents have a number of direct and indirect influences on human activities.

• West coasts of the continents in tropical and subtropical latitudes (except close to the equator) are bordered by cool waters.

• The best fishing grounds of the world exist mainly in these mixing zones.

• There is fog, but generally the areas are arid.

• The mixing of warm and cold currents help to replenish the oxygen and favor the growth of planktons, the primary food for fish population.

• They are characterized by cool summers and relatively mild winters with a narrow annual range of temperatures.

• West coasts of the continents in the middle and higher latitudes are bordered by warm waters which cause a distinct marine climate.

• Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates.

• These areas lie in the western margins of the subtropical anti-cyclones.

• Their average temperatures are relatively low with a narrow diurnal and annual ranges.

Notes of Ch 13 Water (Oceans)| Class 11th Geography

Hydrological Cycle

• Water is a cyclic resource. It can be used and re-used.

• The hydrological cycle, is the circulation of water within the earth’s hydrosphere in different forms i.e. the liquid, solid and the gaseous phases. It also refers to the continuous exchange of water between the oceans atmosphere, land surface and subsurface and the organisms.

• About 71 per cent of the planetary water is found in the oceans.

• The remaining is held as freshwater in glaciers and icecaps, groundwater sources, lakes, soil moisture, atmosphere, streams and within life.

• Nearly 59 per cent of the water that falls on land returns to the atmosphere through evaporation from over the oceans as well as from other places. The remainder runs-off on the surface, infiltrates into the ground or a part of it becomes glacier.

• It is to be noted that the renewable water on the earth is constant while the demand is increasing tremendously. This leads to water crisis in different parts of the world — spatially and temporally. The pollution of river waters has further aggravated the crisis.

Components and Processes of the Water Cycle

Components	Processes
Water storage in oceans	Evaporation Evapotranspiration Sublimation
Water in the atmosphere	Condensation Precipitation
Water storage in ice and snow	Snowmelt runoff to streams
Surface runoff	Stream flow fresh water storage infilitation
Groundwater storage	Groundwater discharge springs

Relief of the Ocean Floor

• The oceans are confined to the great depressions of the earth’s outer layer.

• The geographers have divided the oceanic part of the earth into five oceans, namely the Pacific, the Atlantic, the Indian, and the Arctic.

• The various seas, bays, gulfs and other inlets are parts of these four large oceans.

• A major portion of the ocean floor is found between 3-6 km below the sea level.

• The ‘land’ under the waters of the oceans, that is, the ocean floor exhibits complex and varied features as those observed over the land.

• The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These features are formed, like those of the continents, by the factors of tectonic, volcanic and depositional processes.

Divisions of the Ocean Floors

The ocean floors can be divided into four major divisions:
(i) the Continental Shelf;
(ii) the Continental Slope;
(iii) the Deep Sea  Plain;
(iv) the Oceanic Deeps.

• Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, sea mounts, guyots, trenches, canyons, etc.

Continental Shelf

• The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less.

• The shelf typically ends at a very steep slope, called the shelf break.

• The average width of continental shelves is about 80 km.

• The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc.

• On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.

• Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Continental Slope

• The continental slope connects the continental shelf and the ocean basins.

• It begins where the bottom of the continental shelf sharply drops off into a steep slope.

• The gradient of the slope region varies between 2-5°.

• The depth of the slope region varies between 200 and 3,000 m. The slope boundary indicates the end of the continents.

• Canyons and trenches are observed in this region.

Deep Sea Plain

• Deep sea plains are gently sloping areas of the ocean basins. These are the flattest and smoothest regions of the world.

• The depths vary between 3,000 and 6,000m. These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

• These areas are the deepest parts of the oceans.

• The trenches are relatively steep sided, narrow basins.

• They are some 3-5 km deeper than the surrounding ocean floor.

• They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes. That is why they are very significant in the study of plate movements.

• As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.

Minor Relief Features

• Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.

Mid-Oceanic Ridges

• A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.

• Iceland, a part of the mid- Atlantic Ridge, is an example.

Submarine Canyons

• These are deep valleys, some comparable to the Grand Canyon of the Colorado river.

• They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.

• The Hudson Canyon is the best known submarine canyon in the world.

Guyots

• It is a flat topped seamount.

• They show evidences of gradual subsidence through stages to become flat topped submerged mountains.

• It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Atoll

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.

• It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.

Temperature of ocean waters

Factors Affecting Temperature Distribution
• Latitude : The temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.

• Unequal distribution of land and water : The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.

• Prevailing wind : The winds blowing from the land towards the oceans drive warm surface water away form the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.

• Ocean currents : Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.

Horizontal and Vertical Distribution of Temperature

• The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth.

• The profile shows a boundary region between the surface waters of the ocean and the deeper layers.

• The boundary usually begins around 100 – 400 m below the sea surface and extends several hundred of metres downward.

• This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.

• About 90 per cent of the total volume of water is found below the thermocline in the deep ocean. In this zone, temperatures approach 0° C.

• The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
→ The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid latitudes it develops only during summer.
→ The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
→ The third layer is very cold and extends upto the deep ocean floor.

• In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight. Here, only one layer of cold water exists, which extends from surface to deep ocean floor.

• The average temperature of surface water of the oceans is about 27°C and it gradually decreases from the equator towards the poles. The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.

• The oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere. The highest temperature is not recorded at the equator but slightly towards north of it.

• This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.

• The heat is transmitted to the lower sections of the oceans through the process of convection.

Salinity of ocean waters

• Salinity is the term used to define the total content of dissolved salts in sea.

• It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.

• It is usually expressed as parts per thousand or ppt.

• Salinity of 24.7 %has been considered as the upper limit to demarcate ‘brackish water’.

Factors affecting ocean salinity

• The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.

• Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in polar regions by the processes of freezing and thawing of ice.

• Wind, also influences salinity of an area by transferring water to other areas.

• The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.

Highest salinity in water bodies Lane Van in Turkey (330 o /∞).
Dead Sea (238 o /∞).
Great Salt Lake (220 o /∞).

Horizontal distribution of salinity

• The salinity for normal open ocean ranges between 33°/∞ and 37°/∞ .

• In the land locked Red Sea, it is as high as 41°/oo , while in the estuaries and the Arctic, the salinity fluctuates from 0 – 35°/∞ , seasonally.

• In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70 o/∞.

• The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.

• Salinity decreases from 35°/∞ – 31°/∞ on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region.

• The highest salinity is recorded between 15° and 20° latitudes.

• Maximum salinity (37 o/∞) is observed between 20° N and 30° N and 20° W – 60° W.

• It gradually decreases towards the north.

• The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low salinity due to influx of river waters in large quantity.

• The Mediterranean Sea records higher salinity due to high evaporation.

• Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers.

• The average salinity of the Indian Ocean is 35 o/∞. The low salinity trend is observed in the Bay of Bengal due to influx of river water.

• On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

Vertical Distribution of Salinity

• Salinity changes with depth, but the way it changes depends upon the location of the sea.

• Salinity at the surface increases by the loss of water to ice or evaporation, or decreased by the input of fresh waters, such as from the rivers.

• Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the salt is ‘added.’

• There is a marked difference in the salinity between the surface zones and the deep zones of the oceans.

• The lower salinity water rests above the higher salinity dense water.

• Salinity, generally, increases with depth and there is a distinct zone called the halocline, where salinity increases sharply. Other factors being constant, increasing salinity of seawater causes its density to increase.

• High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.

Notes of Ch 12 World Climate and Climate Change| Class 11th Geography

Introduction

• Three broad approaches have been adopted for classifying climate.

• They are empirical, genetic and applied.

• Empirical classification is based on observed data, particularly on temperature and precipitation.

• Genetic classification attempts to organise climates according to their causes.

• Applied classification is for specific purpose.

Koeppen’s Scheme of classification of climate

• The most widely used classification of climate is the empirical climate classification scheme developed by V. Koeppen. Koeppen identified a close relationship between the distribution of vegetation and climate.

• He selected certain values of temperature and precipitation and related them to the distribution of vegetation and used these values for classifying the climates.

• It is an empirical classification based on mean annual and mean monthly temperature and
precipitation data.

• He introduced the use of capital and small letters to designate climatic groups and types.

• Although developed in 1918 and modified over a period of time, Koeppen’s scheme is still popular and in use.

• Koeppen recognised five major climatic groups, four of them are based on temperature and one on precipitation. Below table lists the climatic groups and their characteristics according to Koeppen.

• The capital letters : A,C, D and E delineate humid climates and B dry climates.

Group	Characteristics
A – Tropical	Average temperature of the coldest month is 18° C or higher
B – Dry Climates	Potential evaporation exceeds precipitation
C – Warm Temperate	The average temperature of the coldest month of the (Mid-latitude) climates years is higher than minus 3° C but below 18° C
D – Cold Snow Forest	The average temperature of the coldest month is minus.
E – Cold Climates	Average temperature for all months is below 10° C
H – High Land	Cold due to elevation

The capital letters : A,C, D and E delineate humid climates and B dry climates.

• The climatic groups are subdivided into types, designated by small letters, based on seasonality of precipitation and temperature characteristics.

• The seasons of dryness are indicated by the small letters : f, m, w and s, where f corresponds to no dry season m – monsoon climate, w- winter dry season and s – summer dry season. The small letters a, b, c and d refer to the degree of severity of temperature. The B- Dry Climates are subdivided using the capital letters S for steppe or semi-arid and W for deserts.

Group	Type	Letter code	Characteristics
A -Tropical Humid Climate	Tropical wet Tropical monsoon Tropical wet and dry	Af Am Aw	No dry season Monsoonal, short dry season Winter dry season
B-dry climate	Subtropical steppe Subtropical desert Subtropical steppe Mid-latitude desert	BSH BWH BSK BWK	Low-latitude semi arid or dry Low-latitude arid or dry Mid-latitude arid or dry
C-Warm temperate (Mid- latitude) Climates	Humid subtropical Mediterranean Marine west coast	Cfa Cs Cfb	No dry season, warm summer Dry hot summer No dry season, warm and cool summer
D-cold snow forest Climate	Humid continental Subractic	Df Dw	No dry season. Severe winter Winter dry and very severe
E-cold climate	Tundra Polar ice cap	ET EF	No true Perennial ice
H-highland	Highland	H	Highland with snow cover

Group A : Tropical Humid Climates

• Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn.

• The sun being overhead throughout the year and the presence of Inter Tropical Convergence Zone (ITCZ) make the climate hot and humid.

• Annual range of temperature is very low and annual rainfall is high.

• The tropical group is divided into three types, namely:
(i) Af- Tropical wet climate;
(ii) Am – Tropical monsoon climate;

(iii) Aw- Tropical wet and dry climate.

Tropical Wet Climate (Af)

• Tropical wet climate is found near the equator. The major areas are the Amazon Basin in South America, western equatorial Africa and the islands of East Indies.

• Significant amount of rainfall occurs in every month of the year as thunder showers in the afternoon.

• The temperature is uniformly high and the annual range of temperature is negligible.

• The maximum temperature on any day is around 30°C while the minimum temperature is around 20°C.

• Tropical evergreen forests with dense canopy cover and large biodiversity are found in this climate.

Tropical Monsoon Climate (Am)

• Tropical monsoon climate (Am) is found over the Indian sub-continent, North Eastern part of South America and Northern Australia.

• Heavy rainfall occurs mostly in summer. Winter is dry.

Tropical Wet and Dry Climate (Aw)

• Tropical wet and dry climate occurs north and south of Af type climate regions.

• It borders with dry climate on the western part of the continent and Cf or Cw on the eastern part.

• Extensive Aw climate is found to the north and south of the Amazon forest in Brazil and adjoining parts of Bolivia and Paraguay in South America, Sudan and south of Central Africa.

• The annual rainfall in this climate is considerably less than that in Af and Am climate types and is variable also.

• The wet season is shorter and the dry season is longer with the drought being more severe.

• Temperature is high throughout the year and diurnal ranges of temperature are the greatest in the dry season.

• Deciduous forest and tree-shredded grasslands occur in this climate.

Dry Climates : B

• Dry climates are characterised by very low rainfall that is not adequate for the growth of plants.

• These climates cover a very large area of the planet extending over large latitudes from 15° – 60° north and south of the equator.

• At low latitudes, from 15°-30°, they occur in the area of subtropical high where subsidence and inversion of temperature do not produce rainfall.

• On the western margin of the continents, adjoining the cold current, particularly over the west coast of South America, they extend more equatorwards and occur on the coast land.

• In middle latitudes, from 35° – 60° north and south of equator, they are confined to the interior of continents where maritime-humid winds do not reach and to areas often surrounded by mountains.

• Dry climates are divided into steppe or semi-arid climate (BS) and desert climate (BW). They are further subdivided as subtropical steppe (BSh) and subtropical desert (BWh) at latitudes from 15°-35° and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at latitudes between 35°-60°.

Subtropical Steppe (BSh) and Subtropical Desert (BWh) Climates

• Subtropical steppe (BSh) and subtropical desert (BWh) have common precipitation and temperature characteristics.

• Located in the transition zone between humid and dry climates, subtropical steppe receives slightly more rainfall than the desert, adequate enough for the growth of sparse grasslands. The rainfall in both the climates is highly variable.

• The variability in the rainfall affects the life in the steppe much more than in the desert, more often causing famine.

• Rain occurs in short intense thundershowers in deserts and is ineffective in building soil moisture.

• Fog is common in coastal deserts bordering cold currents.

• Maximum temperature in the summer is very high. The highest shade temperature of 58° C was recorded at Al Aziziyah, Libya on 13 September 1922. The annual and diurnal ranges of temperature are also high.

Warm Temperate (Mid-Latitude) Climates-C

• Warm temperate (mid-latitude) climates extend from 30° – 50° of latitude mainly on the eastern and western margins of continents. These climates generally have warm summers with mild winters.

• They are grouped into four types:
(i) Humid subtropical, i.e. dry in winter and hot in summer (Cwa);
(ii) Mediterranean (Cs);
(iii) Humid subtropical, i.e. no dry season and mild winter (Cfa);
(iv) Marine west coast climate (Cfb).

Humid Subtropical Climate (Cwa)

• Humid subtropical climate occurs pole ward of Tropic of Cancer and Capricorn, mainly in North Indian plains and South China interior plains. The climate is similar to Aw climate except that the temperature in winter is warm.

Mediterranean Climate (Cs)

• As the name suggests, Mediterranean climate occurs around Mediterranean sea, along the west coast of continents in subtropical latitudes between 30° – 40° latitudes e.g. — Central California, Central Chile, along the coast in south eastern and south western Australia.

• These areas come under the influence of sub tropical high in summer and westerly wind in winter. Hence, the climate is characterised by hot, dry summer and mild, rainy winter.

• Monthly average temperature in summer is around 25° C and in winter below 10°C. The annual precipitation ranges between 35 – 90 cm.

Humid Subtropical (Cfa) Climate

• Humid subtropical climate lies on the eastern parts of the continent in subtropical latitudes. In this region the air masses are generally unstable and cause rainfall throughout the year.

• They occur in eastern United States of America, southern and eastern China, southern Japan, northeastern Argentina, coastal south Africa and eastern coast of Australia.

• The annual averages of precipitation vary from 75-150 cm. Thunderstorms in summer and frontal precipitation in winter are common.

• Mean monthly temperature in summer is around 27°C, and in winter it varies from 5°-12° C. The daily range of temperature is small.

Marine West Coast Climate (Cfb)

• Marine west coast climate is located poleward from the Mediterranean climate on the west coast of the continents.

• The main areas are: Northwestern Europe, west coast of North America, north of California, southern Chile, southeastern Australia and New Zealand.

• Due to marine influence, the temperature is moderate and in winter, it is warmer than for its latitude.

• The mean temperature in summer months ranges from 15°-20°C and in winter 4°-10°C.

• The annual and daily ranges of temperature are small.

• Precipitation occurs throughout the year. Precipitation varies greatly from 50-250 cm.

Cold Snow Forest Climates (D)

• Cold snow forest climates occur in the large continental area in the northern hemisphere between 40°-70° north latitudes in Europe, Asia and North America.

• Cold snow forest climates are divided into two types:
(i) Df- cold climate with humid winter;
(ii) Dw- cold climate with dry winter. The severity of winter is more pronounced in higher latitudes.

Cold Climate with Humid Winters (Df)

• Cold climate with humid winter occurs poleward of marine west coast climate and mid latitude steppe. The winters are cold and snowy.

• The frost free season is short.

• The annual ranges of temperature are large.

• The weather changes are abrupt and short.

• Poleward, the winters are more severe.

Cold Climate with Dry Winters (Dw)

• Cold climate with dry winter occurs mainly over Northeastern Asia.

• The development of pronounced winter anti cyclone and its weakening in summer sets in monsoon like reversal of wind in this region.

• Poleward summer temperatures are lower and winter temperatures are extremely low with many locations experiencing below freezing point temperatures for up to seven months in a year.

• Precipitation occurs in summer. The annual precipitation is low from 12-15 cm.

Polar Climates (E)

• Polar climates exist poleward beyond 70° latitude.

• Polar climates consist of two types:
(i) Tundra (ET);
(ii) Ice Cap (EF).

Tundra Climate (ET)

• The tundra climate (ET) is so called after the types of vegetation, like low growing mosses, lichens and flowering plants.

• This is the region of permafrost where the sub soil is permanently frozen.

• The short growing season and water logging support only low growing plants.

• During summer, the tundra regions have very long duration of day light.

Ice Cap Climate (EF)

• The ice cap climate (EF) occurs over interior Greenland and Antartica.

• Even in summer, the temperature is below freezing point.

• This area receives very little precipitation. The snow and ice get accumulated and the mounting pressure causes the deformation of the ice sheets and they break.

• They move as icebergs that float in the Arctic and Antarctic waters.

• Plateau Station , Antarctica ,79°S, portray this climate.

Highland Climates (H)

• Highland climates are governed by topography.

• In high mountains, large changes in mean temperature occur over short distances.

• Precipitation types and intensity also vary spatially across high lands.

• There is vertical zonation of layering of climatic types with elevation in the mountain environment.

Climate Change

• India witnessed alternate wet and dry periods.

• Archaeological findings show that the Rajasthan desert experienced wet and cool climate around 8,000 B.C.

• The period 3,0001,700 B.C. had higher rainfall. From about 2,000-1,700 B.C., this region was the centre of the Harappan civilization. Dry conditions since then.

• During the Pleistocene epoch, glacial and inter-glacial periods occurred, the last major peak glacial period ago.

• The present inter-glacial period started 10,000 years ago.

Climate in the recent past

• Historical records of crop yield or crop failures, of floods and migration of people tell about the effects of changing climate.

• The worst devastating drought in the Sahel region, south of the Sahara desert, from 1967-1977 is one such variability.

• A number of times Europe witnessed warm, wet, cold and dry periods, the significant episodes were the warm and dry conditions in the tenth and eleventh centuries.

• Variability in climate occurs all the time. The 1990s recorded the warmest temperature of the century and some of the worst floods around the world.

• During the 1930s, severe drought occurred in southwestern Great Plains of the United
States, described as the dust bowl.

Causes of Climate Change

• The changes in solar output associated with sunspot activities. Sunspots are dark and cooler patches on the sun which increase and decrease in a cyclical manner. According to some meteorologists, when the number of sunspots increase, cooler and wetter
weather and greater storminess occur.

• An another astronomical theory is Millankovitch oscillations, which infer cycles in the variations in the earth’s orbital characteristics around the sun, the wobbling of the earth and the changes in the earth’s axial tilt.

• Decrease in sunspot numbers is associated with warm and drier conditions. Yet, these findings are not statistically significant.

• Volcanism is considered as another cause for climate change. Volcanic eruption throws up lots of aerosols into the atmosphere. These aerosols remain in the atmosphere for a considerable period of time reducing the sun’s radiation reaching the Earth’s surface.

• All these alter the amount of insolation received from the sun, which in turn, might have a bearing on the climate.

Global Warming

• Due to the presence of greenhouse gases, the atmosphere is behaving like a greenhouse.

• The atmosphere also transmits the incoming solar radiation but absorbs the vast majority of long wave radiation emitted upwards by the earth’s surface.

• The gases that absorb long wave radiation are called greenhouse gases.

• The processes that warm the atmosphere are often collectively referred to as the greenhouse effect.

• The term greenhouse is derived from the analogy to a greenhouse used in cold areas for preserving heat. A greenhouse is made up of glass. The glass which is transparent to incoming short wave solar radiation is opaque to outgoing long wave radiation.

• The glass, therefore, allows in more radiation and prevents the long wave radiation going outside the glass house, causing the temperature inside the glasshouse structure warmer than outside.

Greenhouse Gases (GHGs)

• The primary GHGs of concern today are carbon dioxide (CO₂), Chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃).

• Some other gases such as nitric oxide (NO) and carbon monoxide (CO) easily react with GHGs and affect their concentration in the atmosphere.

• The effectiveness of any given GHG molecule will depend on the magnitude of the increase in its concentration, its life time in the atmosphere and the wavelength of radiation that it absorbs.

• The chlorofluorocarbons (CFCs) are highly effective.

• Ozone which absorbs ultra violet radiation in the stratosphere is very effective in absorbing terrestrial radiation when it is present in the lower troposphere.

• Another important point to be noted is that the more time the GHG molecule remains in the atmosphere, the longer it will take for earth’s atmospheric system to recover from any change brought about by the latter.

• The largest concentration of GHGs in the atmosphere is carbon dioxide.

• The emission of CO₂ comes mainly from fossil fuel combustion (oil, gas and coal).

• Forests and oceans are the sinks for the carbon dioxide.

• Forests use CO₂ in their growth. So, deforestation due to changes in land use, also increases the concentration of CO₂.

• Doubling of concentration of CO₂ over pre-industrial level is used as an index for estimating the changes in climate in climatic models.

• The CFCs which drift into the stratosphere destroy the ozone. Large depletion of ozone occurs over Antarctica. The depletion of ozone concentration in the stratosphere is called the ozone hole. This allows the ultra violet rays to pass through the troposphere.

• International efforts have been initiated for reducing the emission of GHGs into the atmosphere.
The most important one is the Kyoto protocol proclaimed in 1997. This protocol went into effect in 2005, ratified by 141 nations.

• Kyoto protocol bounds the 35 industrialised countries to reduce their emissions by the year 2012 to 5 per cent less than the levels prevalent in the year 1990.

Notes of Ch 11 Water in the Atmosphere| Class 11th Geography

Introduction

• Water is present in the atmosphere in three forms namely – gaseous, liquid and solid.

• The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration.

• Thus, there is a continuous exchange of water between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation.

• Water vapour present in the air is known as humidity. It is expressed quantitatively in different ways.

• The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.

• The ability of the air to hold water vapour depends entirely on its temperature.

• The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity.

• The air containing moisture to its full capacity at a given temperature is said to be saturated.

• It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage.

• The temperature at which saturation occurs in a given sample of air is known as dew point.

Evaporation and Condensation
Evaporation

• It is a process by which water is transformed from liquid to gaseous state. Heat is the main cause for evaporation. The temperature at which the water starts evaporating is referred to as the latent heat of vapourisation.

• Increase in temperature increases water absorption and retention capacity of the given parcel of air.

• Movement of air replaces the saturated layer with the unsaturated layer. Hence, the greater the movement of air, the greater is the evaporation.

Condensation

• The transformation of water vapour into water is called condensation.

• Condensation is caused by the loss of heat. When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases.

• In free air, condensation results from cooling around very small particles termed as hygroscopic condensation nuclei.

• Condensation also takes place when the moist air comes in contact with some colder object and it may also take place when the temperature is close to the dew point.

• Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air.

• Condensation is influenced by the volume of air, temperature, pressure and humidity.

• Condensation takes place:

(i) when both the volume and the temperature are reduced,

(ii) When the temperature of the air is reduced to dew point with its volume remaining constant,

(iii) when moisture is added to the air through evaporation. However, the most favourable condition for condensation is the decrease in air temperature.

• After condensation the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog and clouds. Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.

Dew

• When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew.

• The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.

Frost

• Frost forms on cold surfaces when condensation takes place below freezing point (0°C), i.e. the dew point is at or below the freezing point.

• The excess moisture is deposited in the form of minute ice crystals instead of water droplets.

• The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.

Fog and Mist

• When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles.

• Fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero.

• In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as smog.

• The only difference between the mist and fog is that mist contains more moisture than the fog.

• In mist each nuceli contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.

• Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by the dust, smoke, and the salt particles.

Clouds

• Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations.

• As the clouds are formed at some height over the surface of the earth, they take various shapes.

• According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types:
(i) cirrus;
(ii) cumulus;
(iii) stratus;

(iv) nimbus.

Cirrus

• Cirrus clouds are formed at high altitudes (8,000 – 12,000m).

• They are thin and detatched clouds having a feathery appearance. They are always white in colour.

Cumulus

• Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000 – 7,000 m.

• They exist in patches and can be seen scattered here and there. They have a flat base.

Stratus

• As their name implies, these are layered clouds covering large portions of the sky.

• These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.

Nimbus

• Nimbus clouds are black or dark gray.

• They form at middle levels or very near to the surface of the earth.

• These are extremely dense and opaque to the rays of the sun.

• Nimbus clouds are shapeless masses of thick vapour.

• A combination of these four basic types can give rise to the following types of clouds: high

clouds:
(i) cirrus, cirrostratus, cirrocumulus;
(ii) middle clouds – altostratus and altocumulus;
(iii) low clouds- stratocumulus and nimbostratus
(iv) clouds with extensive vertical development – cumulus and cumulonimbus.

Precipitation

• The process of continuous condensation in free air helps the condensed particles to grow in size.

• When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface. So after the condensation of water vapour, the release of moisture is known as precipitation.

• The precipitation in the form of water is called rainfall, when the temperature is lower than the 00C, precipitation takes place in the form of fine flakes of snow and is called snowfall.

• Moisture is released in the form of hexagonal crystals. These crystals form flakes of snow. Besides rain and snow, other forms of precipitation are sleet and hail.

• Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet.

• Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.

• Sometimes, drops of rain after being released by the clouds become solidified in to small rounded solid pieces of ice and which reach the surface of the earth are called hailstones.

Types of Rainfall

On the basis of origin, rainfall may be classified into three main types –
(i) the cyclonic or frontal,

(ii) orographic or relief and

(iii) the convectional.

Convectional Rain

• The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed.

• With thunder and lightening, heavy rainfall takes place but this does not last long.

• Such rain is common in the summer or in the hotter part of the day. It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.

Orographic Rain

• When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed.

• The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall.

• After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry.

• The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. It is also known as the relief rain.

Cyclonic Rain

• Cyclonic) Rain is caused by cyclonic activity and it occurs along the fronts of the cyclone.

World Distribution of Rainfall

• Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons.

• In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily.

• Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.

• The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water.

• The coastal areas of the world receive greater amounts of rainfall than the interior of the continents.

• The rain is heavier on the eastern coasts and goes on decreasing towards the west.

• Between the latitudes 35° and 40° N and S of the equator. But, between 45° and 65° N and S of equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.

On the basis of the total amount of annualprecipitation, major Precipitation regimes of theworld are identified as follows:

• The equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone and the coastal areas of the monsoon land receive heavy rainfall of over 200 cm per annum.

• Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive very low rainfall-less than 50 cm per annum.

• The coastal areas of the continents receive moderate amount of rainfall.

• Interior continental areas receive moderate rainfall varying from 100 – 200 cm per annum.

• The central parts of the tropical land and the eastern and interior parts of the temperate lands receive rainfall varying between 50 – 100 cm per annum.

• Seasonal distribution of rainfall provides an important aspect to judge its effectiveness.

• In some regions rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of cool temperate regions.