India Physical Environment Chapter 4 Climate

The average weather conditions of a sizeable area of the earth's surface over a period of time usually spread over a span of at least 30 years.

The regional variation in temperature is described below:
(i)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.
(ii)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.
(iii)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.
(iv)In Kerala and in the Andaman Islands, the difference between day and night temperatures may be hardly seven or eight degree Celsius.
(v)But in the Thar desert, if the day temperature is around 50°C, at night, it may drop down considerably upto 15°-20°C.

The Himalayan Mountains :

Characteristics of monsoonal rainfall:
(i) Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.
(ii) The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
(iii) 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.
(iv) 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.
(v) The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

1. South-west monsoon season.

2. North-east monsoon season.

Two transitional seasons are :

(i)The hot weather season - March to May.

(ii)The cold weather season - December to February.

This easterly jet stream is held responsible for the burst of the monsoon in India.

The three branches of the monsoon winds of the Arabian sea are explained below:

A very strong and steady westerly wind blowing just below the tropopause.

The regional variations in precipitation:

(i) Seasonal reversal of direction in wind system. The monsoon winds blow from land to sea in winter and from sea to land in summer.

(ii) Seasonal heavy rainfall and drought are common features of the Indian monsoon.

(iii) In spite of local variations in climatic conditions, it unites the whole country in a single climatic thread of monsoon type of climate.

The two factors related to the intensity of rainfall over the west coast of India are:
(i) The offshore meteorological conditions.
(ii) The position of the equatorial jet stream along the eastern coast of Africa.

India receives the bulk of rainfall in the months of June, July, August and September. This period is known as rainy season.

Reason :
India receives highest rainfall during the above-mentioned four months; because the south-west monsoon winds are on shore during this period and easterly Jetes streams stress the tropical cyclone to India which causes heavy rainfall.

The three important factors which influence the mechanism of Indian weather are:

(i)Surface, pressure and winds.

(ii)Jet stream and upper air circulation.

(iii)Western disturbances and tropical cyclones.

Nor Westers:
(i)These are dreaded evening thunderstorms in Bengal and Assam.

Cold waves : During the months of November and April, cold and dry winds blow in northwest India. Whon they attain minimum temperature below 6°C from the normal they are called cold waves. Severe cold waves occur in Jammu & Kashmir, Punjab, Uttar Pradesh, Rajasthan and Madhya Pradesh. The frequency of severe cold waves decreases towards the east and south. The southern states are almost free from cold waves.

The variability of rainfall is computed with the help of the following formula :
CV = standard deviation/mean X 100
where CV is the coefficient of variation.

(ii) There are prolonged breaks of rains, in July-August when summer crops need plenty of water.

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

(iv) The rainfall persists more than usual in one part of the country and desist from another part.

(v) The summer rainfall comes in heavy downpour leading to considerable run off and soil erosion.

(i) Heavy rainfall is recorded in hill and forest areas because of altitude and dense forests which make the atmosphere cool and force the raindrops fall. E.g. Assam, the Ganga Delta, the Western Ghats, the Western Coastal regions.

(ii) Areas without elevated mountains only receive scanty rainfall because of their failure to strike against rainbearing clouds (winds) of the monsoon seasons. E.g. Scanty rainfall in Rajasthan due to Aravallis range of mountains.

(iii) Areas falling in the rain shadow or leeward sides receive sparse rainfall. E.g. Eastern side of the Western Ghats or Ladakh.

(iv) Areas falling first in the way of the rain bearing winds receive more rainfall as compared to the areas falling the last. E.g. Kolkata falls first in the way of Bay of Bengal branch of monsoon therefore, it receives more rainfalls than Delhi.

The western disturbances are preceded by warm weather or sudden rise in temperature. These cause light rainfall in the northern plains of India. These cause snowfall in the northern Himalayas. The rainfall as also snowfall of winter season are beneficial to the rabi crops i.e. wheat, barley etc.

The sudden onset of the moisture laden winds associated with violent thunder and lightning is often termed as the break or burst of the monsoon.

Mawsynram in Meghalaya gets highest rainfall in India.

A climatic region is a homogeneous climatic condition which is the result of a combination of factors.
The bases of Koppen’s classification are :

(i) Temperature (ii) Rainfall.

However, when the monsoon trough shifts closer to the Himalayas there are longer, dry spells in the plains while widespread rains occur in the mountainous catchment areas of the Himalayan rivers. This condition causes devastating floods spreading damage to life and property in the plains. The frequency and intensity of tropical depressions too determine, the amount and duration of monsoon rains.

The south-west monsoon is replaced by the north-east trade winds and they pick-up moisture while crossing the Bay of Bengal. These retreating monsoon winds therefore, become rain fetching winds, especially for Chennai in Coromandel coast.

Areas of Moderate Rainfall : Madhya Pradesh, Orissa, Chhotanagpur, West Bengal, Bihar, eastern U.P., north-eastern Punjab, eastern parts of Tamil Nadu and eastern slopes of Western Ghats.

Areas of Low Rainfall : The Deccan plateau, Western U.P., South-eastern Punjab, Eastern Rajasthan and some parts of Kashmir.

Areas of Scanty Rainfall : Western Rajasthan, Kutch, South-eastern parts of Haryana and north-eastern Kashmir.

Another branch of the monsoon i.e. southeast monsoon blows from the Arabian sea. It enters in India and blows over Rajasthan but the Aravalli Hills there cannot check them and so, these winds pass on without raining there. At their access to Himachal Pradesh, these monsoon winds strike with the lofty peaks of Himalayas and cause heavy rains there. The Bay of Bengal branch also joins here and thus, the rain goes on increasing as we go up the Indus Valley.

Conditions of Season : In the month of October and November, the dry winter season starts replacing the rainy season. The sky becomes clear due to removal of monsoons and the temperature starts increasing. It is popularly called the ‘October heat’ or the humidity and heat of Kwar according to the Hindu theology.

The low air pressure of north-west India transfers to the Bay of Bengal in the month of November. During this period, Andaman sea again starts raising cyclones. The summer solstice cyclones are destructive in nature. These strike the deltas of the Godavari, Krishna, Mahanadi and Kaveri rivers of Peninsular India. Sometimes, these reach upto Sunderbans and Bangladesh also. Coromandel coast receives most rainfall due to these cyclones.

1. The coastal regions have moderate temperature throughout the year. Hence, it is recordes 25°C in winter and 35°C in summer.

2. It decreases northwards. Hence, the region lying south of the Tropic of Cancer has 20°C and north of it has 10°C-15°C temperature.

3. The hilly regions remain ice-covered in the winter season.

4. The temperature in the winter season is well-marked in northern India as compared to southern India.

5. The interior parts of the country are oldest areas with the mean daily maximum temperature varying 28°C in the peninsular region to 19°C in north-west.

6. The mean daily minimum temperature shows greater diversity during the season. It

varies from 24°C in peninsular to 5°C in north west parts of the country.

7. The night temperature, sometimes, fall below freezing point under the influence of cold waves which affect the weather conditions in north-west India.

8. The lowest temperature is recorded in Ladakh. It is - 40°C in Dras in Kargil.

Summer Season : (i) The highest temperature is registered in the northwest and west. Barmer is a place where the highest temperature is 50°C.

(ii) The isotherm of mean maximum temperature of 32.5° runs parallel from the Tropic of Cancer of Kanyakumari while the temperature increases from the south to north on the eastern coast. It varies from 35°C to 37.5°C. But it again goes down on Orissa and West Bengal coast.

(iii) Many parts in north-west and central part of India register 42.5°C temperature.

(iv) In northern India, the temperature goes upto 37.5° from Punjab to Chhotanagpur.

(v) The mean minimum temperature does not go below 20°C in northern India.

(i) Monsoon Burst : The sudden onset of rain is termed as monsoon burst. It is the main characteristic of summer monsoon rainfall. It brings rainfall suddenly–a rainfall without warning. This rain brings relief to the summer affected people and land.

(ii) Breaks : The other characteristic of south-west monsoon is the tendency of breaks or spell of dry weather. These breaks are associated with the tropical depressions which have fluctuating pattern. The amount as well as the intensity of rainfall is also determined by the frequency of these depressions.

(iii) Declining trend : The summer monsoon rainfall is decreasing with the increasing distance from the sea such as : Kolkata receives rainfall 119 cm, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.

Following are the reasons for low rainfall at Jaisalmer :

(i)Jaisalmer lies to the west of Aravallis in Rajasthan. The Bay of Bengal branch of monsoon becomes dry when it reaches here.

(ii)This area lies in the subtropical high-pressure belt, where winds have lesser moisture contents which do not give rainfall.

(iii) This area lies away from the high Himalayas and hence does not get rainfall at all.

1. The rainfall declines abruptly from Bengal coast to west and north-west in the northern plains.

2. The rainfall in the peninsular plateau decreases eastward.

Such uneven distribution is determined by the following factors :

1. Presence of geographic features such as the Western Ghats, the foothills of Himalayan mountain and the N.E. hills. They receive annual rainfall more than 185 cm and it represents only 10% of country's area.

2. The direction and distance covered by monsoon branches is another factor creating variation in the rainfall distribution in the northern plains and interior Deccan Plateau. On this basis, 30% of country's area receives less than 60 cm of annual rainfall and remaining 60% of the area receives between 60 cm to the 185 cm.

The examples are elaborated below:
(i)Temperature : In summer in the western deserts, temperature touches 55°C and it drops down to as low as minus 45°C in Leh. Churu in Rajasthan may record 50°C or more in June while in Tewang it is 19°C on the same day.

(ii)Rainfall : Mawsynram receives 1080 cm. in a year while Jaisalmer in Rajasthan rarely gets more than 9 cm during the same period.

(iii)Monsoons : The date of onset of monsoon and withdrawal of monsoon differ in various parts. The west coast has monsoon in the first week of June while the interior parts like Punjab have monsoon by the first week of July only.

(iv)Season : The southern parts of the country have uniform high temperatures throughout the year. There is no winter season in the south. On the other hand, seasonal extremes are found in the north.

(i) The monsoon winds blow from east dropping much of its moisture along the path. When they reach Rajasthan, they are almost dry.

(ii) There is no mountain high enough to check the flow of the Arabian Sea branch of

monsoon winds. The Aravalli mountains are not very high.

(iii) The direction of the Aravalli mountains is parallel to the direction of Arabian Sea winds. Hence, these winds cross over Rajasthan without giving rainfall.

(iv) The intense heat of western Rajasthan desert does not allow the condensation of moisture to take place and hence no rain is there.

1. The highest rainfall occurs along the west coast, Western Ghats, the Himalayan areas and the southern hills of Meghalaya. It is more than 400 cm.

2. In the Brahmaputra valley and adjoining hills, rainfall is less than 200 cm.

3. Southern parts of Gujarat, east Tamil Nadu, northern peninsula, northern Ganga plain and Manipur get rainfall between 100-200 cm.

4. Parts of Andhra Pradesh, Karnataka and Maharashtra get the rainfall below 50 cm.

5. Rest of India gets rainfall between 50 to 100 cm.

Withdrawal of Monsoon : By the end of September the south-west monsoon becomes weak as the low pressure trough shifts southward. By the beginning of October the low pressure centres over northern parts move over the Bay of Bengal and by early November it moves over Karnataka and Tamil Nadu. By the middle of December the centres of low pressure is completely removed from the peninsula. It is the withdrawal of monsoon.

The principal water surplus areas in India are the Western Ghats of the peninsula which provide water to Krishna, Godavari and Kaveri rivers. Other areas are Meghalaya, eastern mountain ranges and Western Himalayas. The northern plains, Karnataka and Andhra Pradesh have almost no water surplus.

The greatest water deficient regions in northwest India include Rajasthan, Punjab and Haryana.

Based on moisture indices the following five climates are identified :

A Per humid : with I_m of 100 or more

B Humid : with I_m between 20 and 100

C₂ Moist sub-humid : with I_m between 0 and 20

C1 Dry sub-humid : with I_m between 0 and -20

D Semi-arid : with I between -40 to -20

E Arid : with I of-40 and less.

Climatic Regions according to koppen’s scheme

(i) North-western India receives rainfall in winter also.

Reasons: The north-western plains of India experience winter rainfall due to the invasion of the western disturbances rising from the Mediterranean sea, steered by the westerly jetstream. As the western disturbances are associated with the cyclonic winds, they bring rainfall.

(ii) A high-pressure centre develops over northwestern India in winter.

Reasons: In winter the sun shines vertically over the Tropic of Capricorn and the north western part of India away from marine influence experiences low temperature and high atmospheric pressure. The minimum temperature is much below than 10° to 12°C. So a high pressure centre develops over this part of the country.

(iii) During the southwest monsoon season, winds in Ganga valley generally blow from east to west.

Reasons:  The Bay of Bengal current splits into two branches—one advances towards North-Eastern India, Myanmar, Thailand and the other crossing the Bay of Bengal directed westward into the monsoon trough of low pressure. The wind in Ganga plain therefore blows from the east to west and the rainfall decreases westward.

(iv) The annual range of temperature is more in Delhi than in Mumbai.

Reasons:  The distribution of temperature is determined by some factors. The places located far inland have a greater range of temperature than those located near the sea. Delhi is located far away of sea therefore it has more annual range of temperature than that of Mumbai as Mumbai is located near the sea.

Southern India : The Tamil Nadu coast in Southern India receives rainfall during October and November. It is caused by the North-east monsoons. During this season, several cyclonic storms develop in the Bay of Bengal which move from the north-east to south-west causing substantial rainfall on the coast of Tamil Nadu.

(ii) These are locally known as mango-showers because these help in the early ripening of mangoes. These are local storms which develop due to high temperatures.

(i) Monsoon winds : India has a monsoon climate. It is controlled by monsoon winds. In summer, the monsoon winds blow from sea to land as S.W. monsoon. These on-shore winds give most of the rainfall to the country. In winter, the Monsoon winds blow from land to sea as N.E. Monsoon. These are dry winds.

(ii) Expanse of the country : India is a vast country. The northern part lies in the tropical region and experiences hot summers and cool winters. The southern part lies near the equator and has hot season throughout the year.

(iii) Location of the Himalayas : The Himalayas act as a climatic barrier. This mountain wall protects India from the cold winds of Central Asia in winter. The Himalayas check the on-shore S.W. Monsoons and force them to give rainfall.

(iv) The Indian Ocean : India lies at the head of the Indian Ocean. S.W. Monsoon originates from this ocean in summer and provides rainfall over most of the country. Cyclones also originate over this ocean.

(v) The Western Disturbances : The western disturbances (cyclones) originate from the Mediterranean Sea and enter India in winter. These provide rainfall in N.W. parts of India in winter.

(vi) Distance from the sea : The coastal areas experience maritime climate. These areas have equable climate. But the areas in the interior experience extreme or continental type of climate such as northwest part of India.

(vii) Relief of the land : Relief influences the distribution of temperature and rainfall in India. Mountainous areas have low temperature. The windward slopes such as Western Ghats receive heavy rainfall while the leeward slopes (Deccan Plateau) lie in the rain-shadow, and are dry.

1. Monsoonal Rainfall: Most of the rain is received from S.W. summer monsoon during the period mid-June to mid-September. It is a seasonal rainfall.

2. Uncertain Rainfall: Summer rainfall is quite uncertain. Sometimes monsoons start early resulting in floods. Often the on-set of monsoons is delayed resulting in drought. The early or late retreat of monsoons also results is serious drought.

3. Unequal Distribution: The rainfall is unevenly distributed over the country. About 10% of the country gets more than 200 cm of rain while 25% of the country gets less than 75 cm of rain and rest of the country gets 100 to 200 cm of rainfall.

4. Heavy Rainfall: Indian rainfall is heavy and downpouring type. It is often said, 'It pours, it never rains in India.'

5. Relief Rainfall: The amount of rainfall is determined by the presence of mountains. High mountains force monsoons to rise and give rain.

6. No countinuity of rainfall: Summer rain is characterized by breaks or dry spells.

7. Variable Rainfall: Most of the areas have variable rainfall, as much as 30 cm plus or minus. Due to variability in areas of low rain, famines occur.

Chennai gets most of rain from retreating monsoon in October and November. These winds after picking up moisture from Bay of Bengal give more rain on the east coast. Thus Chennai gets most of rain in winter when the fury of monsoon is over in most Parts of the country.

The meteorological department, has recognised the following seasons :

(i)The cold weather season 
(ii) The hot weather season 
(iii)The southwest monsoon season 
(iv)The retreating monsoon season

The Cold Weather Season :
(a)It starts in by mid-November in Northern India. December and January are the coldest months. The mean daily temperature remains below 21°C over most part of the country. The night temperature may be quite low, sometimes going below freezing point in Punjab and Rajasthan.

(b)The peninsular region of India does not have any well defined cold weather season. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of the coastal influence of the sea and the nearness of equator. Winds start blowing from North-East to South–West due to low pressure.

(c)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.

Leeward side : The other side of the mountain where winds descend after shedding much of the rainfall.

The failure of summer monsoon results in famines and food shortage. Therefore, monsoon is a gamble for the Indian farmer.

(ii) The rainfall is very low. It is about 100 mm in the month of August.

(iii) Whatever rainfall Jodhpur gets, is in the months of July and August. October to May are dry months.

(i) Rajasthan is under the influence of S.W. summer monsoons. The Aravallis system lies winds parallel to the direction of S.W. monsoon winds coming from Arabian Sea. So this mountain system is unable to check these winds and hence, western Rajasthan is practically dry. The southern parts get some rainfall.

(ii) This area lies at a great distance from the Bay of Bengal. The Bay of Bengal monsoon becomes dry and lose its moisture when it reaches Rajasthan.

(iii) This area is away from the Himalayan region. So it does not come under the influence of monsoon giving rain in the sub-Himalayan region.

A.

A.

B.

D.

A.

C.

Mawsynram in the Khasi Hills of Meghalaya receives rainfall over 1,080 cm in a year.

1. The highest rainfall occurs along the west coast, Western Ghats, the Himalayan areas and the southern hills of Meghalaya. It is more than 400 cm.

2. In the Brahmaputra valley and adjoining hills, rainfall is less than 200 cm.

3. Southern parts of Gujarat, east Tamil Nadu, northern peninsula, northern Ganga plain and Manipur get rainfall between 100-200 cm.

4. Parts of Andhra Pradesh, Karnataka and Maharashtra get the rainfall below 50 cm.

5. Rest of India gets rainfall between 50 to 100 cm.

Monsoons control the atmosphere conditions of the country. Throughout the country, the summers are wet and winters are dry due to on-shore and off-shore winds that blow over every nook and corner of India. The distribution of rainfall is uneven. Agriculture depends upon monsoon rainfall resulting in a subsistence agriculture. Agriculture in India is a gamble on monsoons. The folk songs like Kajari of Bhojpur and Malhar of Braj (Mathura) are associated with monsoonal rains. Thus, monsoons provide a natural base for a uniformity throughout the country.

(i) Temperature (ii) Rainfall.

1. The coastal regions have moderate temperature throughout the year. Hence, it is recordes 25°C in winter and 35°C in summer.

2. It decreases northwards. Hence, the region lying south of the Tropic of Cancer has 20°C and north of it has 10°C-15°C temperature.

3. The hilly regions remain ice-covered in the winter season.

4. The temperature in the winter season is well-marked in northern India as compared to southern India.

5. The interior parts of the country are oldest areas with the mean daily maximum temperature varying 28°C in the peninsular region to 19°C in north-west.

6. The mean daily minimum temperature shows greater diversity during the season. It

varies from 24°C in peninsular to 5°C in north west parts of the country.

7. The night temperature, sometimes, fall below freezing point under the influence of cold waves which affect the weather conditions in north-west India.

8. The lowest temperature is recorded in Ladakh. It is - 40°C in Dras in Kargil.