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PART III – Indian Climate
In the previous parts, we have learnt about the shape and size of our country along with its latitudinal extent. Not only are its physiographic divisions diverse but also far more contrasting in nature. Each one of these factors has an impact on climatic conditions of India, be it temperature, atmospheric pressure, wind system or precipitation.
In this part, we will study regional variations in the climatic conditions of India.
After covering this module, you should be able to:
Understand the factors responsible for climatic variations.
Understand the ‘Monsoon-type’ climate of India.
Influence of Tibetan plateau, Jet-streams and El-Nino on Indian Monsoon.
Understand the distribution of rainfall in India.
Appreciate different seasons of India.
Rainfall distribution of India
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Climate and Weather:
In order to understand the climate of India, we should first know what exactly the climate is.
Weather is the temporary state of the atmosphere, while climate refers to the average of the weather conditions over a longer period of time.
Weather changes quickly, may be within a day or week, but climate changes in imperceptivity and may be noted after 50, 100 years, or even more.
The climate of India has distinct regional variations discernible by the pattern of winds, temperature, and rainfall; further, also in the form of rhythm of seasons and the degree of wetness or dryness.
Factors influencing the Climate of India:
Following factors influence the climate of India-
Location and Latitudinal Extent:
India lies roughly between 6°N to 37°N latitudes. The Tropic of Cancer passes through the middle of the country. The southern parts being closer to the Equator, experience high temperatures throughout the year. The northern parts on the other hand lie in the warm temperate zone. Hence they experience low temperatures particularly, in winter.
Distance from the Sea:
Southern or peninsular India is surrounded by the Arabian Sea, the Indian ocean and the Bay of Bengal, hence the climate of coastal regions of India is equable or maritime. Contrary to this, the climate of the regions located in the interior of the country is cut off from the oceanic influence. As a result, they have an extreme or continental type of climate.
The Northern Mountain Ranges:
The Himalayan ranges in the North separates India from rest of Asia. These ranges protect India from the bitterly cold and dry winds of Central Asia during winter. Furthermore, they act as an effective physical barrier for the rain bearing southwest monsoons winds to cross the northern frontiers of India. Thus, these ranges act as a climatic divide between Indian Sub-Continent and Central Asia.
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Physiography:
The physical features influence the air temperature, atmospheric pressure, direction of winds and the amount of rainfall in different parts of the country.
Monsoon Winds:
Monsoon will be discussed in detail in the later part of this chapter.
Upper Air Circulation:
There are very strong Westerly as well as Easterly jet streams (only during summer) blowing over India. They push the monsoon winds from the sea towards the land.
Indian Climate:
Broadly it is said that India has monsoon type of Climate.
Then the question arises, that what exactly is monsoon?
The word monsoon has been derived from the Arabic word 'Mausim' which means seasonal reversal of the winds during the course of the year.
Monsoons refer to a system of winds in the tropical regions under which the direction of winds is reversed completely between the summer and the winter seasons. Under this system, the winds blow from land to sea in winter and from sea to land in summer. Therefore, most of the rainfall in the regions influenced by the monsoons is received in the summer season while winter season is generally dry. Many theories have been given to explain the phenomenon.
Theory of Differential Heating:
This was the first scientific theory given to explain the phenomenon of Monsoon. According to the traditional belief, the monsoon is caused by the differential heating of land and sea. Due to a higher temperature over the land in summer, a low pressure area develops over the continents and the winds blow from neighbouring oceans towards the land. These winds are of maritime origin and hence cause ample rainfall in summer. On the other hand, the continents become colder than the neighboring oceans in winter. As a result a high pressure area is developed over the continents. Therefore, winds blow from land to sea in winter. These winds, being of continental origin, are dry and do not cause rain. This traditional theory of monsoon
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has been criticized by the German meteorologist Flohn. He argues that the differential heating of land and sea is not enough to cause a seasonal reversal of winds at a global scale.
Monsoon by Differential Heating
Flohn’s Dynamic Concept of Monsoon:
He has explained the origin of the monsoon on the basis of seasonal shift of the pressure and wind belts under the influence of the shift of the vertical rays of the sun. According to this theory, as the vertical rays of the sun shift northwards over the Tropic of Cancer in summer season, the Inter-Tropical Convergence Zone (ITCZ) also shifts to north. This results in the formation of a low pressure area over the northwestern parts of India. This low pressure is further intensified by the high temperatures in this region. This low pressure area sucks the air from the Indian Ocean towards the Indian landmass in the form of Southwest monsoons. In winter season, the ITCZ shifts southwards and a mild high pressure is produced over northern parts of India. This high pressure is further intensified by the equator ward shift of the sub-
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tropical high pressure belt. Due to high pressure over northern India, the winds start blowing from northeast as retreating monsoons.
The complete phenomenon can be understood by the following table.
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According to recent observations, the origin of Indian monsoon is influenced by a number of other factors, besides the differential heating of land and sea and the seasonal shifts of pressure and wind belts.
Some of the Factors which influence Indian Monsoon are-
Jet Streams:
One of the most important factors among these is the system of subtropical westerly and tropical easterly jet streams. The subtropical westerly jet streams blowing over India in winter cause a high pressure over northern India. It thus intensifies the northeast monsoons. This jet stream shifts northwards beyond India in summer season and tropical easterly jets develop over India in this season. The behavior of these jet streams is partly responsible for the variations in the time of onset of southwest monsoons over India.
JET STREAM
Types: Polar + Sub-tropical Jet Stream
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El-Nino and La-Nina:
El- Nino
El-Nino is Cold Ocean current that flows along the western coast of South America. 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. It is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humbolt Current.
This results in two things:
The distortion of equatorial atmosphere circulation
Irregularities in the evaporation of sea water.
Reduction in the amount of Planktons which further reduces the amount of fish in the sea.
It 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.
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Some important data to understand:
India: El Nino years
Effect
Monsoon 2004 Drought 86%
2009
Severe Drought
77% 2013 Neutral 106%
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La- Nina
La-Nina is the condition, opposite to El-Nino.
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Some Facts about Indian Monsoon:
In Kerala and Maharashtra coast, the first monsoon shower leads to blossoming of coffee flowers and mango flowers. This shower is called ‘Blossom shower’ or ‘Mango shower’.
Burst of monsoon on eastern coast (Bengal) leads to destruction of standing crops. This is called ‘KalBaisakh’. The ‘Calamity’ of Baisakh ( the harvest season).
In Assam, Nor Westers are known as Bardoli Chheerha.
In northern plains, hot and dry winds blow during summers. They are called Loo.
Tamil Nadu coast remains dry during the monsoon season because it is situated parallel to the Bay of Bengal branch of southwest monsoon. Tamil Nadu coast receives rainfall during retreating monsoons.
The months of October and November are known as retreating monsoons season.
Cherapunji and Mawsynram (two places of Meghalaya) are world’s wettest place.
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The map showing south-west monsoon and time that it takes to travel upto north.
Source: IMD
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Map showing comparative advance and retreat of monsoon.
The Northeast Monsoon (Retreating Monsoon) and its Effect:
During winter, the weather conditions is influenced by high pressure developed over Northwestern part of the subcontinent. This results in the blowing of cold dry winds from this
regions towards southern low pressure areas lying over water bodies surrounding peninsular India. Since these winds are cold and dry, they do not cause rainfall and weather conditions under their influence remain cold and dry. However; wherever these Northeast monsoon winds collect moisture while passing over the Bay of Bengal, they bring rain along Coromandel coast. Strictly speaking these winds are planetary winds known as North-east Trades. In India they are essentially land bearing winds.
Western Disturbances and winter rainfall in North- India:
The inflow of western disturbances moves under the influence of westerly jet streams from the Mediterranean Sea. It influences winter weather conditions over most parts of Northern Plains and Western Himalayan region. It brings little rain in winter months. This rain is considered to be very good for wheat crops in northern plains.
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The tropical cyclones also develop in the Bay of Bengal. The frequency and direction of these cyclones influence weather conditions along the eastern coast during October, November and December.
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Seasons in India:
By now, you have understood that the complete reversal of direction of winds is
the most striking feature of the monsoons. These changing monsoon winds result
in the change of seasons over the year. It is, therefore, important to understand in
detail, the prevailing weather conditions throughout India during different seasons.
Climatically, the year is divided into following four seasons in India:
1. The cold weather season - December to February;
2. The hot weather season - March to May in south and up to June in the north;
3. The advancing southwest monsoon season - June to September;
4. The retreating southwest monsoons season - October and November.
Cold Weather season:
The important characteristics of cold weather season are:
Low temperatures in the north and their gradual increase towards southern parts of India.
Blowing of cold and dry northeast monsoon resulting in dry weather conditions in most parts. Coromandel coast receives rainfall during winter.
Western disturbances cause light rain in northern plains and snowfall over the Himalayan ranges.
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Hot weather season:
The apparent movement of the sun towards the north increases the temperatures in the northern plains. As a result, the spring sets in soon giving way to the hot weather season which lasts till end of June in this region. The temperatures increase northwards and reach around 45°C in mid-May in most parts of the northern plains. The characteristic features of this season are afternoon dust storms and ‘Loo’ which is a hot dry wind which blows during May and June mainly over the northern plains. These winds cause heat stroke resulting in deaths of hundreds of people every year. The day temperatures at times rise above 45°C in some north-western parts of the country.
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South-west Monsoon season:
It is a rainy season for most parts of India. It starts with the inflow of Southwest monsoons which strike the coast of Kerala normally in the first week of June and cover most of India by mid-July. This weather continues till September. The arrival of these warm moisture laden winds brings a total change in weather conditions. Their arrival causes sudden rains which bring down the temperatures considerably. The decline in temperature is between 5°C to 10°C. The sudden onset of rain is called break of monsoons or the burst of monsoons. The arrival of these
winds may be delayed by a week or two depending upon the pressure conditions over northern plains and over the Indian ocean. The peninsular shape of India divides these Southwest monsoons into two branches - Arabian Sea branch and Bay of Bengal branch.
(We have already discussed about monsoon in detail)
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Retreating Monsoon:
The main characteristic features of retreating Southwest monsoon season are:
weakening of low pressure area over Northwest India;
fall in temperatures throughout India;
shifting of low pressure area to the south; and
origin of cyclonic storms in the Bay of Bengal causing heavy rains and damage to crops and property along the eastern coast of India.
Rainfall Distribution in India:
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Study the map carefully. You will find that the regional variations in average annual rainfall are well pronounced. The distribution map of rainfall shows that northeastern parts of Jammu and Kashmir and extreme western Rajasthan receive a rainfall of less than 20 cm.
On the other hand, the west coastal plains, Sub-Himalayan areas of northeast India including the Shillong plateau receive more than 200 cm. of annual rainfall. Southern slopes of Khasi and Jayantia Hills, particularly the Cherrapunji valley receive the highest rainfall exceeding 1000 cm.
Starting from the southern coast of Gujarat, the isohyte of 200 cm. runs somewhat parallel to the coast of Western Ghats upto kanyakumari. To the east of Western Ghats, the rainfall drops abruptly below 60 cm. over interior Maharashtra and Karnataka.
Most parts of Punjab, Haryana, central and eastern Rajasthan and western Gujarat also receive rainfall below 60 cm. Starting from the southwestern parts of Jammu and Kashmir, the isohyte of 100 cm. moves eastwards up to east of Allahabad from where it bends to the west and south west, running over western Madhya Pradesh, eastern Maharashtra and northern Andhra Pradesh, it joins eastern coast near Visakhapattnam.
To the west and south west of this isohyte, the areas receive less rainfall. Some parts of Coromondel coast receive a rainfall of more than 100 cm. The areas receiving less than 100 cm. of rainfall depend on means of irrigation for agricultural activities.
The distribution of annual rainfall in different parts of India shows the following trends:
The rainfall decreases as one move from Kolkata to Amritsar.
It shows declining trend towards interior from the coastal areas on Deccan Plateau.
Northeastern parts receive more rainfall than north western parts of India.
Areas lying on the windward side receive more rains than the areas lying on the leeward side.
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Climatic Unity of India:
The ‘Climatic Unity’ means that weather conditions over different parts of India are more or less the same during different seasons round the year leaving minor variations as exceptions to
the rule. Indian climate is called ‘monsoon climate’. It explains how much influence the monsoon winds have in bringing climatic unity. This unity in climatic conditions results from the combined influence of regular movements of monsoons (seasonal winds) and the bounding role of the Himalayan mountain system.
The Climatic system of India has the direct impact on the lives of people in the following ways:
Seasonal changes: The sequence of hot, wet and cold seasons affects the life styles and economic activities of the people throughout India.
The farmers all over India start their agricultural activities like ploughing of fields, sowing of seeds, transplantation etc. with or just before the onset of monsoons. Kharif crops - rice and millets, cotton and sugarcane in different areas is an expression of amount of rainfall they receive. During winter, wheat is the major rabi crop in cool and irrigated areas; whereas barley, gram and oil seeds are common crops of unirrigated areas in northern and central India.
The clothes are also affected by seasons. During summer, the people wear cotton clothes whereas the woollen clothes are used in winter season especially in north and central India.
Most parts of lndia have to bear a long dry season; contrary to it, the season of life giving rains is limited to only a few months. This has a far reaching effect on the life style of the Indian people.
Most of the Indian festivals are closely linked with seasons. In north India, Baisakhi is celebrated when rabi crop is ready for harvesting. During winter, when the sun shines vertically over the
Tropic of Capricorn and extreme cold weather conditions prevail over northern plains Lohri and Makar Sakranti are celebrated in the north and west while Pongal distinctly is its southern counterpart. Holi is celebrated in spring after bidding good bye to the prolonged cold winter especially in the north.
The seasonal and regional variations in weather conditions have made different regions capable of producing different crops in varying quantities making all regions completely
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interdependent. This is not a less contribution of the monsoons in promoting underlying unity despite all pervasive diversity.
The paradox of Indian monsoons is that no part in India is spared from occasional or even frequent floods or droughts, if not famines. The drought is common even in the areas of heavy rainfall like Kerala and Assam; likewise, dry areas of Northwest India are not free from floods, be it Punjab or Rajasthan. Consequently, there is need to conserve, control and store water for irrigation, drinking and power generation.
Note: We hope that after finishing this module, you will be able to appreciate the climate and weather phenomenon of India. And cherish its beauty and diversity. ?
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