Climatic Conditions in India
Diverse Climate & Weather Conditions
Variation In Temperature and Rainfall
India, with its vast size and marked variations in terrain, is a land of climatic contrasts. On an extremely hot summer afternoon, the temperature may occasionally shoot up to 55° C in certain parts of Rajasthan and south-west Punjab. And on a severe winter night, the mercury may dip to as low as minus 45° C in a cold arid region such as Kargil.
Similarly, Cherapunji, with its annual rainfall of 1080 cm and Mawsynram (Both in Meghalaya) are known to be the wettest places in the world while the dry regions of western Rajasthan receive no more than 13 cm of annual rainfall. In between these two extremes, there are regions of equable, moderate and uniform climate.
These variations in temperature and rainfall make India a land of diverse climate and weather conditions.
Tropical and Sub Tropical Zones
The Tropic of Cancer divides India into two halves - Southern India lies in the tropical zone and Northern India in the sub-tropical zone, keeping the temperature high all over the country, except in the areas of high altitudes. Besides, some of the phenomena influencing India's weather and climatic conditions lie much beyond its geographical limits. The western disturbances affecting winter weather in northern India originate from the low pressure systems developing in the eastern Mediterranean region.
Temperature and pressure conditions in East Africa, Iran, Central Asia and Tibet affect the behaviour of monsoons. The weather conditions in the rest of the Indian subcontinent, the Indian Ocean and the China Sea also affect the weather conditions in various parts of India. The upper air currents or jet streams too, have their influence on the country's climatic and weather conditions.
Climatic Seasons
The most important factor in shaping India's climatic conditions is monsoons that affect almost all parts of the country with varying intensity and duration and account for seasonal rhythm. An important characteristic feature of the monsoons is the complete reversal of winds which leads to the alternation of season. On the basis of monsoon variations, the year is divided into four seasons.
- Winter (cold weather) Season: December to February
- Summer (hot weather) Season: March to May/June
- South-west Monsoon (rainy) Season: June to September
- Post-Monsoon (retreating south-west monsoon) Season: October and November
The Winter Season
Starting in December, the cold weather season becomes fully established in January and the temperature distribution over India shows a marked decline as one moves from south to north. Generally, the days are bright and sunny but the nights are cold. The generally fine weather of this period, is, however, occasionally disturbed by the western disturbances, which bring light rainfall and severe cold waves.
The Summer Season
The period between March and May is that of rising temperatures and decreasing air pressures as the belt of intense heat shifts from south to north. In March, the day's temperature reaches 35°C in the regions south of the Vindhayas. In April, the heat belt moves further north to Gujarat and Madhya Pradesh and the temperature exceeds 37°C in the northern India. In May, it goes up to 41°C or above and dry hot winds blow over most of the northern region and dust storms of great velocity strike Punjab, Haryana and Uttar Pradesh which are afterwards followed by light showers and cool breeze.
By the end of May, low pressure trough is developed which occasionally attracts the moisture-laden winds. After coming into contact with the hot dryland winds, it causes pre-monsoon rains. Kerala and coastal plains of the west receive a fair share of pre-monsoon showers, commonly known as 'mango showers'.
Assam and West Bengal also receive rain during this season, but north-west India remains comparatively dry.
The South-West Monsoon Season
By early June, the low pressure area over north-western plains becomes highly intense to attract the south-west rain-bearing winds, which approach suddenly with thunder and lightning. Within almost one month's time, these winds overrun almost the entire country.
Air Moves From High Pressure To Low Pressure
The south-west monsoons originate from the Indian Ocean and blow over the land mass of India from June to September. Due to the intense summer heat, a low pressure area is formed over the northern plains of India. But the oceanic region has a low temperature and high pressure centre. Consequently, air starts moving from the high pressure area of the Indian Ocean towards the low pressure area over the land mass of India in the form of rain bearing monsoon winds.
South East Trade Winds
The south-east trade winds, which originate south of Equator, are also sucked into the wind system of the northern Indian Ocean and are deflected towards India. The landmass of peninsular India divides these southwest monsoons into two branches - the Arabian Sea branch and the Bay of Bengal branch.
The monsoon winds arising from the Arabian Sea, strike the Western Ghats and cause heavy rains. Having crossed the Ghats, they advance over the Deccan Plateau and Madhya Pradesh and are joined by a current of winds arising from the Bay of Bengal. Another part of the Arabian Sea monsoon winds cross the coast of Saurashtra and Kutch and passing over Aravalli hills, reach Punjab and Haryana. These winds also join the winds from the Bay of Bengal and cause widespread heavy rains in western Himalayas.
The monsoon winds from the southern Bay of Bengal mainly move towards Burma, but a part of these winds is deflected by the Arakan Hills and moves westward, over the Ganga-Brahmaputra valley. It strikes the northeastern hills and causes heavy rainfall in West Bengal, its adjoining States, sub-Himalayan region and the northern plains.
In all parts of the country, with the exception of the east coast of Tamil Nadu, bulk of annual rainfall is received during the monsoon season. But the distribution of rainfall is highly unequal as the monsoon winds become weaker as they traverse over longer distances. Thus, Kolkata receives 120 cm rainfall, Patna 102 cm, Allahabad 91 cm and New Delhi 56 cm. The windward side of the Western Ghats receives heavy rainfall while the leeward side gets much smaller amount. The intensity and frequency of the cyclonic depressions originating in the Bay of Bengal and their crossing over to the mainland as well as the passage followed by them account for the variations in geographical distribution of rainfall.
Monsoon is a seasonal wind that blows over the northern part of the Indian Ocean, especially the Arabian Sea, and over most of the surrounding land areas.The monsoon blows continually from the southwest from April to October. It blows from the northeast from November to March. Monsoons are generated by the difference in the heating and cooling of air over land and sea. During the summer, radiant energy from the sun heats land surfaces far more than it does sea surfaces. The strongly heated air over the land rises and is replaced by a south westerly wind carrying warm, moist air from the Indian Ocean. Water vapour in the rising air condenses and forms clouds and rain. This process releases large amounts of heat, which helps drive monsoons.
In winter, the land is cooled much more than the sea. The cool air over the land sinks and spreads out to the sea as a dry north easterly wind. The south westerly monsoon brings heavy rains to southern and southeastern Asia, including Bangladesh, Burma, India and Thailand. The strength of the southwesterly monsoon, and the time in April that it begins, affects agriculture in southern Asia. Abnormal monsoons can destroy a region's crops and livestock and disrupt its economy. Monsoons also blow over the coast of northern Australia, eastern Asia, parts of Africa and the southwestern United States.
The Retreating South-West Monsoon Season
The monsoon winds start retreating from Punjab and Haryana by mid-September, reach Ganga delta by late October and the Peninsular India by early November, leaving the land moist and the atmosphere, humid. However, from the middle of October, temperature begins to decline in northern parts of India. The weather during this season is characterised by high day temperature, clear sky and pleasant nights. The fall in temperature continues and the winter season becomes firmly established by December.
During this transition period of October-November, the low pressure conditions disappear from the northwestern India and are transferred to the centre of the Bay of Bengal. These cyclonic depressions in the Bay of Bengal often cross the Southern Peninsula and cause widespread heavy rains along the coastal regions of Tamil Nadu, making October-November as the rainiest months in this part of the country.
North - East Monsoons
The north - east monsoons are the winds blowing out from the landmass of north-western India toward the Indian Ocean during the period of December and February. The low pressure area formed in the Ocean region attracts these winds from the high pressure areas formed during chilly winters over the north-western parts of India. These cold and dry winds move down the Ganga valley towards the Indian Ocean. The winds that move through the Bay of Bengal become moisture laden and strike the Tamil Nadu coast to bring winter rains in that region.