It may conveniently form the basis for dividing the year into different seasons. The most characteristic feature of the monsoons is the complete reversal of winds. It eventually leads to the alternation of seasons. On the basis of the monsoon variations the year is divided into four seasons.
The Monsoon forms the basis for dividing the year into different seasons:
i. The Cold Weather Season: December, January, February,
ii. The Hot Weather Season: March, April, May
iii. The South West Monsoon Season: June, July, August, and September
iv. The Retreating S.W. Monsoon Season: October, November.
(I) The Cold Weather Season (N.E. Monsoons):
The cold weather season starts in early December, and at the beginning of January the north-east monsoon is fully established over India. January and February are generally the typical cold month in most parts of the country. In December, the Sun shines directly over the Tropic of Capricorn. There is high pressure over the continent. N.E. Trade winds blow from the continent to the Ocean.
These winds, being off shore do not give rain. They are dry, except for the branch that blows over the Bay of Bengal and gives rainfall to the east coast of India Because of low temperature pressure is sufficiently high in the northwestern part of India. There is low pressure area in the southern India, Arabian Sea and the Bay of Bengal. Therefore low velocity winds blow from high pressure to low pressure.
The low pressure depressions which originate in West Asia and in areas near the Mediterranean Sea are called Western Disturbance. They visit India after travelling through Iran, Afghanistan and Pakistan and disturb the pressure and wind system. Four to five such a disturbance visit India in each winter season from December to February. It causes a little rainfall in northern Indian states which is beneficial to Rabi crops.
Retreating monsoon winds pick up some moisture while crossing the Bay of Bengal and cause winter rainfall in Tamil Nadu and Andhra Pradesh. It is characterized by clear sky with low humidity, low temperatures and is rainless for most part of India except the east coast and north-west of the country. During this season the temperature distribution over India shows a marked decrease from south to north.
The mean January day temperature in Chennai and Calicut is about 24°-25°C while in the northern plains it is about 10°-15°C. In December, the sunshines directly over the Tropic of Capricorn. The landmass of Asia, including the sub-continent, cools down very rapidly. There is a high pressure over the continent. The Indian Ocean, being warmer, has a relatively low pressure.
N.E. Trade Winds (prevailing winds in the tropical Latitudes), blow from the continent to the ocean. These winds, being off shore do not give rain. They are dry except for the branch that blows over the Bay of Bengal and gives rainfall to the east coast of India. These winds are also known as the N.E. Monsoons or the Winter Monsoons in India.
These western disturbances bring light rainfall, most beneficial to the Rabi crop. This rainfall decreases towards the east and the south. Occasionally, they bring in their wake severe coid waves. The Tamil Nadu coastlands also receive rainfall during this season. The north-eastern winds absorb fresh moisture while blowing over the Bay of Bengal before crossing the coasts south of Madras.
(II) The Hot Weather Season:
From March to May the Sun moves from over the Equator towards the Tropicof Cancer. By June 21, it is directly overhead the Tropic of Cancer. In March, the highest day temperatures of about 38°C occur in the Deccan Plateau. It is characterized by high temperatures over most of India, mostly dry with occasional local storms. Therefore:
(a) Peninsular India, places south of the Satpuras experience temperatures between 35°-40°C. Coastal areas, due to the moderating influence of the sea, have lower temperatures (27°-32°).
(b) Central India, comprising of Delhi and Madhya Pradesh experience temperatures between 40°-45°C.
(c) North-west India, comprising mainly of Rajasthan has very high temperatures (45°C), also due to features like sandy soil, direct insolation and lack of cloud cover.
(d) Places located at high altitude (Shimla) are cooler. Moisture-laden winds are occasionally attracted towards this area giving rise to violent local storms.
(e) In Kerala and the western coastal land the pre-monsoon showers are known as ‘mango showers’. The belt of relatively high air pressure over the Deccan Plateau checks the further incursions or the early advances of the monsoons further inland.
(III) The South-West Monsoon Season:
Because of low temperature pressure is sufficiently high in the north-western part of India. There is low pressure area in the southern India, Arabian Sea and the Bay of Bengal. Therefore low velocity winds blow from high pressure to low pressure.
The low pressure depression which originate in West Asia and in areas near the Mediterranean Sea are called Western Disturbance. They visit India after travelling through Iran, Afghanistan and Pakistan and disturb the pressure and wind system. Four to five such a disturbance visit India in each winter season from December to February. It causes a little rainfall in northern Indian states which is beneficial to Rabi crops.
Retreating monsoon winds pick up some moisture while crossing the Bay of Bengal and cause winter rainfall in Tamil Nadu and Andhra Pradesh. This season begins in June and lasts until September. The low pressure which existed over the Northern Plains is further intensified.
It is strong enough to attract the moisture bearing winds from the Indian Ocean. The S.E. Trade Winds from the Southern Hemisphere are drawn into India as the S.W. Monsoon Winds after they cross the Equator. Due to the triangular shape of India, the S.W. Monsoon Winds are divided into two branches—the Arabian Sea Branch and the Bay of Bengal Branch.
The Arabian Sea Branch:
It gives very heavy rainfall, more than 200 cm; to the windward side of the Western Ghats, and the example is that of Panaji. The Deccan Plateau, which lies on the leeward side of the Western Ghat, receives less than 150 cm of rainfall. Further east, rainfall decreases.
For eg, Hyderabad gets less than 100 cm while Chennai gets even less than 40 cm of rainfall. It does not give much rain to Rajasthan because the Aravalli Ranges lie parallel to the direction of winds and hence condensation does not occur. Therefore, Rajasthan gets less than 25 cm of rainfall. These winds advance northwards, attracted to the low pressure in India. Punjab, at the foothills of the Shivalik, gets Relief Rainfall.
Bay of Bengal Branch:
The Bay of Bengal Branch which also blows from the southwest direction, is deflected by the Arakan Mountains of Myanmar and the N.E. Hills of India (Garo, Khasi and Jaintia). They blow into India as the S.E. Monsoons. The delta of the Ganga-Brahmaputra and the wind- ward side of the N.E. Hills of India get heavy rain. For example, Cherrapunji on the windward side gets 2500 cm of rainfall, while Shillong on the leeward slope gets about 250 cm.
The rainfall decreases as the winds reach the eastern Himalayas and blow westward into the Ganga Plain, attracted by the low pressure in Punjab and Rajasthan. Bikaner, lying in the rain shadow of the Aravalli, gets little or no rain. They give the lower Ganga Valley 200 cm of rainfall, the middle 150 cm and the upper 100 cm. Thus, Kolcata (lower Ganga), gets more than 200 cm of rainfall, Patna (middle Ganga) 150 cm and Amritsar (upper Ganga) 100 cm.
(iv) The Retreating S.W. Monsoon Season:
It starts with the onset of the south west monsoon on land and last from June to September. There is a considerable fall in the temperature. However temperature is still high above 33°C in Delhi. The monsoon winds are active all over the country. India receives about 80 per cent of its rainfall from the south west monsoon winds alone. The sudden onset of rain is often termed as monsoon burst.
The south-west monsoon begins to retreat from the northern India by the second week of September and continue to retreat from the southern India from mid- October to the beginning of December.
The clouds-disappear and the sky becomes clear with the retreat of monsoons. The cyclonic storms which develop in the Bay of Bengal during this season move from south-east to north-west direction and cause substantial rainfall hills, the amount of rainfall is well over 1000 cm. areas of the Western Ghats and Brahamputra.
The retreat of S.W. monsoon is gradual and caused due to high pressure on land and low pressure over the Indian Ocean. Kanyakumari is the last place and Kerala the last state from where the winds withdraw. This season lasts through October and November. The temperature in the Northern Plain begins to decrease as the Sun’s rays no longer fall directly at the Tropic of Cancer.
In September, the Sun shines directly at the Equator. The low pressure over the Northern Plain is no longer strong enough to attract the Monsoon Winds into the heart of India. By end of September, the Monsoon Winds are drawn only upto Punjab, by mid-October upto the Central India and by early November upto Southern India, thus, the S.W. Monsoon Winds seems to withdraw in stages during this season.
That is why this season is known as the Retreating S.W. Monsoon Season. This season is marked by cyclones in the Bay of Bengal. They hit the east coast of India and Bangladesh causing widespread damage to life, property and crops.