According to weather phenomenon, the meteorological department of India recognizes four different types of seasons. They are:
- The Hot Weather Season—March to May.
- The South-West Monsoon Season—June to September.
- The Retreating South-West Monsoon Season—October to November.
- The Cold Weather Season—December to February.
There are regional variations in the length of period of these seasons in India. These differences are significant as we move from the south towards the north.
The Northern India experiences a well-make winter season due to well-marked difference in the angle of the sun’s rays. The Northern India has also a distinct hot weather season.
1. The Hot Weather Season (March to May)
The hot weather season begins in March and continues till the end of May. A rapid increase in temperature is witnessed. Because, the sun moves northward and shines vertically near the Tropic of Cancer, the temperature starts rising by middle of March. May is the hottest month of the season. The highest temperature is recorded in the north-west and west. Barmer in Rajasthan is a place where the mercury touches the 50°C. Many parts of central India register 40°C temperature.
Condition of low pressure develops over north-west and al India. Local Storms of violent type occur during April and May.
2. The South-West Monsoon Season (June to September)
The South-west monsoon generally blows from June to September. They bring rain. For this, the season is known as Rainy Season. The monsoonal rains are responsible for agricultural success in India.
During summer, the maximum isolation is received on the Tropic of Cancer and its vicinity. Consequently, low pressure centre develops over much of the north-west India and central Asia. A comparatively high pressure condition prevails on the Indian Ocean in the south. This results in the movement of winds from the sea to the land. Due to the rotation of the earth, these winds deflect to the right in the northern hemisphere (according to Ferrel’s Law) and blow south-westerly. Thus the condition of the North-Eastern Trade is disturbed and their place is occupied by the south-west monsoons. The intensity of the low pressure is so great that even the normal South-East Trades are also disturbed arid winds from southern hemisphere also cross the equator to join the south-west monsoon. Thus the velocity of the south-west monsoons increases considerably. About middle of June, the south-west monsoon ‘burst’ with storm and rain. The advent of the rain-bearing monsoon is called the ‘Monsoon Burst’: The winds blow steadily and by the July the whole of the country comes under its influence.
As India is of peninsular shape, the south-west monsoons approach the land in two separate currents:
- The Arabian Sea Current and
- The Bay of Bengal Current.
3. The Retreating South-West Monsoon Season (October to November)
By the end of September or early October, the intensity of low pressure and rainfall becomes much less. Conditions which prevailed over north-western part and central part of India begin to decrease. Consequently, the South-West monsoon winds begin to retreat. They leave Punjab plain by mid-September, the Ganga delta by the late October and the south by early November. This act of the monsoon winds in known as Retreat of Monsoon.
During this season, as the south-west monsoon winds retreat, the north-east monsoon winds approach and sometimes they collide, resulting in the formation of Cyclones, particularly on the coastal strip of the Bay of Bengal. In the West Bengal these Cyclones are known as ‘Aswiner Jhar’ or the storms of the autumn. They give some rains.
4. The Cold Weather Season (December to February)
The cold weather season starts in early Dec ember and continues till February. During this season, the sun’s rays do not fall vertically in any [art of the country, because of the southerly movement of the sun. The temperature decreases from south to north. The mean January day temperature in Chennai is about
24°C while in the northern plain it is about 8° to 15°C. The days are warm and nights are cold. Hence diurnal range of temperature is high. The snowfall is common in some parts of the Himalayan range.
The high pressure area develops over the plains in the north-west regions. The winds blow from high pressure area to the low pressure area over the south. These winds are called winter monsoons. They blow in north-east direction.
Generally a fair weather prevails. It is characterized by clear skies, high pressure, low humidity and rain-less days. However, this fine weather is occasionally disturbed by the cyclonic depressions visiting from the Mediterranean Sea. They bring rain in areas of Jammu & Kashmir, Punjab, Himachal Pradesh, Haryana, Rajasthan and U.P. Sometimes, they invade up to West Bengal. The amount of rainfall decreases towards the east. They also cause snowfall in North-Western Himalayan regions such as Jammu & Kashmir, Himachal Pradesh and Uttar Pradesh. As these west-winds come from the west, they are popularly known as ‘Western Disturbances’. The precipitation offered by these winds, is of great importance for Rabi Crops, especially wheat. The average precipitation varies from 20 mm. to 50 mm.
Winter rainfalls also occur along the coastal strip of Tamil Nadu. The north-east monsoon winds absorb moisture while blowing over the Bay of Bengal before crossing the coast. These winds strike against the Eastern Ghats and cause rainfall on Coromandel Coast.