Rhythm of Seasons
The meteorolgists usually recognise the following seasons:
- The Winter Season or cold weather season
- The Summer Season or hot weather season
- The Rainy Season or south-western monsoon season
- The season of retreating monsoon
The Winter Season
Normally the winter season begins in mid-November and ends in March. January is the coldest month. The temperature remains slightly low particulary in north India. The mean daily temperature remains below 21°C in northern India and the night temperature falls well below freezing point at several places. Often there are intervals of unusually cold weather known as ‘Cold Waves’. There is no well-marked winter season in the Peninsular India where the temperature remains above 22°C. In the far southern part of the country, the temperature may be well above 25°C. The mean maximum temperature for the month of January at uvananthapuram is as high as 31°C. There is, however, no appreciable seasonal change in the distribution pattern of temperature in coastal areas.
(b) Pressure Distribution and Winds
Because of low temperature, pressure is sufficiently high in the north-western part of India. Here the pressure varies from 1018 to 1020 millibars(mb). In contrast to this, there is low-pressure area in the southern India, the Arabian Sea and the Bay of Bengal. Consequently, the winds blow from the high-pressure areas of the north-west India to the low-pressure area of the southern India. The velocity of the winds is slow due to low pressure gradient.
This system of pressure and winds is disturbed as a result of the inflow of depressions from the west and the north-west. These low pressure depressions are called western disturbances. They originate in West Asia and in areas near the Mediterranean Sea. They travel eastwards across Iran and Pakistan and reach India during winter season. The jet stream plays an important role in bringing these disturbances to India. On an average, four to five such depressions visit India in each of the winter months. These are generally active from December to February.
The retreating winter monsoons blow from land to sea and do not cause much rainfall. These winds pick up some moisture while crossing the Bay of Bengal and cause winter rainfall in Tamil Nadu, south Andhra Pradesh, south-east Karnataka and south-east Kerala. The western disturbances also cause little rainfall in north-west India. The amount of rainfall gradually decreases from the north and north-west to east. The average rainfall during three months from December to February is about 60 cms in the Himalayan region, 12 cms in Punjab, 5.3 cms at Delhi and 1.8 cms to 2.5 cms in U.P. and Bihar. Although very small in amount, this rainfall is extremely useful for rabi crops, especially the wheat crop. The north-eastern part of India also gets rainfall during the winter months. Arunachal Pradesh and Assam may get as much as 50 mm of rainfall during these months.
The Hot Weather Season
This season starts in April and continues upto June. The sun’s rays are vertical over the Tropic of Cancer which divides India into two almost equal parts. The temperature starts rising by the middle of Marc h and by mid-May it rises to 41°C to 42°C. At some places in north-west India, the day temperature may be as high as 45°C to 47°C. Even the mean daily minimum temperature at night in the month of May is slightly high and rarely falls below 26°C. Because of high temperature, strong hot winds blow during day-time over northern and north-western India. This is known as loo. However, there is some respite from the scorching heat when the temperature is reduced by locally formed dust storms.
The southern parts of India do not experience this extreme hot weather because of comparative proximity to sea. The mean maximum temperature in southern part of India varies from 26°C to 30°C. The temperature falls below 25°C on the places located at high altitude on the Western Ghats. This season is characterised by some local storms described as under
Towards the end of the summer, there are pre-monsoon showers which are a common phenomena in Kerala and coastal areas of Karnataka. Locally, they are known as mango showers since they help in the early ripening of mangoes.
With this shower coffee owners blossom in Kerala and nearby areas.
These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature can be understood from the local nomenclature of ‘Kalbaisakhi’. calamity of the month of these showers are useful for tea, jute and rice cultivation. In Assam, these storms are known as “Bardoli Chheerha”.
Hot, dry and oppressing winds blowing in the Northern plains from Punjab to Bihar with high intensity between Delhi and Patna.
(b) Pressure Distribution and Winds
Because of high temperature in the northern part of India, low pressure conditions prevail there. The lowest pressure is in the north-west India. Contrary to this, there is high pressure in the southern parts of India as a result of low temperature prevailing there. Under the influence of these atmospheric conditions, the winds blow from south-west to north-east direction in Arabian Sea and Bay of Bengal. The winds blow in almost the same direction in Peninsular India also. But, in the northern plain, the winds blow from east to west as a result of orographic control.
The hot weather season over north India is generally dry but not without rain. The locally formed dust storms and thunderstorms bring variable amount of rainfall to different parts of the country. On an average, Delhi receives 2 cms of rainfall during the two dry months of April and May. The total rainfall of the hot weather season is generally less than 2.5 cm over Rajasthan, Gujarat and Madhya Pradesh. It varies from 50 to 14 cm in the sub-Himalayan districts of north-western India, Uttarakhand, Uttar Pradesh, Bihar, west Bengal, Odisha and over the greater part of the Peninsula. It is more than 25 cm in Kerala and over 50 cm in Assam.