Indian Monsoon

India is popularly known as the land of monsoons. Scientifically, monsoon winds are caused due to the difference in land and sea temperatures. The sweep of this wind system is set in the rhythm of summer and winter seasons. The land surface becomes comparatively warmer in summer and cooler during winter while the temperature at sea remains relatively constant. As a result, massive convective air currents are set up over the land in summer.

Cool and humid air from the sea surface travels towards the land to fill up the vacant space in the area of low pressure. The moisture-laden wind starts blowing from the sea towards the landmass during the summer season. In winter, the landmass is comparatively cooler and the slowing down process is called ‘retreating monsoon’.

The onset of monsoon is expected every year normally in the month of June. The Indian masses, especially the rural people, anxiously wait for the arrival of monsoon. The mass media also become careful in heralding the onset of monsoon. The coming of monsoon is in itself a spectacular phenomenon accompanied by heavy dark clouds, violent squeals and thunderstorms.

Surprising climatic incidence takes place during the pre-monsoon period. States like Assam, West Bengal, Odisha, Jharkhand and Bihar experience violent thunderstorms in April and May. This is the well known ‘North-Western Monsoon’, locally called ‘Kal Baisakhi’. These thunderstorms typify strong consecutive motion in the local atmosphere. Whenever pre-monsoon cyclonic activities are severe and frequent, it is difficult to distinguish between pre-monsoon thundershowers and arrival of genuine monsoon rains. The progress of the monsoon in India can be conveniently traced to two of its branches - the Arabian Sea Branch and the Bay of Bengal Branch.

The Arabian Branch normally touches Kerala coast in the first or second week of June and advances northward to reach Mumbai by June 10 to 15. But, it may come earlier or get delayed depending upon the prevailing atmospheric conditions. Such departures are very much expected in the case of arrival of monsoon. In the meantime, the progress of Bay of Bengal Branch is no less spectacular. It moves northward from the Central Bay area and rapidly spreads over most of Assam and Bengal (including Bangladesh) by mid-June. On reaching the southern periphery of the Himalayan barrier, the Bay Branch of the monsoon is deflected westward. It moves towards Ganga plains of the country rather than towards Burma Hills. The remaining part of Western Uttar Pradesh, Haryana, Punjab and East Rajasthan experiences the first monsoon showers by the end of June or in the first week of July.

Mountain Ranges have a profound influence on the Indian monsoon. The Bengal Branch of monsoon, after crossing coast, comes under the influence of the hilly curve of the Eastern ranges of Himalayas. The result is that the southern slopes of Assam hills and Meghalaya experience heavy rainfall. The rest of the monsoon current is deflected westward by the high barrier of the Himalayan mountains. The Southern slopes of the barrier; extending from Sikkim to Kashmir, receive rain almost every day during the monsoon months.

In the peninsular India, the Western Ghats represent a series of hills that abruptly rise from the coastal plain to an altitude of 1 to 2 km. It is an undulating rocky ranges all running parallel to the west coast. The monsoon winds, after hitting these ranges, advance towards the Deccan Plateau, Madhya Pradesh and the Bay of Bengal. Since they have already shed their moisture at the seaward side of the Western Ghats, the quantity of rainfall on the other side of the Ghats is substantially less, forming a rain shadow.

Excessive monsoon rains lead to floods in several areas, while no rain in other parts results in severe drought and famine conditions. Such fluctuations in quantity and timing of monsoon rainfall have attracted the people’s attention in general and farmer’s in particular. Recently, some scientists established the precipitation capacity of monsoon with the movement of an oceanic current known as A1 Nino.