The magic of the monsoon

The monsoon has covered the entire country, reaching Sriganganagar in western Rajasthan — its last outpost — a fortnight ahead of schedule, the India Meteorological Department (IMD) said on Friday. As the Southwest monsoon gets busier over northwest and northeast India, let’s get to know monsoon better in Five Ws and One H.

WHATis monsoon?

Monsoon is defined as the seasonal reversal of the direction of the winds along the shores of the Indian Ocean, which blow from the southwest for the first half of the year and from the northeast for the second half. It brings rain to India and the southeast Asian countries. It is caused due to a difference in temperatures between a land mass and the adjacent ocean. In India, during summer, the land experiences a high temperature. Hot air rises and due to this a low pressure area is created above the land. The sun warms the land and the ocean differently. The temperature over the Indian Ocean is relatively low. So a high pressure region is created over the sea. The wind flows from high pressure area to the low pressure area. Hence by June, the movement of the moisture-laden winds will be from the Indian Ocean to the land mass. The direction of the winds will be from southwest to northeast. Because the winds can’t pass through the Himalayan region, they rise. This rise in altitude of the clouds causes a drop in temperature. As the air is cooled, the atmospheric vapour condenses into raindrops.

WHATare the two
monsoon seasons

India receives 75% of its rainfall from the southwest monsoon (June to September), and the rest from the northeast monsoon (October to December).

The Southwest monsoon

When the southwest monsoon or the summer monsoon reaches India, it splits into two parts around the mountainous region of the Western Ghats. One moves northwards over the Arabian Sea and up the coastal side of the Western Ghats and the other flows over the Bay of Bengal, up through Assam, and hits the eastern Himalayan range. It progresses inland in stages and covers most parts of the country by mid-July.

The Northeast monsoon

During winter, the wind blows from land to sea in the direction opposite to that of the summer monsoon. A part of the northeast trade winds blows over the Bay of Bengal. They gather moisture which causes rainfall on the southeastern coast, while the rest of the country remains relatively dry. The northeast monsoon brings rain to coastal Andhra Pradesh, Rayalaseema, Tamil Nadu and Puducherry.

WHICHinstrument is
used to measure

A cylindrical instrument called Rain Gauge is used to measure rainfall. It is kept exposed to the sky in an open space away from trees and buildings. Rain falls into the cylinder, which usually has an inscribed millimetre scale. The accumulated water level is calculated. Satellites and weather radars are also used for the purpose.

WHATare the
predictors of monsoon
in India?

The five predictors, according to the IMD are – temperature difference between surface temperatures in the North Pacific and the North Atlantic oceans, surface temperature of the Indian Ocean near the equator; mean sea level pressure in the Pacific Ocean near East Asia; land temperatures over northwest Europe; and the volume of warm water in the equatorial Pacific Ocean.

WHYis monsoon
important ?

India’s agriculture is rain-fed. Some of the crops grown a lot of water, and over 40 per cent of our agricultural areas do not have any form of irrigation and are exclusively dependent on rain for water. Seventy per cent of Indians directly or indirectly depend on farming. Hence monsoon affects the household incomes and livelihood.

HOWis monsoon

Annually, India receives about 116 cm of rainfall, out of which 89 cm is received during June to September. These numbers have been considered as the Long Period Average or the LPA. It has been arrived at after taking averages of rainfall received over a 50-year period between 1951 and 2001 by the IMD.

So, when the IMD declares a monsoon normal, it means the amount of rainfall is between 96% and 104% of the LPA. Rainfall between 104% and 110% of the LPA is considered ‘above normal’, and anything above 110% of LPA ‘excess’ and between 90% and 96% of LPA ‘below normal’. Anything less than that is ‘deficient’.

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