Young World

Here comes the rain !

Cooler temperatures. Photo: N. Sridharan  

Not a cloud in the sky; the sun beats down mercilessly. Everyone looks up with great expectations, but there is no hope at hand. Summer days are long and exhausting and the earth and its denizens are parched for want of water. Just when we think we can bear it no longer, dark clouds form on the horizon and before we know it the earth is soaked. The rains bring with it hope of new life and cheer for a good harvest.

In India, this rain, which is much awaited, is the all important monsoon. It is important not only for agriculture but also for flora and fauna and the collection of water necessary for day to day living.

The word ‘monsoon’ came from the Arabic mawsim which means season. It is defined as a system of winds characterised by a seasonal reversal of direction. According to the India Meteorological Department, it is the seasonal reversals of wind direction along the shores of the Indian Ocean, especially the Arabian Sea. It blows from the southwest during half the year and from the northeast during the other half. The monsoon in India is categorised as the Arabian Sea branch and the Bay of Bengal branch.

The monsoon is a typical tropical phenomenon, with the Indian subcontinent lying northwards of the equator. Winds blow in the south west direction (the SW monsoon) from the Indian Ocean towards the Indian land mass from June to September. These are rain-bearing winds. When they touch the southern most tip of the Indian peninsula they split into two branches – the Arabian Sea branch and the Bay of Bengal branch.

Then around October, these winds change direction and begin to blow in the north easterly direction. This is now a land to sea flow – from the subcontinent to the Indian Ocean. At this point they carry less moisture and bring rain to only certain parts of the subcontinent — Andhra Pradesh and Tamil Nadu. This is the North East monsoon and is responsible for maintaining the rice bowls of south India.

There are some essential factors required to cause these rains. The most important is the unique geographical features that the subcontinent enjoys. The presence of abundant water bodies around the land mass is essential. For that we have the Arabian Sea, the Indian Ocean and the Bay of Bengal. These help in accumulating moisture in the winds during the hot season. The presence of mountains or highlands like the Western Ghats and the Himalayas, is also essential. These ranges lie across the path of the southwest monsoon winds. When the winds hit these highlands they are forced to rise causing orographic precipitation. That is precipitation which results when moist air is lifted over elevated terrain.

The Western Ghats are the first highlands that the South West monsoon encounters. These Ghats rise very abruptly from the Western Coastal Plains thus making effective orographic barriers for the monsoon winds. The Himalayas are not only an orographic barrier, they also help confine the winds to the Indian subcontinent. If these mountain ranges were not in place then the winds would blow right into China, Afghanistan and Russia without causing rain. The Eastern Ghats play the role of the orographic barrier for the northeast monsoon.

On the high seas

In the old days when trade depended primarily on sea travel sailors waited expectantly for the “trade winds”. For those who travelled between Africa, India and South-East Asia this was a major concern. Hippalus, a Greek navigator, is credited with discovering the monsoon wind in AD 45-47.

Before the discovery of the monsoon wind, mariners in Orissa are said to have begun the journey to southeast Asian countries during the northeast monsoon (trade wind) and returned during the southwest monsoon.

Arabs also sailed in the Indian Ocean with the help of these winds. The Europeans, particularly the Portuguese, timed their sea journey such that they would reach the Indian shores just before the southwest monsoon and return when the northeast monsoon began.

Discovery!

Hippalus, a Greek navigator and merchant, lived in the first century. He is credited with discovering the direct route from the Red Sea to India through the Indian Ocean. However, Pliny the Elder, a Roman author, naturalist and philosopher, said that Hippalus discovered not the route but the monsoon wind which is also called Hippalus – the south west monsoon wind

Why we need trees

Trees play a crucial role in the monsoon cycle. By seeding clouds that encourage the rain to fall; by trapping it they help recharge the aquifers and hold groundwater in store for the common good. Some water, rising with the cell sap, is returned to the sky by transpiration through the leaves. A well-stocked teak forest gives off the equivalent of 1000 nun of precipitation. Great rainforests act on the atmosphere like tropical seas; they supply it with water vapour and help replenish the rains.

Chasing the Monsoon by Alexander Frater


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Printable version | Nov 27, 2021 11:32:45 AM | https://www.thehindu.com/features/kids/here-comes-the-rain/article3645097.ece

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