In this section, we will talk about the movement of air, how it affects the weather conditions and brings about the rain and monsoon in India.
The movement of air across the earth determines the weather and climate of all regions. The sun’s radiation heats up the land, sea and the air. The land and water bodies also heat up the air over it making it less dense. Here hot air increases and creates a low pressure over that area and cold air sinks and creates a region of high pressure. Air like all fluids likes to maintain equal pressure. To do so, cold air from high-pressure areas flows into regions of lower pressure.
Air also carries water vapor, the amount of water it carries and the temperatures of the air determine how much water can exist as vapor in the air. As more vapor is added, the air cannot hold any more water and just then it begins to rain. The lower the temperature of the air, the less water vapor it carries. Thus as the air cools down, the vapor condenses and causes rain. This whole cycle of evaporation and condensation is called the Water Cycle.
The Wind due to difference in pressures
In India during the summer, the land heats up faster than the sea and a region of low pressure is created over the Indian subcontinent and the Tibetan plateau. By June, there is sufficient low pressure for cooler air from the sea to blow towards the land. As the air from the Indian Ocean blows towards India it picks up moisture and water vapor. When these winds reach India, they begin to cool causing rain. But most of the rain from the monsoon is dumped over hills and mountains such as the Western Ghats and Himalayas. This is known as the Advancing Monsoon. This is because as air hits these mountains, to move towards the lower pressure areas beyond them, it needs to rise and gain altitude.
As the air rises, the temperature falls and the air cools. Unable to hold vapor, the water is released as rain. Since, the Himalayas are so tall, by the time the air reaches the Tibetan Plateau and Ladakh; it has cooled sufficiently to release almost all the moisture it can hold. Hence, Tibet and Ladakh only receive dry winds which equalize the low pressure but bring very little rain.
The Wind due to difference in temperature
As winter approaches, the reverse happens. The land begins to cool quickly compared to the ocean and air begins to flow back to the sea. This air, however, is dry since it has lost all its moisture over land. However, air coming from the North East of India and Bengal pass over the Bay of Bengal before reaching Tamil Nadu. During this transit, the winds pick up sufficient moisture from the sea to cause rain over Tamil Nadu. This monsoon is called the Returning Monsoon. The low pressure is sometimes called a depression.
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