The movement of air is mainly caused by the differences in pressure and temperature. Warm air is lighter and it rises upwards, meanwhile, cold air is denser and hence it moves down to replace the warm air. This phenomenon is experienced as wind.
What causes the movement of air?
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, making it less dense. Here, hot air increases and creates 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 as a result, it begins to rain.
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 the 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 sufficiently 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, especially the Western Ghats and the Himalayas. This is known as the Advancing Monsoon climate. As the air hits these mountains, it tries to move towards the low-pressure areas by rising and gaining 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 the difference in temperature
When 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, though, the 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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