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Why it is cool on the top of hills?

As we are travelling to greater heights away from the surface of the earth, we are nearing to the sun. Hence it should be hotter on the top of a hill. But in reality it is not so. It is “cool on the top of a hill, even on a hot summer day. That is the reason why, people ‘camp’ on hill station like Ooty and ‘Darjeeling’.

To find the cause for the atmosphere to be ‘cool’ on the top of the hills, we should know certain facts about air, earth and atmosphere. Sunrays while searching the earth, pass through atmosphere. Though these rays pass through the air in the atmosphere, the air will not absorb the heat of the rays, as air is a ‘poor absorber’. Thus, earth absorbs more heat than air. When earth gets heated, the temperature of attached or associated closely with it increases and the ‘density’ of that air decreases. This air of lesser density being light travels upwards away from the earth. As one travels up wards away from the earth, the atmospheric pressure decreases.

So the hot air reached that place expands. Any gas that expands suddenly falls in temperature and become cool.

Thus, the hot air which rises away from the earth cools and its temperature falls by 9°c for each kilometer. That is the reason why the atmosphere is cool at the hill stations even in summer.
But, this phenomenon occurs only up to certain height. If one travels to a height more than 40 kilometers it will be once again hot, because the air that gets heated by the earth and travels upwards and in the process expands and become cool, will not reach those heights beyond 40 kilometers.

Prof: Laxmi… Emani
Published date : 21 Feb 2013 02:39PM

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