The sky at the mountains tops is usually very clear and due to this clear atmosphere, the mountain tops cool very rapidly as the heat is radiated back to the atmosphere at a very high rate. The result is that the mountain slopes become much cooler than the surrounding air. The air in contact with the slopes becomes cooler too. As it cools, it becomes dense and heavy because density is inversely proportional to the temperature. Now if it had been a plain area, this cool dense wind would have sinked to the lower level, near the ground, till morning but as in our case it is a mountain
slope, so under the action of force of gravity it slides down the mountain slopes towards the deep valley below and forces the temperature there to become quite cool. The speed of the wind depends upon the gradient of the slope and the temperature at the source area. It is normally in the range of 5-10 knots. As it falls from the mountain top towards the valley it is also called fall wind. Another name for this wind is katabatic wind.
Clear sky devoid of any clouds, little or no wind at the mountain top and low relative humidity are the prime requirements for the mountain or fall wind to set in. Clouds and rains etc. disturb this phenomenon severely.
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