The mountain wind is a high density cold wind that blows from mountain tops towards the valleys making the nights at the valley stations quite chilly.
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.
The effect and case of Gilgit
The station of Gilgit is a low valley situated at about 4500 ft. above mean sea level in the northern areas of Pakistan. It is surrounded by very high mountains with heights 15000 feet or more. The area is generally dry with very little rainfall. The sky above the mountains remains clear with very low relative humidity especially in the summer months of July and August. This makes this station an ideal case for the mountain wind to exert its effect.
In late July to early August, it so happens that Gilgit becomes intensely hot during the day-being out of sphere of monsoon, the air is dry so gets heated up quickly and by mid-afternoon, the temperatures can reach as high as 110 F. The surrounding mountains tops, however, remain 40- 50 F cooler throughout the day. During night, due to clear atmosphere on the mountains, the temperatures start falling rapidly and reach around 50 F. As described above, the air along the slopes becomes very cool and starts sliding towards the Gilgit valley and is most effective just before dawn when they can make the minimum temperature of Gilgit as low as 60 F. So the temperature of Gilgit falls from a high of 110 F to a low of 60 F -a fall of 50 F in 24 hours! This is all due to katabatic wind. This happens typically from late July to mid-August. If we observe the climate of Gilgit closely, the moment western disturbance arrives in this area with its clouds, rain and strong winds, the High of Gilgit becomes 95 F and low rises to 75 F which is a clear indication that the mountain wind effect is disturbed.
In short, mountain or katabatic wind is a cool dense wind that can dramatically change the temperature of the area it affects.