Mountain and Valley Winds

winds in mountainous regions that change direction twice daily: in the daytime valley winds blow up the valleys and mountain slopes, and at night the mountain winds blow in the opposite direction.

Mountain and valley winds blow locally as a result of the differences in the heating and cooling of the air over the mountains and valleys. In the daytime the air over the mountain ridges and valleys becomes warmer than the air at the same levels over the plains and expands more. Consequently, at the higher levels the downward pressure from the mountains to the valleys is reduced (horizontal baric gradient), and air travels in that direction. This flow of air toward the valley below causes a drop in the pressure from the valleys toward the mountains and, consequently, a valley wind. At night the temperature and pressure factors are reversed, so that a mountain wind results. In addition to this circulation of air between the valleys and mountains, there is a downflow of cooled air along the mountain slopes at night and an upward flow of warmed air along the slopes during the day. The speed of mountain and valley winds is inconsiderable, but on occasions it may attain 10 m per sec. In middle latitudes mountain and valley winds occur in the summer when stronger currents, forming part of the general circulation of the atmosphere, do not prevail. In low latitudes (for example, in the Himalayas) they may occur throughout the year. Sometimes only the day (valley) wind or only the night (mountain) wind may occur: sometimes the mountain and valley winds are combined with lake breezes—for example, in the area of Lake Sevan.

In high mountainous areas the layer of air forming the mountain or valley wind extends from the bottom of the valley by 1 km or more and not infrequently up to the mountain crest: at higher altitudes a wind blowing in the opposite direction occurs (counter current). In the lower mountain areas mountain and valley winds are confined to layers over the mountain slopes themselves (slope winds). Mountain and valley winds considerably affect the daily conditions of humidity, cloudiness, and precipitation. In particular, they are a cause of the daily cloud formations and precipitation over mountains and of night clearings. They must not be confused with other winds that are peculiar to mountain areas but do not have a periodic character, such as glacier winds, the foehn, and the bora.