Mountain and Valley Winds

Mountain and Valley Winds These are local winds responding to local pressure gradients set up by heating or cooling of the lower air. During the day when the slopes are intensely heated by the sun, the air in contact with the slopes is also heated by conduction from the ground.

This warmed-up air rises and is replaced by the air moving up from the valley, thus creating an upslope wind during the day. It is particularly well developed when one side of thtt valley is heated much more than the other, for example<; the valleys which have an east-west trend in mid and high latitudes. When the same slopes have been cooled at night by radiation of heat from ground to air, the wind moves valleywards. The wind is at its strongest just before sunrise when radiational cooling is greatest. Valley winds are a kind of anabatic winds.

Katabatic Winds A cold downslope wind caused by the gravitational movement of cold dense air near the earth's surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favourable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.

The strongest katabatic winds are those that blow from an ice cap off the Greenland or Antarctic ice caps. In other parts of the world, these are known by various local names such as the bora in the northern Adriatic coast and the mistral in southern France. The Santa-Ana of southern California is of desert origin, carrying much dust and silt suspension.

Foehn and Chinook These result when strong regional winds passing over a mountain range are forced to descend on the ice side with the result that the air is heated and dried.