Mountain regions display many interesting weather patterns. One example is the valley wind which originates on south-facing slopes (north-facing in the southern hemisphere). When the slopes and the neighbouring air are heated the density of the air decreases, and the air ascends towards the top following the surface of the slope. At night the wind direction is reversed, and turns into a down-slope wind. If the valley floor is sloped, the air may move down or up the valley, as a canyon.

Mountain Breeze

Similar day-and-night changes cause mountain and valley breezes. By day the greatest heating occurs along mountain sides and at the heads of valleys. Cooler, heavier air on the lower levels then blows up the valleys. At night this action is reversed.

KATABATIC WINDS

Because they belong to a category of winds called katabatic winds (from the Greek 'kata', meaning downwards); these winds are observed at every latitude of the globe as soon as the course of cooled air meets a significant slope, but nowhere are they as strong as they are in Antarctica. As we have already seen, the lower layers of the atmosphere that come into direct contact with the ice cap are extremely cold (caused by the reflective properties of the ice and the pureness of the air); and the colder the air is, the heavier it becomes. These masses of cold air contract and become even heavier when they start moving under the effect of the incline of the ice cap - remember that the average altitude of the continent is approximately 2,500 metres and the topography takes the ice-covered surfaces sloping down towards the perimeter, gently at first, then more steeply as the land reaches 100 or 200 kilometres from the coast. So the air mass slips very easily across the surface of the continent where there is any relief; and as the air moves, it has an abrasive effect on the ground, grating and sculpting it into protruding

Chinooks are produced as strong westerly flow rides up and over the mountains. Chinooks are descending, warm, dry winds on the lee side of a mountain range. Chinooks are common east of the Rockies, also observed in the Alps - called the Foehn.