WEATHER CONTRASTS DUE TO FOEHN WINDS
When going to the Alps, you can sometimes observe strongly different types of weather between both sides of the mountain ridge. One moment you enjoy warm and sunny weather, next you drive through a tunnel and suddenly it is rainy when you get out of it. Often, this contrast is caused by foehn winds. After reading this article, you know more about the cause and consequences of these winds and where they occur.
Mostly, foehn winds are caused by low-pressure areas and are most frequent in the autumn and spring. In the Alps, these winds generally come from the north or the south, perpendicular to the mountain ridge. Large amounts of moisture are pushed against the ridge and the air is lifted. As a result, the temperature of this air decreases. Therefore, the water condensates and rains out on the windward side of the mountain ridge. After condensation, the temperature of the rising airdrops with about 0,6℃ every 100m.
After passing the mountain ridge, the air is much drier after most waters rained out. The air flows down again on the other side of the ridge and its temperature increases. Dry air changes faster in temperature than moist air and warms up with 1℃ per descending 100m. Therefore, the valley on the lee-side of the mountains will be much warmer than the windward valley. Also, the weather is very clear there and the view could even be 100 kilometers.
Foehn winds do generally not lead to favorable conditions for winter sports. Because of the high temperatures, the slopes become icy, and the risk of avalanches increases. Especially on top of the mountains, wind gusts could easily exceed 100 km/h. Furthermore, people are sensitive to complaints during foehn, such as headaches, muscle pain, and sleeping problems. Presumably, this is caused by changing air pressure or dry air, but the mechanism behind these problems is not completely clear.
Besides, similar kinds of winds occur at many other places worldwide. These are called Chinook winds in the Rocky Mountains, Fogony in the Pyrenees, Halny in the Carpathian Mountains, and Bergwind in South-Africa, but these are all the same.