III. Weather Phenomena

6. Foehn

A Foehn is a type of dry hot wind that occurs on the lee side of a mountain range.

The reason for the formation of a Foehn is that the air flow that moves perpendicular to the mountain range is blocked by high mountains and is forced upward and cooled (for every 100 meters of elevation the air temperature will drop about 0.65 °C), the water vapor in the air is therefore condensed into clouds and rainfall on the windward slope. When the air travels across the mountain and goes down to the leeward slope it is already dry and the air will pick up temperature due to compression (every drop of 100 meters in altitude will increase the temperature by 1 °C). When the air reaches the land surface its temperature will be much higher than the temperature of the ground, causing a dry and hot wind called a Foehn to form. In Taiwan it is also called a burning wind. In other places it has different names. For example, in the Rocky Mountains (U.S.) it is called a Chinook, while in the Alps it is called a Foehn. When there are typhoons or low pressure systems which pass through the north of Taiwan and the wind is a strong, westerly gale, there will often be Foehn winds in the Taitung regions.