An anabatic wind is warm wind that blows up a mountain slope due to a warmer temperature on the mountain slope compared to the temperature in the air column. Also known as an upslope wind or upslope flow, the name anabatic wind stems from the Greek word "anabatos," which means "moving upward." This type of wind usually occurs on a clear sunny day. The opposite of an anabatic wind is a katabatic wind, which blows down the slope of a mountain. Additionally, katabatic winds blow at night and are caused by reversed temperature differences between the air column and the mountain slope.
Anabatic winds can be useful for gliding and sailplanes. For example, a glider pilot can use anabatic winds to increase the altitude of the aircraft. However, anabatic winds can be dangerous to cyclists, especially when travelling downhill at maximum speed.
Formation of Anabatic Winds
The Sun's rays will heat a hill or mountaintop on a clear day. Consequently, this heat on the mountaintop will then heat the surrounding air. The difference in altitude between the mountaintop and areas around it will result in temperature differences, since they receive varying amounts of sunlight. The difference in temperature between the mountaintop and low-lying areas around the mountain is increased even more if there is shade, which can be created by a number of things, such as the mountain itself or trees.
The difference in temperature between the two areas eventually forces the air at lower altitudes to move up the slope through convention. This air is forced up the slope since the warm air at the top of the mountain creates an area of low pressure after the warm air rises. When the cold air moves up the slope, it is heated and it ascends, since heating causes the air to become less dense. In most cases, this air rises to an elevation at which it cools and eventually forms cumulus clouds, that can lead to rain or even thunderstorms. This cycle then repeats itself. Anabatic winds can reach speeds of between 7 and 11 miles per hour.
Mountain Breeze and Valley Breeze
Mountain and valley breezes are similar to anabatic and katabatic winds, but smaller in scale. A valley breeze occurs when warm air rises up the side of a mountain, while mountain breezes occur when warm air flows down from a mountaintop to the valley. Unlike anabatic winds, mountain and valley breezes are formed the same way as land and sea breezes.
A valley breeze is created during the day when the Sun heats the mountain air faster than it heats the air at the bottom of the mountain. Consequently, the warm air on the mountain rises due to convection. On the other hand, a mountain breeze is the opposite, and occurs during the night, when air on the mountain descends down the valley instead of flowing upward.