Katabatic wind describes the movement of cool air down a hillside into a valley or depression. It is one of several mountain or valley winds which are caused by a difference in ground level atmospheric temperature and density.
The phenomenon is most frequent during the night, and particularly in the early hours just before sunrise, this being the time when night time cooling is at its maximum.
In simple terms, during a clear night in a hilly or mountainous area, upward radiation of residual warm air from just above the earth’s surface will leave a denser, cooler air in its place. This cool air, because of its higher density will tend to move downwards, simply pulled by gravity, flowing along the slope to collect in the lower land below. It is because of this process that so-called frost hollows occur, places where very cold air accumulates in surface depressions during winter nights, causing a risk of frost and potential damage to fruit and other crops.
Observations in different locations have suggested however that katabatic winds do not always cause an accumulation of cold air in valleys. For example in parts of California it has been found that right from the beginning of a nocturnal cooling period, air in valley floors can remain colder than that on the upper slopes, and that downward moving air caused by katabatic drainage cannot penetrate the denser air in the lowest part of the valley, and remains above it.
Sometimes the occurrence of a katabatic wind is so extensive as to place it in the category of a regional wind. The Mistral of south east France is one such example.
This wind, violent and destructive both in strength and low temperature, is an extreme form of katabatic wind, exacerbated by other particular meteorological conditions.
In situations when a depression forms in the Gulf of Genoa, off the coast of Provence, and a strong anti-cyclonic ridge spreads out from the Azores High Pressure area over the Atlantic, the movement of air from the north between the two can be particularly marked.
The katabatic effect will cause dense, cold air to move down from the upper Alpine slopes around Lake Geneva, funnelling it into the Rhone valley. The steep pressure gradient caused by the offshore low and the ridge to the west will cause this air movement to accelerate, rushing down the Rhone valley to the Mediterranean coast and affecting coastal areas in both directions.
Mistral winds of over 11 meters per second can occur on up to a third of the days in any year, the effect on Rhone valley crops made clear by the west to east lines of trees across the valley floor between fields and vineyards.
