A wind catabatic , Greek katabatikos which wants to say descending the slope, is a gravitational Vent produced by the weight of a cold mass of air descending a geographical relief. Various weather conditions are necessary for its release: an inversion of temperature in altitude and a weak gradient of pressure possibly accompanied by a depression downstream. Once the engaged process, the cold mass of air accelerates and the speed of the wind can be extremely violent (more than 300 km/h), definitely more than the Vent anabatic.

One finds the most spectacular winds this type in the Antarctic or Arctique with the Greenland, in particular around Tasiilaq where it takes the name of Piterak. However, it can occur everywhere where the conditions are met. The Mistral and the Bora are thus two winds catabatic known Européens but the wind which goes down from the mountains by one night released and cold is another example.

Principle

One can cool the air in two ways: by radiation or modification by the layer under-jascente. In the first case it is about a Masse of air which loses its energy towards the roadbases as in the case of the temperature of surface which descends the night at the time of a clear sky. In the second case, the air passes on a colder surface and will gradually take the temperature of this one as in the case of the air on a glacier.

In both cases, if the gradient of pressure in the area generates a wind, the mass of cold air will move with this one and no wind catabatic is possible. On the other hand, if an inversion of temperature keeps the cold air in a layer close to the ground and that the wind is low or zero, one will attend the formation of a drop of cold air. The cold drop creates a local high pressure. If it is at an altitude higher than hotter air, this last forms a low local pressure and the cold drop will descend the slope to replace it. The latter being denser than of the hotter air, it will be involved in bottom of this one at the time of the contact. The force of the wind depends on the difference between the two masses of air, on the slope and the friction caused by the relief.

The wind catabatic is distinguished from the winds of the type fœhn which also descend the slope. The latter are the result of a forcing by the wind of the air over the relief and its temperature at the top of the obstacle is not due to a local cooling.

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