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downdraught


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A relatively confined, descending current of air within a cumulonimbus cloud or other storm system. Generally initiated by the drag exerted by the severe precipitation in a shower cloud or thunderstorm, downdraughts are colder than the surrounding air, because they warm at the saturated adiabatic lapse rate, which is less than the environmental lapse rate. The air may also be cooled by evaporation of precipitation, thus becoming denser and either starting the downdraught or increasing the rate of descent. Strong downdraughts fan out on reaching the ground, and may extend tens of kilometres ahead of the system, as well as producing violent squalls. See also updraught.

Subjects: Meteorology and Climatology.


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