What weather phenomenon occurs when a large downdraft lasting approximately 5 minutes crosses an area of roughly 3 kilometers?
Refer to figures.
Microbursts are down currents in the cloud that also move outwards by reaction from the ground, having speeds considerably in excess of 1000 feet per minute downwards (up to 6000 fpm) and 50 kt horizontally. The wind shear (headwind to tailwind) may be between 50 and 90 kt.
They are largely caused by descending raindrops, which cool the surrounding air by evaporation, and their higher density accelerates the downdraught even further.
They are concentrated in a burst that is up to 4 km in horizontal length and have a lifetime of less than 5 minutes. A microburst is a similar event but over a bigger area.
Microbursts are most likely to occur in summer air mass thunderstorms in low-latitude regions with dry surface conditions.
They cause extreme turbulence and severe wind-shear conditions. A warning sign of wind shear is virga, which is streaks of precipitation from below the cloud that does not reach the ground.
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