A microburst is an intense, but short-lived, downdraft.

Microbursts are a significant concern during low-altitude phases of flight.  The downdraft experienced can be as much as 6,000 feet per minute.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 19-6
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge 12-11

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