A microburst happens when a column of cool air sinks rapidly through a thunderstorm and spreads out when it hits the ground. In a wet microburst the sinking air drags a mass of rain with it, so from a distance it looks like a bomb of water falling from the cloud. Microbursts usually cover less than about 4 km and can produce damaging winds, which also makes them a hazard to aircraft. A storm chaser filmed this one over Tucson, Arizona, in August 2015.
via Downburst | Wikipedia