Downdraft
A downdraft is downward moving air, usually the direct result of air convection within the thunderstorm. Downdrafts can be extremely powerful, and some aircraft have crashed as a direct result of downdrafts.
The downdrafts that caused Delta Air Lines Flight 191 to crash after takeoff were presumably caused by an extremely powerful type of downdraft - Microburst Wind shear, prompting the Federal Aviation Administration to research and deploy new storm detection radar stations at some of the major airports.
