NOAA's National Weather Service Weather Forecast Office

Downbursts

Inside a thunderstorm there are powerful updrafts and, as the storm matures, downdrafts (what goes up must come down).  The updrafts can reach many tens of miles per hour (sometimes as high as 100 mph).  Turns out, storm downdrafts can be equally intense.  The downdrafts are caused by factors such as the drag from heavy masses of rain and hail, and especially the fact that falling precipitation evaporates and cools the air, making it heavier than its environment.  Most thunderstorms generate downdrafts, the cooling outward-rushing air that often breaks the heat of any oppressively hot summer afternoon. The leading edge of the downdraft is called the gustfront.  It is sometimes marked by spectacular cloud features called shelf or roll clouds (a long frontal roll cloud pictured below).
On some occasions, downdrafts can become very intense, slamming into the surface with wind gusts well in excess of hurricane force.  That is a downburst. The smallest of these is called a microburst, some of which may be only several hundred yards wide.  Recent research has shown that much storm damage once ascribed to tornadoes is actually the result of microbursts.  Their winds can equal that of small tornadoes and, to the untrained eye, the damage looks as if a tornado went through the area.  They can also be accompanied by very loud roaring noises. 

Wind speeds above 120 mph in downbursts are not that uncommon.  In 1995, downburst winds were clocked at 136 mph at Grissom Air Force Base, Indiana, with some estimates as high as 140 mph in Miami County, Indiana.  As you can see, downbursts pose a significant hazard to the aviation industry.  Just five minutes after President Reagan landed in Air Force One at Andrews Air Force Base near Washington D.C. the highest recorded thunderstorm microburst wind ever clocked--gusts of 149.5 mph--struck there, causing considerable damage.   One of the most noteworthy accidents in U.S. aviation history was the crash of a Delta Lockheed L-1011 at Dallas-Ft.Worth Airport on August 2, 1985.  The accident took the lives of 137 people, including the driver of one car that was struck on the ground.  Wind speeds of 80 mph were noted with a storm passing to the north of the airport as the plane made its approach.

As far as the wind element of a thunderstorm, the National Weather Service classifies a thunderstorm as severe if it produces wind gusts of 58 mph or higher.  If you begin to experience wind speeds of this magnitude you should move inside a substantial structure until the wind has subsided.  Consult our handy reference chart to estimate the speed of the wind at your location.