August 27, 2005
This photo was taken on July 17, 2005 in Memphis, Tennessee and shows an outflow boundary from a severe thunderstorm. Outflow boundaries form when a rain-cooled down flow of air reaches the ground and spreads out. As with a cold front, people in the vicinity experience a sudden change in wind direction, an increase in wind velocity, and a sudden drop in temperature. If the atmospheric moisture layer is quite deep, as it often is in Memphis in the summer, the boundary is the focus for development of new storms.
In the photo, the outflow had passed my position and was continuing in a northerly direction. One can see, as manifest by the posture of the clouds, a "wedge" of cooler air lifting the warm humid layer it's undercutting. While this outflow boundary was actually quite docile, with the winds peaking at about 20 mph (32 kph), other outflow boundaries can result in severe straight line winds. Memphis experienced such a storm in July of 2003 which had winds that were measured greater than 80 mph (128 kph). Informally dubbed "Hurricane Elvis," this storm resulted in electrical power being lost for over 300,000 people, with some homes being without power for two weeks.Related Links: