Most pilots equate wind shear to turbulence or convection. Certainly some forms of wind shear are indeed turbulent and convection can also induce dangerous wind shear. If the wind shear occurs close to the surface when the aircraft is landing or departing, it may be in excess of the inertial capabilities of the aircraft making it difficult for the pilot to recover resulting in an accident that is often fatal. But don’t be fooled; some of the most dangerous wind shear situations come out of rather benign-looking convective environments.
Perhaps the most dangerous wind shear occurs in a convective downburst where downdrafts in thunderstorms have been estimated to be greater than 100 miles per hour. These high-speed winds strike the surface and spread out creating a gust front that can also be very dangerous especially if it occurs when landing or departing. When that downburst occurs in a very small area spatially (about 2 miles) it is referred to as a microburst similar to the one in this image. Most microbursts are ephemeral lasting no more than five minutes in most cases. If you are not convinced of the power of a microburst, check out this video. This demonstrates just how much energy can be driven out of a convective outflow.
At this point in time, several large turbojet aircraft have succumb to the forces associated with a microburst. The one that is especially memorable is Delta Airlines Flight 191 that encountered a microburst on approach to the Dallas Fort Worth International Airport (KDFW) in 1985. Delta Flight 191 didn’t fly through or under an intense supercell thunderstorm as you might imagine. In fact, a thunderstorm with a cloud base of nearly 10,000 feet was the culprit. High-base convection with a heavy rain signature should be of particular concern to pilots since they signal a deep mixed layer with a high lapse rate and plenty of precipitation to fuel a strong downdraft. Pilots can be fooled because high-based convection does not seem particularly threatening (especially to pilots flying large turbojet aircraft) which makes them even more likely to stumble into the path of a downburst or microburst.
Here’s a quote from a paper written by Captain William W. Melvin entitled Wind Shear Revisited, Air Line Pilot Magazine, Nov 1994.
Many pilots have been trained to avoid large supercell-type thunderstorms in the belief that this will prevent encounters with microbursts. Yet no evidence exists that any of the known microburst encounters have occurred in supercell storms. Dr. Ted Fujita and Dr. Fernando Caracena recognized authorities in this field have repeatedly emphasized that microbursts are frequently generated from benign-appearing cells. Many “experts” who disagree with Drs. Fujita and Caracena have emphasized the supercell storms with warnings of dangers of gust fronts. These so-called experts are leading pilots down the primrose path for microburst encounters.
In a high-based thunderstorm there’s typically an extremely dry environment between the cloud base and the surface. Initially, little or no rain may reach the surface. As the rain falls out of the cloud into this very dry atmosphere it evaporates quickly which causes a cooling effect relative to the air around the precipitation. Such cooling makes the air much denser, and therefore negatively buoyant, effectively creating a very intense downburst with winds that may exceed hurricane force, even approaching the speeds found in a weak to moderate tornado.
The next time you see what appears to be a very inviting situation in and around active convection, think twice about departing or making that approach to land. Microbursts can happen quickly before any visual cue may exist. Especially avoid areas where you see a circular ring of dust at the surface that may be curled around the edges or any kind of bulge (piling up) of precipitation at the bottom of an evolving rain shaft. These are all characteristics of a intense downdraft, microburst or gust front.
