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Wind shear - A case of benign-looking convective environments

Updated: Oct 26, 2021

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 difficult for the pilot to recover resulting in an accident that is often fatal.       

Wind shear is simply a change of wind direction and/or wind speed over a given distance. There's always some kind of wind shear present in the atmosphere on any given day or time. But, it is the gradient of wind shear (how rapidly the wind direction and/or speed changes over a specified distance and/or time) that's the most important.    

Shear is evident when there's a change of wind direction or wind speed in the horizontal and/or vertical.   We often see shear created when the wind is gusty.  As the wind gusts, the speed increases suddenly (horizontal speed shear) and the wind may even shift direction during a gust.

Perhaps the most dangerous wind shear occurs in a 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 when landing or departing. When that downburst occurs in a very small area spatially it is referred to as a microburst as can be seen in this amazing video of back-to-back microbursts that occurred in Tuscon, Arizona. Most microbursts are ephemeral lasting no more than five minutes (although there is evidence that some do last longer or can be followed by a second microburst as shown in the video). Elapsed time in this video is 35 minutes according to the photographer who took the video footage.

A microburst was defined by Dr. Ted Fujita in 1985 as a downburst with the spatial scale on the order of a runway length.  That would involve no more than 4 kilometers (2.2 nautical miles) over the surface.  The smaller spatial scale of a microburst converts into tighter wind shear gradients that are experienced by penetrating aircraft as more rapid changes in wind direction and/or wind speed.  For many microbursts, this high wind shear gradient will be in excess of the