Downbursts

Downdrafts are generated when rain-cooled, more dense air sinks inside a thunderstorm. Also some of the strong winds aloft are carried down with the downdraft by a process called "momentum transfer". As precipitation begins to fall, it drags some of the air with it. This "precipitation drag" initiates a downdraft. The downdraft is intensified by evaporative cooling as drier air from the edges of the storm mix with the moist air within the storm.

These processes lead to a rapid downward rush of air. As the air impacts the ground it is forced to spread out laterally causing the gusty winds associated with thunderstorms. Occasionally, thunderstorms will produce intense downdrafts that create damage as the wind spread out along the ground.

Downbursts and Aircraft

Downbursts can create hazardous conditions for pilots and these events have been responsible for several disasters.

  1. As aircraft descend (right) into the airport they follow an imagery line called the "glide slope" to the runway (solid light blue line).
  2. Upon entering the downburst, the plane encounters a "headwind", an increase in wind speed over the aircraft. The faster wind creates lift causing the plane to rise above the glide slope. To return the plane to the proper position, the pilot lowers the throttle to decrease the plane's speed thereby causing the plane to descend.
  3. As the plane flies to the other side of the downburst, the wind direction shifts and is now from behind the aircraft. This decreases the wind over the wing reducing lift. The plane sinks below the glide slope.
  4. However, the "tailwind" remains strong and even with the pilot applying full throttle trying to increase lift again, there is little, if any, room to recover from the rapid descent causing the plane to crash short of the runway.

Since the discovery of this effect in the early to mid 1980's, pilots are now trained to recognize this event and take appropriate actions to prevent accidents. Also, many airports are now equipped with equipment to detect downbursts and warn aircraft of their location.


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