http://img17.imageshack.us/img17/6619/microburstnasa.jpg


The above drawing shows a good view of what the down burst looks like. The down burst is caused by the collapse of the updraft part of the cell. The storm can no longer keep the column of air rising and as the air begins to fall the rain cooling speeds the descent of the air mass. Down bursts are generally of short duration, but super cells can produce fairly large down bursts of several minutes. As you see in the drawing the column hits the ground and spreads out in all directions. If the down burst is right over the approach end of the runway this spells the worst for the approaching plane. As the plane flies inbound on the ILS, the pilot firsts sees an increase in airspeed with a ballooning upward. The pilot adjusts pitch to control the climb and pulls back on the thrust. As the plane nears the approach end of the runway it enters the area of the shear with the vertical component. The aircraft now begins a rapid sink rate and looses the head wind. If the pilot makes it past that, he encounters the loss shear part of the down burst and unless the plane can power out of the shear it will impact the ground.

Delta 191, a Lockheed L-1011 had just such a down burst landing in KDAL several decades ago. Eastern Airlines also lost a 727 landing KJFK runway 22L during a similar event.

Thunderstorms in the eastern part of the U.S. generally have low ceilings and poor visibility, whereas storms in the west have high base cells with more visibility. Don't confuse Virga with a down burst. Virga is a form of precipitation that evaporates before it hits the ground. Low humidity and high temperatures can cause rain to evaporate completely shortly after its release from a cloud. Desert areas often have clouds showing virga. In fact, the precipitation often starts out in the form of ice crystals and never reaches the ground.