Downbursts
Where a tornado is a destructively powerful updraft that features intense winds spiraling inwards and lifting up into the storm with a sucking action, a downburst is the opposite -- a massively powerful downdraft that can cause damage almost as intense as that caused by a tornado. A downburst also typically occurs in a thunderstorm, and may in fact happen while the same storm is producing a tornado nearby. On the other hand, downbursts often occur without other dangerous weather forms present -- the downburst may be the thunderstorm's only damaging feature, and hail and tornados may well be totally absent.
A downburst is created when hail or large raindrops fall into dry air and melt or evaporate, cooling the air very abruptly and intensely. This intensely cooled air becomes much heavier than it was a few moments before, since cold air is denser and heavier than warm air. This pillar of cold air plunges towards the ground, literally falling under its own weight, and flows out of the bottom of the thundercloud. When it strikes the earth, the wind blasts out in every direction as a straight-line wind, reaching speeds of up to 150 miles per hour.
Needless to say, a straight-line wind of this intensity is able to cause considerable damage, snapping trees and wrecking lightly-built structures. It is always easy to tell tornado damage from downburst damage after the event, however, because tornado debris exhibits a whorled pattern from the rotating winds, while a downburst's damage radiates outward from a central point.
Sizes and varieties of downbursts
Downbursts come in various sizes, and may affect an area little larger than a single small meadow, or a region covering several square miles. The size of the downburst depends on how large the downdraft is that causes it, which in turn depends on how much air is cooled by the evaporating rain or melting hail. Downbursts can also be wet or dry. A dry downburst can be invisible except for its effects, while a wet downburst can be seen clearly even if it fails to actually strike the ground.
- Microbursts, which make up the majority of downbursts, cover an area of less than two and a half square miles. They are just as intense as macrobursts, but their area is much more limited. Microbursts which occur near airports can cause extreme difficulties for pilots attempting to take off or land and have been known to cause aircraft to crash.
- Macrobursts are fortunately quite rare, since they can affect areas larger than 2.5 square miles -- and sometimes much more. A series of macrobursts which struck northern Wisconsin in July of 1977 leveled an area of forest 17 miles wide and nearly 200 miles, caused one death, and proved strong enough to shatter the trunks of four-foot-thick trees.
- Wet downbursts occur mostly in the southeast of the United States, and are clearly visible because of the large amount of rain and sometimes hail that fills them. They may even bear a vague resemblance to a tornado, although observation can quickly tell you that are they are not because of the outward flow from the base of the downburst across the ground.
- Dry downbursts are the most common type in the American West and Midwest. Some precipitation is visible under the cloud at the point where the downburst is descending, but the downburst itself carries very little and can be most invisible except for debris blown along at ground level and the bending of trees away from the downburst center.
Derechos are similar to large-scale, long-lasting downbursts in many ways, but differ from them in others. Scientists who study one often study the other, however, since many of the principles are the same. Downbursts are not as well-known as tornados, but can be nearly as destructive, so efforts are also being made to educate the public about these weather phenomena in order to increase the general level of safety. They are also interesting in that they show that a downdraft in a thunderstorm can be just as dangerous as an updraft.
