Montana Earth Science Picture of the Week

Although Montana does occasionally experience tornados, a more common source of destruction associated with thunderstorms in our state is something called a “downburst”. These are exceptionally strong downdrafts that, upon reaching the Earth’s surface, diverge horizontally like water streaming from a garden hose nozzle that has been aimed at the ground. Downbursts can occur with or without rain. They can blow down trees, flatten crops, and destroy buildings. Sometimes downburst damage is wrongly blamed on an unseen tornado. The photo shows damage at Culberston High School caused by a downburst that swept through northeastern Montana on July 15, 2005.

Understanding downbursts is “no sweat”. . .
Downbursts are caused by the same thing that happens every time a molecule of water evaporates from your skin. As sweat evaporates it helps cool your body because the water molecules absorb heat as they change from liquid to vapor. The same thing can happen with a thunderstorm. As rain falls through very dry air, much (or all) of the rain may evaporate. As this happens the water molecules absorb heat from the surrounding air, making that air much colder. . . . The more evaporation, the colder the air gets. Anyone who has ever opened a refrigerator door knows that colder air is heavier than warmer air. As this air (cooled by the evaporation of rain) gets heavier, it plunges toward the ground like a lead weight. When the microburst reaches the ground surface winds may exceed 150 miles per hour.

Watch the "Latent Heat Captured on Video" to see how a phase change can release heat.

The smaller ones can be more dangerous . . .
Technically, scientists refer to the smaller downbursts as "microbursts". Downbursts, which can also be caused by evaporation due to dry air that rises into a thunderstorm, are classified according to size and duration. The smaller ones (microbursts) typically have a path of destruction that is 2.5 miles or less, can have winds approaching 170 m.p.h., and last less than 10 minutes. Microbursts are especially dangerous at airports where there small size defies detection and they play havoc with planes that are trying to land. Larger downbursts are called “macrobursts”, or simply "downbursts". The downburst that caused the damage shown in the photo had a swath of destruction that measured 50-by-20 miles.

Click here to watch a 42-second video called "The Microburst Demonstration".

Patterns of destruction . . .
Tornado scientist Ted Fujita discovered downbursts while surveying starburst patterns of wind damage as he was flying over an area in West Virginia after a tornado outbreak in 1974. (See photo at bottom of this page.) Fujita recognized that some areas displayed damage that caused by straight-line winds extending out from a center area, much different from the chaotic messes caused by tornadoes. As a young man Fujita had observed first-hand a similar pattern of damage in the aftermath of Hiroshima and Nagasaki where the first nuclear bombs had been dropped to end WWII.

Below: This photo, taken by Tom Kotynski in the Little Belt Mountains of central Montana, shows trees that were blown down by a microburst. To see more great photos by Tom, visit his web site.

Etc. . . .
1. Fujita came to the USA because he was fascinated with tornados. Among other things he developed the Fujita scale for rating tornados based on the damage they cause. The “F” in F-5 stands for Fujita.

2. Fujita’s discovery of downbursts changed the way pilots land planes in the presence of thunderstorms. These changes have probably saved thousands of lives.

3. With some thunderstorms you may notice that the rain isn’t making it all the way to the ground. This rain that falls, but evaporates before reaching the ground, is called “virga”.

Macroburst . . .
In July of 2005 a macroburst hit northeastern Montana, causing severe damage. To learn more about this storm, click on the Hot Link below.

Terms: microburst, virga