Montana Earth Science Picture of the Week

Evaporation to Blame for Microbursts

Photo courtesy of Tanja Fransen
National Weather Service Forecast Office in Glasgow, Montana

Although Montana does occasionally experience tornadoes, a more common source of destruction associated with thunderstorms in our state is something called a "downburst" (smaller ones are called "microbursts"). 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. Their straight-line winds can blow down trees, flatten crops, and destroy buildings. Sometimes downburst damage is wrongly blamed on an unseen tornado. The photo above shows damage at Culberston High School caused by a downburst that swept through northeastern Montana on July 15, 2005. This downburst had a swath of destruction that measured 50-by-20 miles

Photo courtesy Tracy O'Connor, Poplar Airport Manager
CLICK HERE to see a larger version of the photo.

Deadly Microburst of 2010 . . .
A series of thunderstorms that moved through north-eastern Montana June 16, 2010 produced the microburst that flattened this modular home 3 miles east of Froid, killing one of the inhabitants. The home was lifted from its foundation, thrown 30 feet to the north-northeast and flipped over. A man and a woman were in the home when the microburst hit at about 9 pm. The lady, who was found over 250 feet north-northeast of the main debris area, died and the man was hospitalized with injuries that included broken bones.

Experts estimate wind speeds needed to cause such destruction were in the 110 to 125 mph range. Unlike the 2005 storm that affected Culbertson, the worst damage from this event was isolated to a small area east of Froid. Neighboring farms had no significant or obvious damage. According to Tanja Fransen of the National Weather Service Forecast Office in Glasgow, microburst winds of 50-70 mph occur between 10 to 15 times a year in eastern Montana, with more devastating winds occurring once or twice a year. However, these usually go unnoticed due to the rural nature of the region. . . There simply aren't many people or buildings in this part of the state.

Understanding downbursts and microbursts 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 should know 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". .

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 20 miles north of Butte, shows trees that were blown down by a microburst that occurred in 1999. To see more photos of this area CLICK HERE.

Below: Check out this video of a microburst that happened near Tucson, Arizona in the summer of 2015.

Etc. . . .
1. Fujita came to the USA because he was fascinated with tornadoes. Among other things he developed the Fujita scale for rating tornadoes 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.

Term: virga


Tornado AND Microburst Hit Crazy Mountains (article and photos 7-28-2010)
*More about downbursts
*More about that storm of July 14, 2005 (National Weather Service site)
Watch and Listen to a Microburst PenCast
Next Picture: Rhinos in Nebraska

Below: This photo, taken by Dr. Ted Fujita, shows the type of straight-line wind damage caused by a microburst that hit the ground at an angle. The trees are laid out neatly in one direction rather than in a starburst pattern. The photo was not taken in Montana.

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