March 14, 2010 in Idaho Voices

Downbursts, tornadoes cause different damage

Michelle Boss

Spring must be getting close. It seems like the latest weather news stories have had more to do with tornadoes and wind damage than blizzards and wind chills.

Last Monday, a downburst near the western Oregon town of Scappoose brought down trees and power lines. On that same day, an EF-2 ranked tornado (on a damage scale of EF-0 to EF-5) hit the town of Hammon, Okla., destroying several barns and homes. While damage from both the downburst and tornado was caused by strong winds, the two types of storms are very different.

A downburst is a strong downdraft that causes damaging winds on or near the ground. It is caused by rain cooled air descending from the mid and upper levels of thunderstorms. Downbursts can be put into two different categories based on their size.

A macroburst may have damaging winds which extend more than 2 1/2 miles, last for five to 30 minutes, and produce widespread tornado-like damage with winds as strong as 134 mph. A microburst will have a damage swath of less than 2 1/2 miles, last anywhere from five to 15 minutes, but may have damaging winds as high as 168 mph.

One of the ways meteorologists can tell the difference between tornado wind damage and downburst wind damage is by the orientation of the debris on the ground. Downbursts cause what is called “straight line” wind damage. As the descending air from the downburst hits the ground, it spreads outward with a force that can take out buildings and down trees. This debris will be laid out in straight lines parallel to the outward wind flow.

With a tornado, winds are flowing into the storm (as opposed to outward). Due to the swirling winds of a tornado, debris will be lying at angles. The swirling winds from a tornado can also be much more intense than thoset from a downburst. During the Moore, Okla., tornado in May 1999, one of the strongest tornadoes ever recorded, the DOW (Doppler on wheels) measured wind speeds of 318 mph.

While downbursts can occur in almost any thunderstorm, most tornadoes are spawned by the rotation of winds in what’s called a super-cell thunderstorm, which is much rarer – especially in this part of the country. For every one tornado, there are approximately 10 downburst damage reports. When wind damage occurs here in the Inland Northwest, while many folks automatically assume it must have been a tornado, the culprit is usually straight-line winds from a downburst.

Michelle Boss can be reached at weatherboss@

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