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Backpacker Magazine – May 2008

Phenomenon: Microbursts

These vertical hurricanes are rare–but deadly.

by: Chris Ladd, Illustrations by Bryon Thompson


Microbursts
They develop within a few minutes and strike the ground with the force of a runaway freight train. They demolish forests, wreck houses, kill wildlife–and have even been responsible for a half-dozen plane crashes. But for years they were so mysterious that, until 1981, meteorologists didn't even have a name for them. They are microbursts: turbulent weather conditions that transform a mass of rapidly cooling air into a high-speed downdraft up to 2.5 miles in diameter. Fortunately, most backpackers only observe the aftermath of these violent events. If you ever come across a smashed-up forest with no obvious cause, here's what might have gone down.

Vertical Hurricanes
Microbursts can develop anywhere across the United States; they often appear as wet microbursts in the East and South, and as dry microbursts over the Great Plains, Rocky Mountains, and the Western Plateau. The two types differ dramatically. In the East, microbursts are spawned by large thunderstorms producing heavy rain, lightning, and hail. Their drier, Western counterparts are more subtle, often arising inside small, innocent-looking, cottonball clouds that lack visible precipitation. Swirling dust storms beneath these clouds are sometimes the only clue that downdrafts are occuring.

microburstDry
In the West, rain often falls from high cumulonimbus clouds into lower, drier air, where it evaporates. This creates virga–misty curtains that hang from cloud bottoms but never reach the ground. Like all liquids, the rain loses heat as it transforms into a vapor. (The same process causes fuel bottles to cool when powering stoves.) This evaporative cooling produces a dense mass of colder air within the cloud.

microburstWet
In the South and East, heavy rain generated by powerful thunderstorms drags cold, dry air from high in the cloud to lower layers of the storm. As the water and ice suspended in that mass evaporate, the cooling air descends faster through the cloud.

 

 

microburstDescent
In both types of microbursts, the cold, dense air mass sinks because it is 20 to 50 percent heavier than the surrounding air. As it descends, it continues to cool and accelerate, eventually plummeting toward the ground at speeds between 15 and 50 mph–and in rare cases up to 150 mph. The entire process can occur in five minutes.

microburstImpact
When the fast-moving air strikes the ground, it acts like a bucket of water poured on a concrete floor, says Steven Vasiloff, a meteorologist at NOAA's National Severe Storms Laboratory in Norman, Oklahoma. The blast effect and resulting vortexes can flatten trees at the impact site, as well as strip branches and damage vegetation and soil up to several miles away. Unlike tornadoes, which twist and scatter wreckage, microbursts create a radial pattern of destruction, with downed trees and debris oriented outward from the impact zone.



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anonomus
Apr 01, 2009

this is a reliable source.

Ian
Jul 31, 2008

I was camped on an island ready to start a trip down the Petawawa river in Algonquin park(Ontario Canada) when our area was hit by fierce microbursts. It started out as the wind suddenly reversing directions. Then pine cones were flying sideways. Then you couldn't hear anything but the wind. If someone screamed in your ear you wouldn't hear it. Looking out at the mainland it was like watching god reach down and run his fingers through the trees as they colapsed in streight lines. Trees 1.5-2 feet in diameter were completely leveled, not one stayed up. When it was over the patch of island we were camped on still stood, but the whole other half of the island was gone. Not a tree stood. I will always remember it like it was yesterday.

A note to anyone who has the weather suddenly change to something resembling a huricane, don't shelter in your tent!!! Find the nearest clearing and wait it out. Some of our group came out of their tent to find a large tree less than a foot from them. They heard the branches brush by.

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