Thursday, July 20, 2023

Blowdown in SW Montana - aftermath of 2019 Microburst

On the afternoon of Sunday, August 11th 2019, a microburst occurred in the Tobacco Root Mountains of southwestern Montana, blowing down the estimated 200 to 250 acres of trees shown in the photo above. This blowdown resulted in the blockage of trails leading back to Granite Lake stranding 4 people in the backcountry due to trees blocking trails leaving the area. Personnel from the Madison County Search and Rescue, National Forest Service fire team, and two military helicopters were dispatched to the area to rescue those stranded in the backcountry.

The sky is falling!
So, why do microbursts happen? Like tornadoes, lightning, hail, and flash floods, microbursts are associated with severe thunderstorms. They’re caused by the same cooling effect that happens when sweat evaporates from your skin. As sweat evaporates, it cools your body because the water molecules absorb (and remove) heat when they change from liquid to vapor. The same thing can happen in a thunderstorm. As rain or hail falls through very dry air, or the thunderstorm draws in dry air, much (or all) of the precipitation may evaporate (called sublimation when hail changes to vapor). Water molecules absorb heat from the surrounding air to make this phase change, making the air much colder. The more evaporation, the colder the air gets. Anyone who has opened a refrigerator door knows that colder air is heavier than warmer air. As this air (cooled by the evaporation of rain, or sublimation of hail) 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 – strong enough to blow fully grown pine trees over. Interestingly, the opposite phase change (vapor to snow) releases heat, contributing to the warming effect that causes Chinook winds.

Watch this 1-minute video of a microburst in Tucson, Arizona.

Mr. Tornado.
Microbursts were discovered in the 1970s by tornado scientist Ted Fujita who developed the famous Fujita scale for rating tornado intensity. No doubt, you’ve heard of an F5 tornado – the ‘F’ stands for Fujita. The scale was revised several years ago – now called the ‘Enhanced Fujita Scale’ (EF0 – EF5). Fujita proposed that microbursts were for real, and suggested they were responsible for a number of mysterious aircraft crashes that had happened in the past during takeoff or landing beneath thunderstorms. Confirmation of his hypothesis ultimately led to a reduction in aircraft accidents and saving of lives.

Term: sublimation

For more about the area shown in the photo, go to