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Fri Dec 14, 2012, 03:16 PM

Microbursts and asteroids, oh my!

Phil Plait has a great article on his new blog location at Slate.com: Montana microburst flattens trees like toothpicks:

Over a century ago, on June 30, 1908, an unearthly visitor came to our planet. A chunk of rock (or possibly ice) about 30 meters across—the size of a house—barreled in at a speed probably 50 times that of a rifle bullet. Ramming through the Earth's atmosphere, incredible forces compressed it, crumbled it, and when it reached a height of just a few kilometers above the ground, those forces won. In a matter of just a few seconds the energy of its immense speed was converted into heat, and it exploded.

This was the famed Tunguska event. The fireball created a huge forest fire over hundreds of square kilometers of the Podkamennaya Tunguska River region of the Siberian forest ... but then the immense shock wave from the blast touched down. It blew the fire out and swept down those trees like a rolling pin, knocking down untold millions of them. It took 20 years to mount an expedition to the remote location, and I've often wondered exactly what the explorers saw when they got there.

Phil includes a couple of pictures of the destruction. I'm reluctant to include them in this post, due to the fact that Phil had permission from Rod Benson (the photographer) to use them. I don't; fair use or not, I'll respect the rights of the photographer.

Those aren't toothpicks, those are trees, my friends, lodgepole pines, knocked down like so many twigs in 1999. This picture was taken by Rod Benson, and it shows trees in Montana felled by a microburst, a rare weather phenomenon that can create powerful winds, characterized by a huge and sudden downward rush of air from a thundercloud. The air accelerates down, then slams into the ground and spreads out at high speed, easily hitting 150 kph.


And as huge and devastating as that is, it's peanuts compared to what happened in Tunguska in 1908. The explosion of the interplanetary debris was equivalent to about a 15- to 20-megaton nuclear weapon—15 to 20 million tons of TNT detonating. The explosive yield of the blast was estimated in part by the size of the area with trees blown down. Incredibly, studying weather phenomena like microbursts helps us learn about even more extreme phenomena like asteroid impacts.

There's a fantastic image of the Tungaska explosion by space artist Don Dixon. Once again, the image was 'used with permission,' and once again you'll have to go to Phil's blog to see it. It's worth the trip.

I would also recommend checking out Phil's earlier post: 100 years ago today: KABLAM!!!!!. Phil has some great comments on the continuing danger from asteroids:

We’ve come a long way since that hot, muggy Russian morning on June 30, 1908. And let’s be clear: if another Tunguska-class object had its sights set on us, we wouldn’t know it until we, like those Russians, saw a flash of terrible light in our sky, felt the burning heat, and were knocked down by the blast.

The odds of it happening any time soon are low, very low. I don’t lose any sleep over it — I’m not worried, I’m concerned. But this anniversary is a sobering reminder that it can happen again, and it will, unless we do something about it.

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