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A Pyroclastic Cloud is a term borrowed from volcanoes. It is also used to refer to a restroom occurence, most applicable for men. This would be when you are in need of performing a bowel movement, and it is urgent for one reason or another (this usually happens on a hot day), and you rush to the restroom, and to save time you pull down your shorts while you are bending to sit down, your nose travells directly through the plume of groinal sweat odors released from your underwear.
Bill: Hey Jim, I just ran to pinch of a load and got stuck in the pyroclastic cloud.

Jim: Oh man, thats sick, slow down next time, let it disperse.
by Shake Zula May 23, 2005