Photo of the Week, Vol. 2

Not mine, but by Robert Sumner of the Joliet Herald-News. Caption as follows:

The sky over Will County turned eerie Tuesday afternoon as sagging, pouch-like clouds that looked like cows' udders or upside-down soup bowls moved in. They are known as mammatus clouds, and they are fairly rare in this area, said Casey Sullivan, a meteorologist with the National Weather Service in Romeoville. "They're the only clouds that form from descending motion in the atmosphere," Sullivan said. Mammatus clouds are typically found on the undersides of very large thunderstorms known as supercells. A supercell moved southeast across Illinois on Tuesday, though the hail, high winds and heavy rains passed to the west and south of Will County.