Put simply, pictures of clouds. While terrestrial objects (the ground, trees, other miscellany) are allowed, the majority of the image should be occupied by clouds.
You can also view the photos that didn't make the cut for this contest – the B-Sides.
Contest links on Fark.com: All Entries | Voting Results Click on any photo to view larger size.
Shafts of sunlight over Molokai
Sunset after an Arizona thunderstorm
Painted clouds in Oklahoma