This is an awesome picture. Supercell Thunderstorm by Sean Heavey, nationalgeographic: A supercell thunderstorm rolls across the Montana prairie at sunset. This was a winning shot in the 2010 National Geographic Nature Gallery Photo Contest.
Virga clouds look a bit like jellyfish floating in the sky. The "tentacles", which you can see hanging down, are called "virga". They are essentially rain, or snow, that evaporates away before ever reaching the ground. Virga common over Arizona