Stargazers in parts of Europe were stunned by an unusual sight on Monday evening: a large, glowing blue spiral in the sky.

High altitude cirrus clouds on a fine day in the Adirondacks ... But some cycles are visible — at least in part. On a warm, blue-sky day, I found myself gazing up at condensing water in the sky in the ...

Noctilucent, or "night-shining", clouds can be seen on Mars, just as they are on Earth. Nasa's Curiosity Mars rover imaged these high-altitude clouds ... sight in the sky above the UK.