SpaceWeather.com has a growing photo gallery of noctilucent clouds (NLCs), mysterious clouds that occur at very high altitudes, near the edge of space. Some scientists suggest that they're caused by space dust, others pin them on rocket exhaust or global warming. Noctilucent, meaning "night-shining," clouds are wispy, tendrilous, and often glow acid blue. (Image seen here captured by Paul Evans on Saturday in Northern Ireleand.)
Link to 2006 Noctilucent Cloud Gallery, Link to 2003 NASA article with more background (Thanks, Paul Saffo!)
