Starwatch: time to look for noctilucent clouds at the edge of space

The late spring/early summer is a good time to look for noctilucent clouds. These are the highest known clouds that form in Earth’s atmosphere. They exist at a height of about 80km; to all intents and purposes this is the edge of space. The clouds glow as white or pale blue tendrils across a dark sky. This allows you to distinguish them from cirrus clouds, which are visible only in daytime because they do not glow.

The best time to see noctilucent clouds is about 30 minutes after sunset. Look to the west. The clouds themselves are formed of water ice crystals up to about 100 nanometres in diameter, although scientists still don’t fully understand the way in which the water gets that high in the atmosphere. Adding to the mystery is that no one appears to have reported seeing noctilucent clouds before 1885. Although most displays appear to be natural, rocket launches can also cause artificial displays of noctilucent clouds.