Clear summer nights offer the opportunity to view rare noctilucent clouds. Visible in northern skies from late May to August, these are thin, streaky clouds with large billow or wave patterns. The highest clouds in the atmosphere, they form approximately 50 miles up. It is their height which allows them to be visible after the sun has set. Thought to be made of ice crystals, they are usually bluish or silvery but may also be orange or reddish in colour. Noctilucent clouds can be seen from approximately an hour after sunset. As a rule they move from east to west, which is the opposite way to typical weather clouds. Peak brightness is reached quickly then fades until the point where the sun has passed its lowest point below the horizon. Pre-sunrise displays then brighten, and may be even more striking than evening appearances.