The highest known clouds in Earth’s atmosphere can be seen soon after sunset in late spring and early summer
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.