Noctilucent clouds. Space shuttle image of the limb of the Earth, showing noctilucent clouds (polar mesospheric clouds, across top) in the upper atmosphere. These are transient clouds, usually seen in the summer months at high latitudes. Even when twilight falls, they stay bright because they are still in sunlight at their great height (75 to 85 kilometres). This image was obtained as Space Shuttle Atlantis, on mission STS-117, was passing over north-central Asia, on 10th June 2007.