Polar mesospheric clouds. International Space Station (ISS) image of the limb of the Earth, showing polar mesospheric clouds (noctilucent 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 just before dawn, as the ISS was passing over western Mongolia. Photographed on 22 July 2008 by a member of the ISS's Expedition 17 crew, as part of research for the International Polar Year.