Noctilucent clouds, seen from the International Space Station (ISS). These clouds (white streaks across centre) are very thin and form high above the Earth at heights of 75-90 kilometres. Because they are so thin and high, they can only be seen at twilight, shining in the glow of the setting Sun, and only at high and low latitudes. The lower three layers of the atmosphere (from bottom) are the troposphere (orange, 15 kilometres thick), and the stratosphere and mesosphere (both pale blue & 35 kilometres thick). Above these, the atmosphere fades into the darkness of space. A crescent Moon is also seen. The ISS orbits some 380 kilometres high. Photographed on 27 July 2003, over Mongolia.

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