Noctilucent clouds at sunset over trees. Noctilucent clouds 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 usually only at high and low latitudes. Photographed at 22:22 UTC on 9 July 2010, from northern Hampshire, UK.