The clear, bright twilight of the northern summer night offers to a keen observer a number of beautiful natural phenomena. Occasionally, about an hour after sunset, a delicate veil of clear bluish-white colour appears over the northern horizon, so tenuous that only a trained observer would be aware of its presence. Sometimes, however, this so-called noctilucent cloud exhibits much brighter billows, bands and other features reminiscent of common cirrus cloud formations. Noctilucent clouds consist of a layer of particulate matter, concentrated by some mechanism in the vicinity of the mesopause (the temperature minimum) at an altitude of 80 to 85 km. Because of their great height they are illuminated by the sun's rays when lower layers of the atmosphere at normal cloud heights lie in the earth's shadow; they can be seen when there is sufficient contrast between the light scattered by the cloud particles and the light scattered by the air molecules in the line of sight. These conditions are fulfilled when the sun is between about 4° and 12° below the horizon.