Photographs of ordinary clouds are a dime a dozen, but a spectacle seldom witnessed is the appearance of noctilucent clouds. While everyday clouds form low in the troposphere as water vapour condenses on dust, noctilucent clouds, as their name suggests, form astonishingly high in the mesosphere, where ice gathers on minute particles at the very edge of space. However, this is where the similarity to ordinary clouds ends. These night-shining clouds, known to scientists as polar mesospheric clouds, are a much rarer phenomenon, appearing only at particular times of the year and limited by geographic latitude.

A distinct combination of astronomical and meteorological conditions is required in order for noctilucent clouds to appear. For one, the observer must be in deep twilight, with the sun sitting between six and sixteen degrees below the horizon, so that its rays still light the lofty clouds while the ground below lies in darkness. Moreover, the mesosphere must be extraordinarily cold, colder than any air near the surface. Finally, there must be tiny ice crystals present, which gather on specks of dust and the debris of burnt-up meteors. Under these precise conditions, a viewer facing the twilight glow may observe the pale threads of a noctilucent display.