The majority of our weather occurs in the bottom 4% or so of Earth’s atmosphere, in the troposphere, which extends around 10 miles (16 km) up from the surface. One type of cloud, however, is far more remote and exists in the upper parts of the mesosphere, a layer of our atmosphere reaching over 50 miles (80 km) into space. These clouds get their name (noctilucent, i.e. luminous at night) from the time at which they are visible and are usually seen a couple of hours before sunrise or after sunset, when their higher altitude allows the sun to reflect off them brightly, while lower clouds and the sky turn darker.
Noctilucent clouds are visible in both the northern and southern hemispheres from mid-latitude regions (50-60 degrees north or south from the equator) and only during the summer months. In the northern hemisphere, observation of these rare clouds is usually confined to between late May and August when, counterintuitively, the temperatures in the mesosphere are at their lowest and cold enough (lower than -120C) for water vapour to transition into very small ice crystals. Despite the very thin and pure atmosphere at these heights, the crystals are able to form on small impurities, thought to be either from volcanic ash particles or extra-terrestrial meteoric dust.
