The second in an occasional series on meteorological terminology: the rare and luminous clouds that form in an upper layer of Earth’s atmosphere
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.