Noctilucent clouds (NLCs, from the Latin noctis lucens, "night-shining") are the highest clouds in Earth's atmosphere, forming at altitudes of 80-85 km in the mesosphere, far above conventional weather clouds (0-12 km). They are composed of tiny ice crystals barely 100 nanometres in diameter.
Observation and appearance
They are only visible during deep twilight, when the Sun is between 6° and 16° below the horizon: at that time, the lower layers are in shadow but the mesosphere is still illuminated by grazing solar rays. They appear as tenuous veils of electric blue or silvery white with a wavy structure, generally toward the north (in the Northern Hemisphere). Best observed between May and August at latitudes 50°-70° N.
Their existence was first documented in 1885, two years after the Krakatoa eruption. Since then, their frequency and extent toward lower latitudes has increased, which some scientists link to climate change: increased methane in the atmosphere generates more water vapour in the mesosphere, while paradoxically the mesosphere cools as the troposphere warms.
In Spain they are extremely rare, only occasionally sighted from northern Iberia during exceptional summers. They are a purely visual phenomenon with no direct meteorological impact, but they serve as a valuable indicator of the state of the upper atmosphere. See also: cirrus.