Noctilucent cloud
A cloud of ice crystals in Earth’s mesosphere, roughly 80 kilometers above the surface, that remains visible after sunset by reflecting sunlight.
Mesopause: The cold boundary between the mesosphere and thermosphere, where atmospheric temperatures reach their minimum. Its exceptionally low temperatures allow water vapor to freeze into the crystals that form these clouds.
Otto Jesse: A German meteorologist who first systematically observed and named noctilucent clouds in the late nineteenth century. His observations established noctilucent clouds as a distinct atmospheric phenomenon.
Atmospheric remote sensing: The study of atmospheric properties using measurements made remotely, including from satellites and ground instruments. Noctilucent clouds offer a visible feature for probing otherwise difficult-to-measure upper-atmosphere conditions.
Polar mesospheric clouds: High-latitude ice clouds in the mesosphere, including the noctilucent clouds observed from the ground. These are the polar manifestations most often associated with noctilucent displays.
Cirrus cloud: A thin, wispy cloud composed mainly of ice crystals in the upper troposphere. Unlike cirrus, noctilucent clouds lie far higher and can remain sunlit after the lower atmosphere darkens.
Ice nucleation: The initial formation of ice crystals around particles or through freezing of supercooled water. Noctilucent clouds require ice crystals to form despite the mesosphere’s extremely low water content.
Berlin: The capital city of Germany, situated in northeastern Germany. Jesse’s early observations from Berlin followed the 1885 Krakatoa eruption.
AIM (spacecraft): NASA’s Aeronomy of Ice in the Mesosphere spacecraft, launched in 2007 to study polar mesospheric clouds. AIM measured cloud occurrence, structure, and environment from orbit.
Northern Hemisphere: The half of Earth north of the equator. Northern summer brings frequent noctilucent-cloud displays at high latitudes.
Polar stratospheric cloud: A cloud that forms in the cold polar stratosphere, often enabling ozone-depleting chemical reactions. These clouds occupy the stratosphere, well below the mesosphere where noctilucent clouds form.