Noctilucent clouds (polar mesospheric clouds)

(Section 2.6.3)

Definition

Clouds resembling wispy Cirrus, but usually with a bluish or silvery colour; they stand out against the dark night sky well past summer sunset because they are in the mesosphere.

Physical constitution

Noctilucent clouds are composed of very small water ice crystals formed on minute particles of dust, possibly of cosmic origin from micrometeors. The average diameter of noctilucent clouds particles is thought to be about 0.3 μm. Noctilucent clouds form only during the summer when temperatures in the mesosphere are coldest. Temperatures lower than about −120 °C are required for formation of noctilucent clouds.