noctilucent clouds (NLCs)
Complex interwoven streaks or knots of cloud, generally white or pearly-blue
in color, sometimes with a golden lower edge, best seen from latitudes 50
to 60° during the deep twilight of summer with the Sun 6 to 16°
below the horizon. Although not completely understood, they are believed
to result from water vapor condensing on small particles (possibly volcanic
or meteoritic debris) at an altitude of about 82 km. Although superficially
resembling cirrus clouds, NLCs form close to the mesopause
– far above the tropopause, to which ordinary clouds are confined
– and are distinguishable by their delicate herring-bone structure.
| Image credit: Pekka Parviainen (NCWG/U.
PHENOMENA AND STRUCTURES