noctilucent clouds (NLCs)
Image credit: Pekka Parviainen (NCWG/U. Colorado).
Noctilucent clouds (NLCs) are 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 kilometers. 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.