Nacreous cloud season is underway. The temperature of the polar
stratosphere has dropped below -85 C, the threshold for formation
of tiny ice crystals that diffract sunlight to produce flamboyant
"mother-of-pearl" (nacreous) colors. Icy nacreous clouds
are best seen from cold places such as Scandinavia, Iceland and
Alaska during the twilight hours after sunset or before sunrise.
See also January 2008.