Noctilucent clouds are supposed to be electric blue.
Yet during the extreme NLC outbreak of June 21st, some observers in Europe saw a different color: red.
“It happened around midnight,” reports Marek Nikodem of Szubin, Poland. “The top of the clouds developed a reddish-purple canopy.”
Others saw them, too, including over Piwnice, Poland:
And over Deventer, The Netherlands:
What creates this color?
The answer is lack of ozone.
Much of the sunlight hitting noctilucent clouds first passes through Earth’s ozone layer. Ozone absorbs red light, while allowing blue to pass – hence the usual color of the clouds. When the sun is hanging very low, however, reddened sunlight refracted through the dense troposphere can paint the tops of NLCs red, overwhelming the usual “ozone blue.”
But ozone isn’t the only reason noctilucent clouds are blue. The clouds consist of tiny ice crystals about the size of particles in cigarette smoke. Cigarette smoke looks blue, too, and for the same reason. Tiny particles scatter blue light better than other colors.