Noctilucent Clouds photographed by NASA.
In November 2013 you recall Comet ISON passed close to the Sun. Scientists hoped it would remain intact and provide a spectacular comet tail visible to us on Earth. Unfortunately it broke up and was never visible to the naked eye.
In mid January Earth passed through a strip of very fine debris from Comet ISON. These particles were falling at 125,000mph towards the Earth. Because they were small they were stopped by the upper atmosphere and with time they will gently drift down. It might take months before the dust settles down on earth. A manifestation of this invisible rain of comet dust could be an occurrence of noctilucent clouds.
Noctilucent clouds are very fine, wispy formations made out of ice crystals that occur high up in the mesosphere (50-53 miles) and are the highest clouds in the earth’s atmosphere. Noctilucent clouds form mostly near the Polar Regions, where the mesosphere is coldest. A recent study from NASA suggests that these clouds are seeded by space dust. Tiny dust acts as condensation nuclei for ice crystals to form.