These clouds are so high in the atmosphere that they are only visible at dusk.    These elusive clouds were thought to have been caused by methane reaching the 50 mile high altitude, but after careful review of sightings it was determined these happen within a few days of a shuttle launch.   http://www.sciencenews.org/view/generic/id/45954/title/Comet_likely_culprit_in_Tunguska_blast