noctilucent clouds, also known as night shinning clouds, can be seen when illuminated by the setting sun as it drops bellow the horizon. found high in the mesosphere, near the edge of space, the clouds consist of tiny ice crystals about the size of particles in cigarette smoke. science, however, has yet to understand how these ice crystals are able to form in such an arid area of the atmosphere.

ice crystals in clouds need two things to grow: water molecules, and dust to which those molecules can stick. this process, known as nucleation, happens all the time in ordinary clouds. but it’s hard to waft wind blown dust all the way up to the mesosphere, leaving some to hypothesize this dust must come from the debris left by comets and asteroids. it’s also hard for water molecules to get that high, which is why noctilucent clouds typically only appear in the summer.

where they were once confined to latitudes above fifty degrees, thanks to global warming – which actually serves to lower temperatures in the upper atmosphere, and thus facilitate ice crystal formation – they can now be found as far south as colorado and utah.