noctilucent clouds

siamak sabet captured this lovely shot of the allusive noctilucent ("night-shining") clouds, over Mt. Sabalan... an extinct volcano in iran.


these clouds exist very high in the atmosphere, nearly 90 km above earth's surface, above almost all of our atmosphere! they are not seen very often, and their existence remains a bit of a mystery. notice the distinct blue color of the clouds. the Aeronomy of Ice in the Mesosphere (AIM) satellite mission has confirmed that the particles making up the clouds are tens of nanometers in size... just the right size to scatter blue sunlight, making it uniquely visible to our eyes. but the mission also found even smaller ice crystals, which makes no sense at such an altitude since it's so dry! why and how are there clouds so high up?

to read more, here's an interesting article (and more pictures) from NASA called strange clouds at the edge of space.