Who hasn't spent an afternoon dreaming dragons and castles in the clouds? Watched a thunderstorm build from nothing into towering anvils and bizarre colors? Some of the most beautiful sights on earth begin with clouds sailing over the moon or sun. Simple water vapor and ice crystal collections do some spectacular things.
But if all you've seen are the old standbys of cumulus, cirrus, and stratus, you've missed out on some truly incredible sights. Let's take a walk through the skies and observe some of the rarest clouds around.
Noctilucent (night shining) clouds form so high in the atmosphere - over fifty miles in some cases - that scientists still don't completely understand them. They were first observed after Krakotoa's eruption in 1885, and there's some talk that their increased prevalence could be a harbinger of global warming. Their ice crystals are so tiny they don't scatter light efficiently, and so they're only visible when the sun is below the horizon.
Click the picture for a great NASA story on them.
Mammatus clouds, though not an everyday sight, aren't quite so rare. They're opportunists, forming under a wide variety of cloud types - not to mention jet plane contrails and volcanic ash clouds. They're another poorly understood lot. Wikipedia lists no less than ten proposed mechanisms for their formation.
Lenticular clouds have personally freaked me out before. Living near a mountain, you have a good chance of seeing these every once in a great while, and it's bizarre. They don't look like they could have possibly formed from natural causes. They form on the downward side of warm, moist air flowing over mountains and creating standing waves.
Kelvin-Helmholtz clouds look like nature went a little crazy with the scroll art. They form when two different layers of air moving at different speeds make wave structures. These look like stylized ocean waves because it's pretty much the same mechanism that forms both: Kelvin-Helmholtz instability. Yes, air does indeed behave like a fluid.
Nacreous clouds form 9-16 miles high, and put on a spectacular show, lit by the invisible sun after sunset or before dawn. Gorgeous, yes, but also associated with ozone depletion: they support the chemical reactions that allow ozone holes to form. Bad, bad, beautiful clouds!
And, finally... regular old water vapor clouds. These look totally ordinary, don't they? And so they are, except for one thing: they're from Mars:
As northern summer ends on Mars, water vapor from the north pole comes down to lower latitudes making clouds, frost and even fog possible. That is what we are starting to see at the Mars Phoenix landing site.
Isn't that absolutely awesome? The ordinary is extraordinary again. Click the image to watch the Martian clouds go by on a late summer afternoon.