In 2009, a previously unclassified type of cloud became official, the first since 1951: Undulatus Asperatus. It looks really cool:
The Cloud Appreciation Society has a whole bunch of pictures. The only explanation I've found so far is they're caused by "Turbulent motions between differing air masses."
Hole-Punch Clouds
From Wired: "As propeller or jet airplanes pass through the right atmospheric conditions, they make liquid water droplets freeze and immediately drop as snow, leaving a circular fissure behind... Normally, snowflakes or raindrops need a speck of dust or another imperfection to form around. But supercooled water can freeze instantly, if it cools quickly to around minus 35 degrees Fahrenheit."
Kelvin-Helmholz Waves
These are some of the coolest clouds I've ever seen. They're formed when two layers of air of different densities move past each other at different speeds, creating shearing at the boundary.Parts of the boundary move up and parts move down. Because one layer is moving faster than the other, the shear causes the tops of the waves to move horizontally.
Cloud physicist Patrick Chuang's got some words: "They are some seriously weird looking clouds."
He goes on to say, "In mammutus clouds, evaporation causes pockets of negative buoyancy as it cools the air inside the cloud. This makes the clouds puff downward instead of up like cumulus clouds, and they end up being like upside-down bubbles."
Night-Shining (Noctilucent) Clouds
These ones are by far my favourites. Noctilucent clouds are ice crystals about 45 nanometres thick (that's 4.5 x 10^-8 metres) . For comparison, the diameter of a DNA helix is 2nm, the height of a hark disk head over the disk, 3nm. 40nm is the wavelength of extreme ultraviolet waves. These ice crystals form at around -145C without needing dust particles, 80 kilometres up -- that's way up near the end of the friggin' Mesophere, higher than any other clouds around. You get the most during the Summer, when the Mesophere is coldest (see why) .
Here's a diagram to show at what time of day you can see them:
The article where I found most of these shows a few other cool clouds, too. It's worth checking out.
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