Mammatus clouds are strange, pouchy formations often occur in association with a thunderstorm. In mammatus 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 looking like upside-down bubbles.
Lenticular clouds are usually created by gravity waves. Some kind of topography, like a mountain, that gets in the way of air flow. As the air comes down the side of the mountain, it tends to overshoot and then springs back up. It oscillates like this for a while, and on the upward part of the waves, clouds form as rising air cools.
These clouds that look like a row of crashing waves are known as Kelvin-Helmholz waves. They form when two layers of air or liquid of different densities move past each other at different speeds, creating shearing at the boundary.
Pileus clouds are caused by strong, relatively fast upward motion. Situations where this occurs include quickly growing thunderheads, volcanic eruptions and even nuclear explosions. In each case, something pushes warm, moist air upward quickly.
Noctilucent or “night shining” clouds are formed by ice at the boundary of Earth’s atmosphere and space, 50 miles high. They shine because they are so high they remain lit by the sun even after it has gone below the horizon.
Roll clouds typically occur in the lower atmosphere ahead of a storm front. Warm updrafts in the storm front push cold air up, which then flows down along the sides of the updraft. The cold downdraft then bounces back up a bit setting up a wave-like structure in front of the storm. On the upswing, the cold air forms a cloud. Evaporation of the cloud causes a downdraft on the edges that erodes the cloud, forming a roll. If the wave continues, a series of roll clouds can form, called a street.
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