One of the most awe-inspiring aspects of the storm was the appearance of mammatus clouds. These pouch-like formations hung ominously from the sky, resembling a series of upside-down bubbles.

Formed by cold air sinking down, mammatus clouds are usually seen when there is turbulence in a cumulonimbus cloud. Two astronauts are returning home after nine months in space. Nick Beake of the ...

they do signify unstable air within the storm. Unlike most clouds, which are formed by rising air, mammatus clouds are formed from sinking air inside the cloud. A scud cloud, or fractus cloud ...