Falling clouds
 Q:
I grew up in Pennsylvania when it snowed a lot. I've always wondered how there
could be clouds when it was below freezing. Should they not just freeze and fall
out of the sky? I would hate to go to my grave with this great wonder hanging
over me. Someone, World
Cirrus clouds with 'fall streaks'. Photo courtesy of Piccolo Namek,
Wikipedia
A: Actually, the temperature of many clouds is below freezing, even when
it's not freezing on the ground. So, clouds are likely composed of liquid water
droplets that are extremely cold, below freezing. Even though the droplets do
indeed fall, they fall at a slow speed that updrafts easily overcome. Thus,
they don't fall out of the sky, because updrafts lift them for the short
lifetime (about a half hour) of a cloud.
Moreover, "if a cloud droplet freezes, its weight remains exactly the same,"
says physicist and meteorologist
Craig
Bohren, distinguished professor emeritus at Pennsylvania State University.
Some clouds, like cirrus clouds, are made mostly of ice particles, and these
particles usually are small enough for updrafts to keep them from falling. The
ice crystals range from about a hundredth of a millimeter (white-blood cell
size) to a millimeter (poppy-seed size).
Ice crystals "can grow, though, and become large enough to fall at a velocity
greater than typical updraft velocities," says Bohren. Then, they fall out of
the cloud. Winds aloft may blow the falling ice, forming 'fall streaks'
trailing from a cirrus cloud. See figure.
Further Reading:
Cirrus clouds, University of Illinois, the weather world 2010 project
Jet trails and
cirrus clouds by Dennis Klocek, DocWeather.com
(Answered Oct. 16, 2006; updated Nov. 2, 2007)
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