How large is ice cloud formation in the stratosphere and why doesn't this ice fall? I had a student ask me, and I wanted to find the answer for him. Thanks!

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Cara Nelson

MIKE MOSS SAYS:      Cara,     If you are really asking about stratospheric ice clouds, those are called nacreaous clouds and are very rarely seen outside of polar latitudes (another name for them is "polar stratospheric clouds"). They occur at altitudes of around 50,000 to 80,000 feet above ground and, depending on the type of cloud (some are composed of water ice, while others are composed of a solution of water, nitric and sulfuric acids) may range from around 1 micron to around 10 microns in diameter. At these very small sizes, they fall at a very low speed, so that seen from a distance they appear to be suspended in place.

This is also true to some extent for the more common cirrus clouds in the upper troposphere to near the bottom of the stratosphere. They are principally composed of ice crystals at altitudes from around 20,000 to 30,000 feet. These ice particles may range in size from around 5 to over 50 microns in diameter, and occasionally will grow large enough to comprise snow crystals and snow flakes. In all cases (true of any cloud), the particles actually do fall through the air, but the speed of their descent may be partially or completely offset by rising motion of the air that they are formed in. In some cases, you will see cirrus clouds with a wispy arc-like appearance. The arc shape usually means the crystals in the cloud are falling through layers of air having different wind speeds.Other times, you may notice high clumps of cirrocumulus clouds that have a "bearded" look at the bottom. Again, this represents some of the crystals growing large enough to have a fall velocity greater than the remainder of the cloud, and greater than the rate of ascent of the air around them. Usually, these crystals fall into drier air below the cloud and evaporate (also alled sublimation, since the water molecules go directly from an ice to vapor state) which is the reason for the bearded or streaky appearance, with the "fall streaks" or "virga trails" ending in mid-air a few hundred to a few thousand feet below the cloud bases.

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