Glaciating Altocumulus Clouds

November 09, 2005

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Provided and copyright by: Harold Sherrod, Jr.
Summary authors & editors: Harold Sherrod, Jr.

The photo above showing dual rows of mid altitude glaciating altocumulus clouds was taken over Las Vegas on September 15, 2005. Their jellyfish like appearance is partially due to virga (precipitation which evaporates before reaching the surface). In this case, it's not rain drops but rather tiny ice crystals falling from the clouds. According to David Lynch, coauthor of "Color and Light in Nature," these glaciating altocumulus clouds are composed of supercooled water droplets, and when they freeze, the resulting ice crystals grow very rapidly because the vapor pressure of water vapor over ice is small compared to water vapor over water. As a result the crystals become quite large, and fall rapidly compared to the tiny water droplets in the cloud.

The scene is made more interestng for 2 additional reasons. One, the Sun had already set but was still illuminating the clouds' under-surfaces. Two, though the general movement of the clouds was from south to north, the wind direction at the altitude of the virga was easterly (from east to west).

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