Glaciating Altocumulus Clouds Over Port Orchard, Washington

November 24, 2011

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Glaciating altocumulusPhotographer: Jonica Smith
Summary Author: Jonica Smith; Jim Foster

The photo above featuring a sky-scape of odd-looking parachute-like clouds was taken over Port Orchard, Washington on July 23, 2011. These are glaciating altocumulus clouds that formed shortly before sunset. Their unusual appearance is caused by virga -- precipitation that evaporates before reaching the surface. However, even though it was midsummer, ice crystals not water drops are falling from these clouds. When the supercooled water droplets that compose altocumulus clouds freeze, the ice crystals that result can grow very quickly due to the comparatively low water vapor pressure over the crystals compared to over water droplets having the same temperature. Note that the brighter clouds are closer to the horizon and still illuminated by the Sun.