Make a snowflake

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The curious growth of a snow crystal
Temperature and humidity affects the shape of snowflake crystals. The temperature of formation determines the original crystal shape. Large (“dendritic”) flakes grow best between -10° and -12° C. Plates grow at warmer or colder conditions. Humidity — water vapor pressure in the cloud — affects the growth rate due to deposition. A flake that passes through dry air may sublimate (lose mass by converting from solid to gas). Because temperature and humidity change as a flake bounces around in a cloud, the basic shapes can blend into countless crystal shapes. An ice crystal may also collide with another crystal and aggregate into a snowflake of a different shape. Or crystals may grow as they accrete (collide and adhere) tiny drops of liquid water in the cloud.
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The crystal shape of a snowflake depends on several parameters, including temperature. In this applet, you can explore how snowflake crystal shapes depend on temperature — click repeatedly in one of the temperature zones (between the lines). Then move your cursor to a different temperature zone and click some more. You may combine up to 3 different shapes before resetting (by clicking below the cloud).

This applet is Copyright © 2008, 2011 and 2013 by Tom Whittaker. Snowflake and cloud images by S.V. Medaris.
Physics adapted from K.C. Young, Microphysical Processes In Clouds Oxford Univ Press (1993), and from Bob Crowder, The Wonders of the Weather Australian Govt Publishing Service (1995).