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Ice-Crystal Mechanisms
the formation of freezing rain
Freezing rain can
develop either through ice crystal processes
or supercooled warm-rain processes.
Ice crystals high in the atmosphere
grow by collecting water vapor molecules,
which are sometimes supplied by microscopic evaporating cloud
droplets. In the figure below, the blue line represents the
temperature of the atmosphere and the black line represents the 0C (32F)
isotherm (a line of equal temperature).
When the blue line is to the right of the black line, the atmosphere is
warmer than 0C and when the blue line is to the left, the atmosphere is
colder than 0C.
As the snow falls, it encounters a layer of warm air
where snow and ice particles completely melt and collapse
into raindrops.
As the raindrops approach the ground, they encounter a layer of cold air
and cool to temperatures below 0C. However,
since the cold layer is so shallow, the drops themselves do not freeze, a
phenomena called supercooling (or forming
"supercooled raindrops"). The supercooled raindrops are raindrops that are
colder than 0C and freeze on contact when they strike the ground.
Freezing Rain
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warm-rain
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