TEMP 19.1°C Ι DEW POINT 17.4°C Ι FEELS LIKE 20.0°C Ι HUMIDITY 90% Ι PRESSURE 998.7mb Ι WIND 5.4mph 180° S
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Published on: February 4, 2012, by: STW Editor

How does it snow?

Most precipitation that falls to the earth actually starts off as snow. The process of water forming into snow begins high in the earth’s atmosphere.

I have often heard the term “too cold for snow“.  This is in fact a myth, how else would the poles have so much of the white stuff?  As long as there is some source of moisture and some way to lift or cool the air you can get snow. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground, typically -9°C (15°F) or warmer, since air can hold more water vapor at warmer temperatures.  However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake.  As a general rule though, snow will not form if the ground temperature is above 5°C.

Snow forecasts are better than they used to be and they continue to improve, but snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make night and day differences in snow forecasts.

This is part of the fun and frustration that makes snow forecasting so interesting.

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