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Too Cold To Snow
Mon Feb 16, 2004
Listen in RealAudio 
Hi, I'm Bryan Yeaton and this is The Weather Notebook. Many of us have heard the phrase: "It's
too cold to snow." Is that true?
At first inspection, that seems to agree with experience. The coldest days and nights are
clear and dry, right?
Well generally, that's true - large, frigid arctic air masses are very dry. But, it's not the
cold that keeps the snow from falling, but the general lack of ascending air to start the
precipitation process.
Even at very cold temperatures, some water vapor is present in the air. But the colder the
air, the less vapor it contains. For example, if we saturate parcels of air, the maximum water
vapor content drops by about half for each twenty-Fahrenheit degree decrease in air
temperature, say from 20 degrees to zero degrees.
Since the amount of snow that potentially can be wrung out of the air is dependent on the
vapor content at saturation temperature, the potential snowfall amounts also drop with air
temperature.
As a result, snowfalls under extreme cold conditions are generally light and accumulations
small. Barrow, Alaska, for example, totals only eight inches during its four deep winter
months while six inches falls in just September, when the temperatures are warmer. Antarctica
is actually a polar desert-one of the driest deserts on earth.
Ironically, at temperatures of 40 below zero and colder, snow can actually fall out of the
clear blue sky, without any clouds. At such temperatures, water vapor in the air spontaneously
forms ice crystals, which slowly settle to the earth. When these falling crystals are caught
in the light, they sparkle like gemstones, a weather condition known as "diamond
dust."
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is
brought to you through the good graces of The National Science Foundation, and Subaru of
America.
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