Occasionally people remark that “it’s too cold to snow.” Is this ever really the case? Also, while walking outdoors on cold, cloudy days I’ve noticed the air seems to warm slightly when it begins to rain or snow. Is this my imagination or does precipitation cause the temperature to rise? –M.H., Arlington, Texas
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As with many bits of folk wisdom, the idea that it can be too cold to snow is an unholy mix of fact and fantasy. It’s true that the colder air gets, the less water vapor it can carry. At 5 degrees Fahrenheit air can hold less than half as much vapor as it can at 23 degrees. The less water vapor you have, naturally, the less snow you’re going to get. Eventually things can get to the point that air can carry virtually no water at all, and not only will it not snow, you’ll barely get clouds. At minus 25 degrees Fahrenheit or below the closest to clouds or snow you’ll see is generally something called “diamond dust,” familiar to many mountain skiers. This consists of tiny ice crystals that twinkle in the sunlight.
There is, however, at least one circumstance when the colder a northerly wind is, the more snow you’ll get. That’s what happens during a “lake effect” storm, such as the one that struck the Chicago area in late February. A cold north wind traveling over Lake Michigan picked up so much warmth and moisture from the water that it was able to dump as much as 18 inches of snow on parts of Chicago and northwest Indiana. Meanwhile, communities 35 miles to the west enjoyed a sunny day. Had the north wind been colder still, it would have been able to work up even more of a running jump, so to speak, picking up more moisture, forming deeper clouds, and producing more snow.
