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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

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.

So far so good. The problem is that when most people say it's too cold to snow, they're probably talking about it being 10 or 15 degrees out, not 25 below. It's perfectly possible for snow to fall when it's 10 or 15. Nonetheless the widespread impression, to some extent borne out by the facts, is that it usually snows when it's warmer.

The reason for this has to do with the cycle of alternating warm and cold fronts that regularly pass over much of the U.S. Most wintertime precipitation is brought by warm tropical air masses carrying moisture up from the Gulf of Mexico. Snow occurs when the tropical air smashes into a cold front heading down from the north. The cold air cools the warm air and wrings out its moisture, causing a snowstorm. Once the cold front passes through you generally get a few days of cold, dry northerly winds that bring no snow. The northerly winds are dry mainly because their moisture-carrying capacity is less, but also because they travel mostly over land or frozen ocean and thus have less opportunity to pick up moisture. In other words, snow isn't physically impossible with a cold north wind, it's just less likely.

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.

As for whether it gets warmer when rain or snow begins . . . well, Cecil has been consulting his friends in the meteorology biz, and so far as we can see, the answer is no. It's true that when rain or snow forms, some heat is given up to the atmosphere. But this happens at the point of formation, i.e., in the clouds, not ground level. The most memorable type of snowstorm, the classic fat-flake variety, occurs when the winds are calm and the temp just below freezing, so maybe it merely seems to have gotten warmer in retrospect, somehow. Then again, maybe snow reminds you of your childhood, a la Citizen Kane, causing you to feel all warm and cozy inside.

What is the official name for those little plastic thingies on the ends of shoelaces? --Katie Allison, Mesa, Arizona

Whatsamatter, crossword puzzle got you stumped? They're called "aglets." In the same vein, an Anglo-Saxon slave is an "esne" and an East Indian tree is a "dhak." Now you can go finish your crossword puzzle in ink. Maybe someday you'll get to the point where you can do 'em like me. I type.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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