HOW COME? More than cold weather needed for snow

Why doesn't it snow when the temperature is much below 32 degrees? asks Olivia Ellis, a student in Brookville

While it must be cold to snow, frigid weather isn't enough. Parts of Kentucky and Tennessee can get more than 20 inches of snow a year, but the average snowfall over frosty Antarctica is -- surprise -- a measly 6 inches. But it does snow around the South Pole, even though the average temperature in Antarctica's anterior is a face-numbing -70 F.

So while Earth's southernmost continent lies under an icy covering, it's a shell built up over many centuries. Antarctica is considered a desert, and its annual precipitation makes it the driest continent on our planet.

It's a puzzle: Since water freezes into ice crystals at 32 degrees F., it seems that the colder it is, the more it should snow. But beyond the big chill, moist air is a key ingredient in the snowy-day recipe.

For a really good snowfall, conditions must be just right. Snow, after all, is frozen, crystallized water. So a big parcel of water-saturated air is essential. And warmer air is able to "hold" more water vapor than the same-sized parcel of colder air.

How much more? At typical winter-day temperatures, saturated air holds very different amounts of moisture. At 32 degrees F., a cubic meter of air contains nearly a tenth of an ounce of water. But at zero degrees F., the same-sized parcel of air just two one-hundredths of an ounce. And at -40 degrees F. -- a typical temperature in Antarctica -- a cubic meter of air contains only two one-thousandths of an ounce.

So it's actually the dryness of the air that leads to less-impressive snowfalls at lower temperatures. But there's more: Scientists say that there is less evaporation from the ground in frigid weather, making air extra arid. Plus, water-saturated air must rise and expand before it forms snow-making clouds. And at very low temperatures, air doesn't rise as readily.

As the temperature plunges, the kind of snow that falls changes, too. Each snowflake is made from two to about 1,000 individual ice crystals. Snow crystal shape and how readily crystals clump together vary with temperature. As the temperature approaches zero degrees F., we see less snow and smaller flakes.

Scientists say that we're most likely to see fat flakes fall from the sky when the temperature ranges from about 24 to 32 degrees F. According to CalTech physicist and snow expert Kenneth Libbrecht, by the time the air temperature drops to -4 degrees F., the snowflakes have already mostly fallen.

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