by David Murphy
This is a common notion that goes around. The belief is that one inch of rain would translate into 10 inches of snow, were it cold enough to make snow. This idea comes from the ratio meteorologists use to determine the amount of snowfall expected from a storm, given that computer models only tell you how much liquid precipitation is expected. In other words, models don't predict snowfall amounts. They only tell you how much water is expected from the storm. Since snow is made of ice crystals which don't combine as well as drops of water, the accumulation of snow is always greater than the computer-predicted rainfall. It's up to the meteorologist to determine how much greater and on average, the 10-to-1 ratio often works out to be a good predictor of actual snowfall.
But it's not always the correct ratio. In colder storms, which produce a drier but fluffier snow, a 15-to-1 ratio is not uncommon. And in warmer, wetter storms, a thicker, heavier snow is produced and the ratio might drop to 7-to-1, or even lower. The ratio used all depends on the temperature of the air in which the storm is churning.
But here's my problem with the way people tend to interpret this ratio...
Sometimes, after a winter rainstorm, you'll hear people saying, "Yo, today's storm gave us 1 inch of rain---we're lucky it wasn't snow or it would've been 10 inches!" In reality, a storm that produces 1 inch of rain would probably not produce 10 inches of snow. This is because colder air (which, of course, is required to get snow) can't hold as much moisture as the milder air required for rain. In the above example, the rain storm would have to have its temperature lowered before it could produce snow and in doing so, it would actually have to lose much of its moisture as rain before the snow could begin---this would happen automatically, in fact. The result? There wouldn't be enough moisture left over to produce the 10" of snow. All things being equal, it would probably take a rainstorm that's already producing many inches of rain in near freezing conditions to then go slightly colder and give you the ten inches.
I even hear meteorologists oversimplifying this sometimes, so don't feel bad if you have, too.