Let it Snow

When I was younger, I always wondered why the weatherman could tell us with absolute certainty that it would rain on Thursday, there’d be a thunderstorm at 4:30 on Saturday, and it would be sunny and warm all day Sunday. But when winter sets in there’s snow in the forecast, it was always a lot of “maybe” and “it could”. Now that I’m older and wiser, I can see that a lot of forces are at play when it comes to building a snowflake. What determines whether the weather will be snow, sleet, or just cold rain? Snow requires the right amount of moisture at the right temperature, plus cold enough temperatures from cloud to ground.

Ideally, air temperature should be at least 15̊ F, but not higher than about 40̊ F for it to snow. Warmer air holds more moisture, making it more likely for snow to form, as long as the temperature stays within that narrow threshold.

Part of the uncertainty regarding snow, I think, is my location. All my life I have lived relatively close to the ocean, that great body of temperature-moderating water. The ocean slowly absorbs summer heat, then slowly radiates that stored heat through the winter, keeping the nearby air relatively warm, compared to the middle part of the continent. While it may be cold enough for snow to form in the clouds and stick on the ground, sometimes that warm ocean air forms a layer in between. When that happens, the snow melts and refreezes into sleet pellets or remains liquid rain and freezes on contact with the surface- freezing rain. 

Elevation is one of the factors that matters in predicting precipitation type. Usually, higher elevations are cooler. In my Pennsylvania life that doesn’t matter much, but in Washington it could be the difference between a blizzard and a downpour. There, the snow level (elevation at which snow becomes rain) is a moving target, changing by the day or even hourly. Forecasts from fall through spring frequently mention the snow level, and occasionally it drops to sea level. I once witnessed the spectacle of the snow level moving down the side of a mountain across the road from my cabin. I stood in the rain, waiting for the school bus, as the snow steadily marched downhill. By the time the bus arrived it was snowing where I stood.

Today's snow information comes from the National Snow and Ice Data Center. That nifty chart up above is from Weather Wiz Kids. And don't forget to follow Nature Minute on Facebook if you haven't already.