This time of year, the burning question on everyone’s mind is always about when we will get our first snow. It’s interesting to note that during the two really “bad” winters of 2007-’08 and 2008-’09, November was rather snowless, with Spokane measuring only 2.9 inches in November 2007 and a meager 1.5 inches in November 2008.
Last November was the snowiest of the past three years, with Spokane picking up 4.7 inches of snow (average is 6.4 inches), but of course it was followed by one of the least snowy winters on record. This year, at least as of last Wednesday, nary a flake had been seen across the valleys of Eastern Washington and North Idaho.
Most folks put too much emphasis on the high- and low-temperature forecast, when looking for the possibilities of snow. While temperatures below freezing certainly don’t hurt our chances for the white stuff, and definitely tend to keep what does fall from melting as fast, sub-freezing temperatures at the elevations in which we live aren’t necessary to have snow. That is because the precipitation is not forming near the ground where we are, but way up in the clouds, where it is much colder.
The idea that snow is rain that has frozen is a common misconception. Frozen rain is called sleet. In reality, rain is merely melted snow. This is true even during the summer months. Rain falls, not because liquid drops of water crash into each other in the clouds and become too heavy to remain suspended. Rain occurs when supercooled liquid drops (drops of water that remain in liquid form at temperatures below freezing), along with water vapor, are deposited on ice crystals within a cloud. When these ice crystals, or snowflakes, become large enough they begin to fall. The form of precipitation that we eventually see on the ground then depends on the temperature profile that the ice crystal is falling through.
If temperatures are below freezing for the whole trip to the ground, then of course the snowflake would remain a snowflake. But even when temperatures at our level are well above freezing – even in the 40s – it is still possible to have snowfall, because the melting is gradual.
Here is an interesting fact about snow. Melting snow can actually serve to cool the surrounding environment, allowing for less melting of subsequent snowflakes. Why? The process of melting a snowflake takes heat energy. That heat energy has to come from somewhere, and it comes from the surrounding air. When snow is melting, heat energy is being removed from the air, resulting in a net cooling effect (in the absence of warm advection). This is sometimes why we can have precipitation that first falls as snow, goes to rain, and then back to snow again!