Temperature layers determine precipitation type

During summer, it can sometimes be challenging to determine not only if there will be precipitation, but also when, where and how much there will be. Along come cooler temperatures, and another variable gets added to the mix. If we get precipitation, what kind will it be?

One of the reasons weather balloons are launched twice daily at upper air stations across the United States is so that meteorologists can get a vertical profile of the atmosphere. It would be simple if the atmosphere were like a cold, tall glass of milk – the same temperature at the bottom as it is at the top. But in reality, the atmosphere can be as complicated as a toasted club sandwich, with many layers at different temperatures. These different layers in the atmosphere are what contribute to the many different types of precipitation we see in the cool season.

Contrary to popular belief, snow is not frozen rain. The opposite is true, however, as rain is merely melted snow. In the mid-latitudes (in other words, not in the tropics), all precipitation starts out as snow up in the clouds. What happens to that snow on the way down determines the type of precipitation we eventually see. If temperatures remain below freezing most of the way down, we will see snow. Notice I said most of the way down. It is actually possible to see snow when surface temperatures are in the 40s, because that temperature is likely measured just a few feet off the ground. Snow would begin to melt as it hit the above-freezing temperatures, but we’d still likely see some heavy, wet flakes as they fell through the sky. If temperatures are above freezing in a deep enough layer above the ground, all the snow melts and we get plain old rain.

In a standard atmosphere, temperatures usually decrease as altitude increases. That is one reason it snows more in the mountains than the valleys. It is not always that simple, though. Sometimes a layer of warm air will get sandwiched in between two layers of cold air. When that happens, snow falling from the clouds will first melt into rain, and then refreeze into what we call sleet. What is more hazardous though, is when a very warm layer of air sits atop a very shallow and chilly layer of air at the surface. Snow falls from the clouds, melts into rain, and stays liquid until it hits the very cold ground, or any other subfreezing object near the ground. The liquid rain quickly freezes, forming that layer of ice we call freezing rain.

So how is it that one storm might produce more than one type of precipitation? This can happen because the layers of air are not stagnant. Cold air in one layer might be replaced by warmer air or vice versa. The precipitation type then reflects the temperature changes that are occurring. Although forecasters often use the phrase “rain changing to snow,” what is really occurring is a cooling of the layer of air that was previously above freezing. As temperatures drop due to evaporative cooling or just the movement in (advection) of colder air, the snow passing through that layer stops melting, and stays in snow form all the way to the ground.

Speaking of snow, November 1973 was a record-breaker with nearly 32 inches for the month. That year, Coeur d’Alene saw more than a foot of snow from Nov. 4 to 6, with another big storm dropping 7 inches on the Nov. 25.