Temperature layers determine precipitation type
Now that temperatures have really cooled off, and nighttime readings frequently dip below the freezing mark, most folks will be keeping an eye on the seven-day forecast for signs of snow. I have a pet peeve with television weathercasters (not just here, but nationwide) which involves forecasting snow, as opposed to other types of precipitation, based only on a forecast temperature of 32 degrees or below. Let me explain why this is bad methodology.
Precipitation comes from clouds. With the exception of fog (which is just a cloud that is on the ground), clouds reside aloft where temperatures are much cooler than on the ground. For the most part, the higher the elevation, the cooler the temperatures. Even in the summertime the temperatures at cloud level are often below freezing. Despite the subfreezing temperatures, the cloud is not made up entirely of ice. Supercooled water droplets can exist in temperatures below freezing, and as cold as minus 40 degrees Fahrenheit. In the midlatitudes, precipitation forms in a process that involves supercooled water droplets and ice crystals in the cloud. The end product, in what is called the Bergeron process, is a snowflake. The fate of this snowflake and the type of precipitation we eventually see, are determined by the different temperatures the snowflake encounters on its way to the ground.
Think of the atmosphere as a sandwich with many layers at different temperatures. If the temperature in all the layers is below freezing, the snowflake will remain a snowflake as it reaches the ground. If the snowflake falls into a thick enough layer where temperatures rise above freezing and stay that way, the snowflake will melt and we will get rain. It is very common to get snow when surface temperatures are above freezing, because even as a snowflake encounters warmer air it doesn’t melt instantaneously. A rule of thumb for snow forecasting is that you can get accumulating snow about 1,000 feet below the freezing level. At even warmer temperatures, you may still see snowflakes, but these would rapidly melt before getting a chance to accumulate. Believe it or not, it is even possible to have snow at a temperature as warm as 50 degrees, though that occurrence is rare.
Other types of precipitation can form when surface or near-surface temperatures are at 32 degrees or below. Sleet is formed when the snowflake melts in a warm layer, and then refreezes as it enters another subfreezing layer closer to the ground. It is possible to get freezing rain when a falling raindrop encounters a very shallow layer of subfreezing air near the surface. Instead of turning to sleet, the raindrop becomes supercooled and only freezes when it comes in contact with an object on the ground forming a hazardous layer of ice.
In reality, a raindrop does not ever “change” into a snowflake. What is happening when we say “rain changing to snow,” is that by one mechanism or another, the air is cooled sufficiently for snowflakes to cease melting before they hit the ground.
Local weather roundup
We managed to get our first snow on the record books for the year Oct. 29. The average date for the first plowable snow (several inches) for Coeur d’Alene is Nov. 14. Overall, the month of November averages 7.8 inches, though in 1973 a record-breaking 31.6 inches fell. Next week, we’ll take a more in-depth look at snow and other types of winter weather. I’ll also explain how on a really cold day, you can get precipitation out of a clear blue sky.