I have heard the question many times. What IS the difference between the snow level and the freezing level? Aren’t they one and the same? The answer is…NO!
Professor Cliff Maas from the University of Washington has written very eloquently about this very subject. I have summarized and paraphrased his article below.
“Freezing level: In most midlatitude locations, particularly in winter, precipitation starts aloft as snow. As the snow falls from the colder upper atmosphere into the warmer air below, it often reaches a level at which temperature warms to freezing (32 degrees F), the freezing level.
Snow level: Below the freezing level, snow starts to melt, but it takes a certain distance to do so—on average about 1,000 feet (300 meters). Wet snow, but still snow. Since melting snow stays at freezing, the melting layer is often at a uniform temperature of 32 degrees F. Eventually the snow melts completely and we reach the snow level, below which only rain is observed.
Both the freezing level and snow level can change in time as precipitation falls, and the direction is usually down. The reason? Cooling due to evaporation and melting.
First, evaporation: The air below the cloud is often unsaturated, which means the relative humidity is less than 100%. As the snow falls into that layer there is evaporation (actually sublimation), which results in cooling. If the snow turns into rain there still can be evaporation and cooling. Such cooling continues until the air is saturated, and can cause the freezing and snow levels to drop quickly and substantially (hundreds to even thousands of feet).
And then there is melting: When snow falls into air warmer than freezing, it melts. But it takes energy to melt the snow, and thus as melting occurs the surrounding air cools. Heavier precipitation results in more melting and more cooling. Such cooling can occur even after evaporation has stopped (because the air becomes saturated). Melting thus causes the freezing and snow levels to fall. The good news about this: If you are up in the mountains and it starts to rain on you, there is a good chance, particularly if you are near the snow level, for the rain to turn into snow!”
