Anyone interested in winter recreation cares deeply whether they are in rain or snow, snow generally being preferred, of course. So it is important to know the current and future elevations of the snow level, the height separating snow from rain.
And there is another closely related term that is used in weather forecasts: the freezing level, the altitude at which the temperature drops to freezing.
Freezing level: The freezing level is the elevation at which the temperature drops to 0°C. 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 (0 degrees C), the freezing level.
Snow level: Snow levels is lower than freezing levels because snow can still fall at temperatures slightly above freezing. The elevation where the snow turns to rain is the snow level. It takes time for the snow to melt, and that is why we will often see precipitation falling as snow even when it is +1°C or +2°C (and sometimes even more) at the surface. If the snow level is above the ground, it will still be lower than the freezing level as snow often takes a few hundred metres to melt as it falls toward the ground.
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!
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