Saturday, January 3, 2015

Freezing Level Versus Snow Level

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.

The snow level can be pretty obvious, as seen in this image of Capitol Hill in Seattle.

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.

So let's get educated about these important levels.  What exactly do they mean?  How are they related? And how do they change in time?

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 (32F), the freezing level.   Below that level the snow starts to melt, but it takes a while to do so--on average about 1000 ft (300 meters).    Wet snow, but still snow.   Since melting snow stays at freezing, the melting layer is often at a uniform temperature of 32F.

Eventually the snow melts completely and we reach the snow level, below which only rain is observed.  
The snow level in the mountains is often obvious, with white-clad trees 
above and bare trees below.

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!

Freezing and snow levels can also rise as warm air floods a region, particularly as a warm front approaches.  The highest freezing levels in the western U.S. are generally associated with atmospheric rivers: warm, moist currents of air originating in the tropics and subtropics.  In such events the freezing level can rise to 5000-8000 ft, even in the winter!

 The National Weather Service forecasts often talk about freezing and snow levels and how they will change in time:  information worth being aware of.