Sunday, March 6, 2011

Skiing in the Transition Zone

It was a fascinating day of skiing in the Wasatch today, one that provided a great lesson in how snow density, ski quality, and precipitation type change in the transition zone, the layer in which snow is warming, melting, and turning to rain.

The transition zone.  Based in part on
Ron Stewart's 2008 AMS/MSC Mountain
Weather Workshop Presentation

The schematic above provides a simple conceptual model of the transition zone.  The freezing level is the highest level at which the temperature is 0C.  Above the freezing level, it is below freezing throughout the troposphere.  It is possible for there to be multiple freezing levels in winter storms, although that is somewhat rare in the Wasatch Mountains and was not the case today.

The freezing level and the snow level are not the same because ice does not melt instantly when it falls into the transition zone.  Instead, it begins to warm and melt, gradually changing from dry snow to wet snow to slush (part ice part water) and finally rain.  Because snowflakes are not all the same size, the transition zone features a mixture of these precipitation types.  

In addition, because the melting of snow cools the atmosphere to the melting point (0C), there is often an isothermal 0C layer in the transition zone, as depicted above.  This layer can be quite deep, especially if precipitation rates are high.  Because of this effect, the snow level may lower some when the precipitation rate increases and rise some when the precipitation rate decreases.   

The snow level is traditionally viewed as the altitude above which snow is accumulating on the ground.  I've never been a fan of this definition because such accumulations are also dependent on the temperature of the surface on which the snow is falling.  One might instead use the level at which at least half the falling precipitation is wet or dry snow, as depicted above.  

Dramatic changes in snow density and ski quality occur in the transition zone.  If you are above the freezing level, where dry snow is falling, the skiing can be quite good and indeed that was the case today.  The snow density here was a bit high compared to the average here in Utah, but it skied quite well and even provided a few face shots from time to time.

At 9000 feet, Neil Lareau is above the freezing level and
gets the goods in the Wasatch backcountry.
On the other hand, the snow density increased dramatically as one descended into the transition zone where the dry snow became increasingly wet and higher density with decreasing altitude.

A few minutes later, at 7400 feet, Neil Lareau is near the bottom
of the transition zone.  There are no faceshots skiing the wet snow,
 but dodging rollerballs adds an element of excitement.
One other factor that greatly influenced the snow conditions today was sublimation of the snowflakes below cloud base.  This was particularly important this morning and, at least in Mill Creek Canyon, led to much less precipitation below about 8000 feet than above it.  

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