On Sat, 1 Apr 2006 11:31:55 -0500, Miguel Naughton 
<[log in to unmask]> wrote:

>On Sat, 1 Apr 2006 10:40:42 -0500, Scott Braaten <[log in to unmask]> wrote:
>> Snow doesn't like to stick around when the
>> dewpoint is that far above freezing.
>What's the reason for this?  I have never heard this before.

When the dewpoint is below freezing, it can keep the snowpack in a "cold" 
state through sublimination (phase change from solid straight to gas).  
Large temperature differences can occur within a substantial snowpack, as 
the top layer is warmed by the air temp and sunlight.  The lower layers 
are still frozen solid and could care less what the outside temp is.  
There is little melt from solid to liquid but the snow instead just 
evaporates.  You could see this during last week as creeks and rivers were 
not running very high even in drainages with a substantial snowpack.  Not 
much water was being introduced into the ground system.  

As soon as the dewpoint rises above freezing, the air begins to hold more 
and more moisture.  In our world, 32F or 0C is the magic number and as 
dewpoints rise, the amount of water it holds increases exponentially.  The 
5F dewpoint rise between 10-15F results in much less actual moisture 
change than the difference between 50-55F.  A dewpoint above freezing 
will "ripen" the snowpack and transition it from a "cold" snowpack to 
a "warm" snowpack.  The snow will not go from a solid to a gas but instead 
from a solid into a liquid.  Some will still evaporate, but much more 
actual "melting" will take place.  That water then needs to drain out 
through the rest of the snowpack or pool on top.  A feedback process 
begins as liquid water starts making its way through the snowpack warming 
it up.  As more melts, more liquid water forms and melts the bonded layers 
in the snowpack.  More weight is added, compressing the snowpack, and 
heating it further.  It is closer to saturation.  Rain does the same thing.

While skiing, on a day with a dewpoint of 15F and an air temp of say 60F, 
like the dry days we had earlier this week, you'd notice you can still ski 
on the snowpack, with it peeling off 1" at a time.  On a moist day, with a 
dewpoint of 40F and a temp of 60F, you'll find you do more skiing in the 
snowpack.  You'd sink in, the snow becomes more "manky," and just 
generally more moist.  

I'm not sure if I explained that well, or at all, but its a stab at it.  I 
think as far as skiing goes, people on the list can tell the difference 
between a "cold" snowpack and a "warm" one...although they might not 
necessarily know it.  If you look through all flood information, its 
usually mentioned that periods of high dewpoints cause accelerated 
melting.  Plus, with a low dewpoint, overnight temps will fall below 
freezing usually.  


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