At 08:22 AM 12/29/03 -0500, you wrote:
>ok, i was going to write to cecil adams, but maybe you guys have the
>straight dope.  twice in the last week or so, i've heard that fog eats
>snow.  is there any truth to this, or is it just a byproduct of the melting
>snow itself?


Um, neither.  It's a function of ambient temperature.  You can have fog at
low temperatures (it's called "clouds" if you're high enough up on the
mountain) with no snow loss. I've seen zero-zero conditions in man-induced
fog on cold days, produced by snowmaking guns (much to the ire of
management, in my Killington patrol days I kept Outer Limits closed for
part of one day because you couldn't see a damned thing, and it was about
10 degrees out... the snow was pretty sweet, too - it was one of the
strangest runs of my life, but also one of the most memorable; if I
stopped, I felt I was still moving - and several times I think I was, due
to snow creep.  But I digress).

Yes, the 'snow eating fog' phenomena does appear to exist, at least
anecdotally.  But it's also important to remember that the conditions that
create 'snow eating fog' in New England are those that also typically bring
warm temperatures, rain and a fairly steady southerly wind.  And that's a
deadly combination for snowpack.  It's possible that the greater mass of
the air in these conditions - and the heat contained in that mass - does
put a bit more heat in contact with the snow. making it melt a little
faster, but I suspect the additional speed is pretty negligible when
compared to the other factors in place.

Another thing to bear in mind:  the thermal mass of the snow.  Early in the
season, when snow is largely uncompacted (except on trails) the snow has
fairly low thermal mass and is thus more prone to rapid melt out,
especially in the woods.  Packed trails (either by machinery or skier
traffic) are covered with snow that's compacted, and therefore has greater
thermal mass, as does machine made snow.  The greater the thermal mass, the
slower the melt.  Which is simply a function of basic physics; it takes a
certain amount of heat to get a certain mass of water to change state; a 1
kg block of ice will melt more slowly than 1 kg of fluffy snow.  Among
other things, the snow has a much greater surface area to come in contact
with the heat.

Don't overlook the impact of skier traffic and grooming on this apparent
phenomena, either.  Snow gets moved by both, and is generally pushed
downhill.  Groomers push snow up to make sure that water bars are covered,
which typically results in a thinner area of snowpack over the top of the
water bar and, generally, a thinner area of snowpack either just uphill or
just downhill of it.  If you're looking at an average 3' snowpack on a
given section of trail, the area near the water bar may have a foot or less
of coverage.  If the prevailing weather conditions during a thaw leads to
loss of a foot of base depth - not hard to do, particularly with
not-so-compacted snow - the result is an average of 2' of snowpack on most
of the trail and bare spots across the trail at the water bar, leading to
trail closure.

It takes highly knowledgeable cat operators to keep a very even snowpack on
a given trail - operators who know the trails intimately and know where
every change in elevation is.  These guys (and gals) can lay down snow
that's remarkably even...  sort of like evenly buttering toast.  The best
example of this I can think of occurred about seven years ago when I was
still at Sunday River; Right Stuff was the last trail open and it was
immaculately groomed wall to wall.  In May, a two-day warm spell - with
rain, wind and fog - visited upon the mountain; when the fog lifted, the
trail was COMPLETELY bare of snow.  A fascinating demonstration.  And there
were still large patches of snow elsewhere on the mountain, on trails where
snow hand not been so smoothly distributed.

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