Sunday, December 5, 2010

Fog, the evil snoweater

Unfortunately the fog has now enveloped downtown and the Avenues, with visibilities below 1/4 mile in locations.

View from the Avenues, 1430 MST 5 Dec 2010
Of course, if you don't like the damp, you can just head up to the mountains where one can still peer down on the murk below.

View from top of Wildcat Chair, Alta, 1230 MST 5 Dec 2010
Some of you may have noticed a dramatic loss of snow in the valley this morning.  I certainly noticed that the snowbanks that previously lined my driveway are shrinking rapidly.

The warm temperatures are contributing, but the presence of fog is also important.  When temperatures are above freezing, fog becomes an extremely ravenous snoweater.  Think of how it changes the surface energy balance. Fog is very opaque in the longwave portion of the electromagnetic spectrum.  As a result, it provides lots of downwelling longwave radiation.  Second, although fog does reflect some sunlight back to space, a shallow layer allows for some shortwave to penetrate to the surface.  Finally, you don't get any sublimational cooling when the dewpoint is above freezing.  Instead, you get warming as water vapor condenses onto the snow.

I suspect that the energy input into the snowpack may be maximized in the presence of a shallow fog layer.  If anyone knows of a good study that has examined this issue, please pass it along.

1 comment:

  1. It seems like the dense fog and drizzle that we had last night should help to clear out the air pollution. Doesn't that precipitate out of a lot of the particulates in the air?