Friday, March 23, 2012

Finding Powder

Incredibly, even though the high temperature at the Salt Lake City International Airport was 71ºF yesterday and the weather station at the top of the Collins Chair at Alta (10,400 ft) reached 43ºF, there was still some settled powder to be found in the Wasatch Mountains this morning.

Where?  Up very high on an aspect that faces due north.  I mean right on the north compass point.  Going even just a smidge off that compass point and you'll be dealing with a sun crust or manky snow depending on the time of day.

How is it that powder can survive when the free air temperature is above freezing?  First, the air is very dry.  The dewpoint temperature at the top of the Collins Chair at Alta, which we'll use as roughly representative of the highest elevations in the Wasatch, has lingered in the mid teens for the past couple of days.

As a result, while there is some heat flux from the atmosphere to the snow, there is also a net loss of energy because the snow is sublimating.  That means that ice is converting to vapor.  Sublimation leads to cooling, just like evaporation does.  If the dewpoint were higher, the sublimation would be weaker and it is more likely that the snow would become wetter on the north aspects.

Second, these aspects are in the shade.  Most of the energy to melt snow comes from the sun, so cutting direct solar heating out of the surface energy balance makes a big difference.  This only works on northerly aspects that are clearly shaded all day.  Lower-angle northerly slopes can still see the sun and that's all it takes to trash the powder.

I don't know if the powder we found today will survive the winds that are coming, but there is another storm coming in on Monday.  It looks to be a warm one and potentially a quick hitter, but perhaps it will give us another quick flirtation with freshies.

1 comment:

  1. It looks like that guy just reached the powder/glop transition and is about to go over the handlebars!