#SkiColorado

I spent the last week in Steamboat attending a meeting known as the Weather Summit and helping out a bit at Storm Peak Lab, a major meteorological observing facility at the top of the ski resort that is now operated by the University of Utah after it was transferred over from the Desert Research Institute. 

We arrived late Tuesday, which turned out to be the only day we saw the sun.  

The meeting was structured to allow for morning turns, which featured dust on crust Wednesday morning and cream on crust Thursday morning.  On Wednesday morning, as the airmass warmed, there was a transition in snow crystals from lightly rimed dendrites to rimed crystals. Steamboat has trademarked the phrase Champagne Powder (don't tell the French about this) and the morning snow was of such quality (although not deep).  By mid morning though, the dendrites were heavily rimed, as illustrated by the snow crystals on my shell.

A lot of work has been done using data from Storm Peak Lab to examine cloud characteristics and snow growth processes.  A favorite paper of mine is Hindman et al. (1994) which examined the concentration and size of cloud droplets over a multiyear period at the lab.  Out of 274 samples, 241 (88%) exhibited what we would expect in clouds with "continental" characteristics where as 33 (12%) featured clouds with "maritime" characteristics.  

Source: Hindman et al. (1994)

Over the continents, there tends to be a lot more very small particles in the air that are referred to by scientists as aerosols. Some of these aerosols can serve as cloud condensation nuclei, or CCN.  These become the "birthplace" of cloud droplets.  Because there are a lot more CCN, continental clouds tend to feature more cloud droplets per volume of air.  These droplets also tend to be smaller.  In contrast, in maritime clouds, there are fewer CCN, there tend to be fewer cloud droplets, and they tend to be larger.  All else being equal, those larger droplets more efficiently rime falling snowflakes, leading to heavily rimed snow crystals (such as those pictured above) or graupel.  

There are other pathways to creating rimed crystals or graupel in clouds even in continental clouds, but clouds with maritime characteristics do it more efficiently.  I didn't take a look at the aerosol concentrations on Wednesday, but on Friday they were low, so perhaps this was a contributing factor to the development of rimed crystals on Wednesday.