In honor of the coming snows, Eli thought it would be interesting to read a June paper from Nature, "The importance of feldspar for ice nucleation by mineral dust in mixed phase clouds" by Atkinson, et al, including another Tamsin, Malkin. (for those interested in different names there is great fun at Lawyers Guns and Money this weekend). There appears to be at least one follow on from this group at Atmospheric Physics and Chemistry, and there is a horse race, involving a UToronto group
The issue is whether there is a difference in the freezing temperatures of water droplets nucleated around small dust particles (solid aerosols). This is one of those things that at first glance you think WTF, but as soon as you know the result the reason is pretty obvious. So these folks made up a bunch of aerosols using different species of mineral dust, like the kind that comes off deserts and other dry places, drove the dust through water vapor until they picked up enough water to be between 14 and 16 microns diameter and then cooled them at 1K/min. They could observe the freezing very Millikan like through a microscope.
Eli is not going to get too deeply into the modeling, but indeed the idea is interesting. Obviously something is going on at the interface which causes water molecules to bind tightly. Turns out that there is a lot of oldish work on dielectric constants and dissolution of minerals, and IEHO the answer lurks there. For example, from 1980 a paper on dielectric constant and loss tangent angles of Indian rock samples by Singh, Singh and Lal showing a significant enhancement of the dielectric constant of moist feldspar dust and not so much for stuff like marble and quartz. Also, comments here and there about how the alkalai ions at the surface of the feldspar can be solubalized, leaving a charged surface.
A geologist could probably say something more than this handwaving, but it sure looks like water molecules can bind strongly to the feldspar surface providing excellent nucleation sites for freezing.
UPDATE: Just pointing out for the aerosol fans, that Eli has been on a bit of an aerosol kick lately
Ashes to ashes,
Dust in clouds
If you ain't sneezing
Then the ice is freezing
(Horatio does it better)