I just found this via a post on sci.physics:

Based on research into the physics of mixing, someone has found a
critical temperature for size and mass

"The researchers initially considered a system of same-size particles,
and equated the kinetic energy of each particle, which is directly
related to the ambient temperature in kinetic theory, to its equivalent
potential energy.

They found that a critical temperature exists below which a layer of
particles 'condenses out' at the bottom of the vessel. This critical
temperature corresponds to a particular ratio of mass and diameter. The
condensed particles vibrate in a confined space but cannot swap places
with their neighbours or re-enter the 'fluid' portion of the system.

From a range of simulations, Hong's group constructed a phase diagram
of different combinations of mass and diameter ratios. The diagram maps
the onset of segregation and the flip from the 'Brazil nut problem' to
the 'reverse Brazil nut problem' - and so predicts how certain
combinations of particles will mix."

http://physicsweb.org/articles/news/5/4/5

So now we can look forward to finding out just exactly when it is too
cold to snow.