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Wind Chill and Melting Snow
Name: Michelle C.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2/9/2004
Question:
I understand that wind affects the melting of snow because the runoff can be
driven into existing snow making it denser, therefore more difficult to melt. But wouldn't stronger
winds also contribute to quicker melting in above freezing conditions due to the removal of the
insulating blanket of cold air over the snow? Also how much does wind contribute to the evaporation
of melting snow?
Our local weatherman tends to make statements like "There is a lot of wind today, so we will not be
seeing much of the snow melting."
Why would wind chill have anything to do with the melting of snow?
Wind blows away the layer of heat around us, how does it work with snow?
Replies:
The process of melting snow is very complex. Temperature (both air and ground), relative humidity,
wind speed (can work either way), snow density, presence of salt and other "de-icers". Wind chill
plays a factor because it is a measure of how much warm air is swept away from the snow. The snow
melting process is an engineering nightmare!
Vince Calder
Michelle,
Gravity is probably more effective in allowing melt runoff to percolate down through the snow than
wind is.
Strong winds contribute to increased melting (or sublimation) of the snow by rapidly removing water
vapor from the snow surface,
allowing drier air to come along and thereby remove more water vapor. Sublimation is the change of
state directly from a solid (like ice) to the vapor phase (water vapor), skipping the liquid phase.
Sublimation contributes to snow removal, but is not really melting. The greater the wind speed, the
more water vapor can be removed, and therefore the faster the snow will be removed. This effect is
probably more important than the "wind chill" effect, which implies that the greater the wind speed,
the more energy is removed
from the snow surface.
Wind chill is only important when the surface being cooled is warmer than the surrounding air. If the
air is warmer than the snow (which is at around freezing all of the time), it will contribute energy
to the snow to help melt it, as well as serve to remove water vapor from the snow surface. If the air
is cooler than the snow, no wind chill effect occurs and the wind simply serves to remove water vapor
from the surface. The exception to the latter occurs for very strong winds, where the air moving over
the snow "deposits" energy at the snow surface because of frictional effects, thus, in a minor way
helping to melt the snow.
As for the weatherman's statements, when he says that there will be a lot of wind, perhaps this will
be accompanied by heavy clouds; if so, the snow melt will be slower because the Sun is not shining
on the snow. If he is thinking that high winds will reduce the effect of the Sun on the snow (by
keeping the snow surface cooler), he is partially correct (as long as the air is not much warmer
than the snow); however, the enhanced removal of water vapor from the snow surface by the wind will
be much more dominant.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
Dear Michelle-
Lots of factors affect snow melting, and wind velocity is but one. The temperature of the
ground, and even the composition of the ground can affect the rate at which snow melts.
Melting can occur even when the air is below freezing, if the snow is heated by the sun.
And the albedo of the surface of the snow affects the melting rate also. Snow can also
evaporate, when air temperatures are below freezing.
In most cases stronger winds with above-freezing temperatures will hasten melting, due to
the mixing of the water vapor just above the snow surface with drier air higher up.
Wendell Bechtold, Meteorologist
Forecaster, National Weather Service, St. Louis MO
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