Name: Sandra D.
When snow melts, does it melt from the top surface as a
result of the sun and/or air temperature or from the ground surface. the
snow does not appear to change in its' surface appearance and this
creates a debate in our family each spring.
The melting occurs at either the top or the bottom depending on the
temperature of the air and the ground respectively. If the ground temp is
below freezing, the snow will not freeze, correspondingly if the
temperature at the ground is above freezing the snow will melt at the
interface of the ground and snow. Same arguments apply for the air.
Dr. Harold Myron
Both mechanisms operate depending upon conditions. Snow reflects a lot of
incident sunlight, but not all, and warm air also melts ice. Both these
occur from the top ----> down. As the water trickles through the snow it too
melts ice from the inside if it is warm enough, or it may re-freeze
depending upon the snow temperature. On the other hand, if there is any
blacktop, ground, or even concrete nearby, it heats up rather quickly and
the heat is conducted through the underneath "stuff" and the snow bank melts
from the bottom up.
Sandra, just to stir up even more debate at the next family dinner, toss this
out: some snow sublimates. That is a fancy way of saying that some of the
solid snow on top goes directly from the ice (solid) state to the gas
(evaporated) state, without melting (you know, like dry ice goes from solid
to gas without passing through the liquid phase). This can happen even if
the air temp stays below freezing. Ever notice how some snow will disappear
even though it has not really warmed up? Enjoy :)
Paul Mahoney, PhD
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Update: June 2012