Q It seems like whenever it’s really cold and dry, there’s less snow on the ground, but it’s not melting. Is it evaporating?

A Not exactly, but close. The snow is “sublimating,” through a process that is similar to evaporation. “Whenever there is an interface of air and water, either liquid or solid, you have molecules trying to leave the water,” said Steve Ackerman, a professor of atmospheric and oceanic sciences at the UW-Madison and one of the “Weather Guys” in the Wisconsin State Journal and Wisconsin Public Radio.

The constantly vibrating molecules in a liquid or solid are restrained by other forces, but a water molecule will escape the water and enter the air when it moves violently enough. “We call that evaporation from a liquid, and sublimation from a solid,” Ackerman said.

Sublimation is obvious in your freezer, where the ice cubes tend to shrink with time. It’s also visible in snowfields that eventually waste away without melting. “You notice there’s no liquid water, none of the hard ice that results from melting and refreezing, but the snow still shrinks,” Ackerman said.

On humid days, water vapor in the atmosphere may be captured by snow, adding to its mass, although not to its volume.

Low relative humidity, as often found on cold days, speeds sublimation, and so does strong sunlight. Snow absorbs much of the near-infrared portion of sunlight, and that energy accelerates its water molecules, speeding sublimation. “This helps explain why a shaded snowfield stays around longer,” Ackerman said.

— Produced in cooperation with University Communications

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