Why does snow disappear?

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Why does snow disappear?

ENLARGE

Closeup of melting snow

Sublimation in action.

When the snow builds up on my patio, it starts to evaporate after a few days, even though the temperature is still below freezing. On average, what percentage of our snowfall each year evaporates back to the air?

The transition of water from the ice phase (snow) to the gas phase (water vapor) is called sublimation. Sublimation is a common way for snow to disappear in cold, dry winters.

On days when temperatures are above freezing, we can see the melting process as snow turns to liquid water, which evaporates, gets absorbed into the ground, or runs off. We do not see sublimation because the snow turns directly into water vapor without first melting, but we do notice that the snow is decreasing and may even disappear on cold winter days.

The rate of sublimation is a function of the weather. It takes a lot of energy to turn ice into water vapor: about seven times as much energy as would be needed to boil that water. This energy comes primarily from the sun, so sunny weather is best for sublimating snow. Wind also helps, as it removes the water molecules that have left the snow. Low humidity also helps to increase the rate of snow loss.

So the amount of snow that sublimates depends on the winter weather at your particular location.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.