I notice that when the snow builds up in 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 into the cold, dry air?
The transition of water from the ice phase (or snow) to the gas phase (or water vapor) is called sublimation. Sublimation is a common way for snow to disappear in Wisconsin winters.
On warmer days, when temperatures are above freezing, we can see the melting process as snow leaves water behind on surfaces, which then evaporates or gets absorbed by the ground. We do not see the sublimation process because the snow goes directly into water vapor without first melting into liquid water. However, we do notice that the snow amount is decreasing, so snow sometimes seems to disappear on cold winter days.
The rate of sublimation is a function of the weather conditions. It takes a lot of energy to turn ice into a gas called water vapor: about 7 times the amount of energy needed to boil that water. The energy needed to sublimate the snow off your patio comes primarily from the sun. So, sunny weather is the best weather for sublimating snow. Windy days are also good, as the wind helps to remove the water molecules once they leave the snow and enter into the atmosphere. A low humidity also helps to increase the rate of snow loss. So, the amount of snow that sublimates back into the air depends on the typical winter weather for a given location.