FARGO — We are taught very early in school that water boils at 212 degrees, turning the liquid water into vapor. We are also taught the water cycle and how water evaporates. But if we think about it, when water is evaporating in summer the air temperature is certainly not 212 degrees. What really happens is that when water reaches a temperature of 212 degrees, it all boils away into vapor. But at cooler temperatures, some of the water, molecule-by-molecule, is leaping off the water surface and becoming vapor. This is evaporation on a smaller scale than boiling.
A glass with a half-inch of water will be dry in less than a week in your dry winter house. A similar thing happens to ice. Molecule-by-molecule, ice particles will leap off a patch of ice on a sidewalk and escape into the air as vapor. This is called sublimation. During sunny weather, it is possible to see ice patches on pavement slowly shrink through this process.