From the Weather Center: February 2021

by John Simon

This time of year, people may notice that the snow sometimes disappears even on days when the temperature does not rise above freezing. This is due to a process known as sublimation.

Sublimation is most often used to describe the process of snow or ice changing into water vapor in the air without first melting into water. Sublimation occurs more at higher altitudes, where the air pressure is less than at lower altitudes. The opposite of sublimation is “deposition”, where water vapor changes directly into ice—such a snowflakes and frost. In addition to snow, a known thing that often undergoes sublimation is “dry ice.”

Both snow and dry ice can sublime, although more slowly at temperatures below the freezing/melting point temperature line at 0 °C for partial pressures below the triple point pressure of 612 Pa (0.0006 atm).

The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the upper layers of snow. It isn’t easy to observe while it’s happening, but the effects of sublimation on snow are plainly visible if you are looking for them.