As winter settles in for the long haul in many places, early-rising pilots long for spring when the rising sun has enough oomph to melt away any frost coating the aircraft on flight school flight lines.
That melting process is unlikely in January, when temperatures may sit in a regrettably reduced range for days on end. Yet on those mornings, frost may diminish even if temperatures never rise above freezing—that is, the temperature at which the state of water changes from ice to a liquid. (And if an aircraft that is airborne on an instrument flight plan has begun icing up while flying in clouds, upon breaking out into clear air, the pilot may be happy to observe that the leading edges of wings, struts, and other surfaces have begun to lose their ice.)
A formal definition of sublimation is a “process by which a solid is changed to a gas without going through the liquid state.”
The Pilot’s Handbook of Aeronautical Knowledge notes in Chapter 12 that water can exist in those three states within the atmosphere’s temperature range, and that “sublimation is the changing of ice directly to water vapor, completely bypassing the liquid stage.”
If that’s difficult to visualize, the discussion offers an analogy: “Though dry ice is not made of water, but rather carbon dioxide, it demonstrates the principle of sublimation when a solid turns directly into vapor.”
Another point to take away from the sublimation discussion is that although water’s transitions from one state to another are driven by the processes of “evaporation, sublimation, condensation, deposition, melting, or freezing,” only evaporation and sublimation add water to the atmosphere. (By which process did the frost that’s coating your trainer right now get there? Deposition.)
Pilots don’t have to become water wonks to soak up basic ideas and explain how water’s behavior relates to aircraft and aviation weather, as meteorologist Jack Williams engagingly does in his weather columns.
But getting back to dry ice for a moment, you may have read that dry ice is a potentially dangerous substance to transport by aircraft. Why?
Sublimation again: The sublimation of dry ice into CO2 gas can displace air containing more oxygen, raising the risk of hypoxia for an aircraft’s occupants.