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 Question 1554: Frost forms only when the relative humidity reaches 100%

 We flew from SeaTac to Maui last week. Because of snow on the ground and not enough deicer, many planes were unable to take off. On the return trip, the flight had a realtime listing on their t.v. screen of where we were (showing the progress we were making) and also showed altitude, flight speed and outdoor temperature. I noted that the outdoor temperature at 36,000 feet was 60 degrees below zero! So then I wondered....if planes can't take off without deicer at 32 degrees Fahrenheit, how can they "fly" at even colder temperatures? — VN, Anacortes, Washington
The problem for planes isn't the temperature, it's the humidity. When the air reaches 100% relative humidity, moisture in that air begins to condense on objects such as plane wings. The moisture can also condense into rain, snow, or sleet and then fall onto those plane wings.

If the temperature of overly moist air is 32 F or below, planes preparing for takeoff can accumulate heavy burdens of ice. When water vapor condenses as ice directly onto the wings themselves, that condensation process is called deposition and is familiar to you as frost. Deposition is a relatively slow process, so most of the trouble for planes occurs when it is actually snowing or sleeting. Removing the ice then requires either heat or chemicals.

When the plane is flying at high altitudes, however, the air is extremely dry. Even though the air temperature is far below the freezing temperature of water, the fraction of water molecules in the air is nearly zero and the relative humidity is much less than 100%. That means that an ice cube suspended in that dry air would actually evaporate away to nothing. Technically, that "evaporation" of ice directly into water vapor is call sublimation and you've seen it before. Think of all the foods that have experienced freezer burn in your frost-free (i.e., extremely dry air) refrigerator or the snow that has mysteriously disappeared from the ground during a dry spell even though the temperature has never risen above freezing. Both are cases of sublimation — where water molecules left the ice to become moisture in the air.