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When I was a young student pilot, long before the FAA prohibition on polished frost went into effect, an instructor left a six-inch square of frost on the wing to demonstrate how fast it would disappear after takeoff. Sure enough, we hadn't even reached pattern altitude and the frost was completely gone. Furthermore, in my experience frost never forms on an airplane as long as it's moving, or as long as there's sufficient airflow moving across it.

I've never been able to find a satisfactory explanation for this. Why doesn't frost form on a moving airplane?

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    $\begingroup$ After your takeoff, had the ambient air temperature plus the ram air temperature rise been less than freezing, the frost would not have disappeared quickly. It would still have sublimated eventually, but that would have taken more than a few minutes. $\endgroup$ – Terry Jan 8 '15 at 1:55
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    $\begingroup$ Dry air will make frost disappear through sublimation. However moist air will create frost on the plane. $\endgroup$ – ratchet freak Jan 8 '15 at 2:07
  • $\begingroup$ I've had my wings ice up in VMC at night - very scary! $\endgroup$ – Dave Gremlin Mar 30 '19 at 17:56
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Frost can be formed in two ways. On a colder object by water directly desublimating on its surface or on any object when air is saturated with water.

Frost can form on a moving airplane, but only the second way. Why would they have anti-ice on pitot tubes, propeller and leading edges otherwise?

When the air is not saturated with water, ice will sublimate. If the air is moving it carries away the sublimated vapour and thus increases the sublimation rate. When flying in visual meteorological conditions, that's generally the case, so frost won't form in VMC, thogh watch out when dew point is close to temperature; in such case light haze may form that already causes some icing while visibility is still above the VMC limit. Ram and friction heating also help sublimating the ice a bit.

However when you fly into a cloud in freezing temperature, the water droplets will freeze on the aircraft when they hit it. They will hit it on forward-facing surfaces: wing and stabilizer leading edges, propellers, engine inlets, pitot tubes and nose and windscreen. That's why these components have anti-ice, either using heating or deicing boots.

VMC-only aircraft will often only have pitot tube heating and carburettor heating, because those can sometimes gather frost in sufficiently moist air that is not yet forming cloud. This lack of anti-ice makes flying into IMC even more dangerous in winter than it already is for all the other reasons.

Icing only happens when the cloud contains supercooled water droplets, which occurs in clouds where the ambient air temperature is between freezing point (0°C/32°F) and around -40°C/-40°F when the droplets freeze to ice pellets. The exact point the droplets freeze varies somewhat because it depends on other parameters like amount of dust, but pilots can check whether they are accreting any ice by looking in the corners of the windscreen. Whether freezing conditions are to be expected will also be mentioned in the detailed weather forecast.

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    $\begingroup$ I agree with almost everything you've said but i want to make sure we're talking about the same thing. If frost won't form in VMC, why will an airplane sitting on the ramp on a clear calm winter night quickly frost up? $\endgroup$ – Steve V. Jan 8 '15 at 13:44
  • $\begingroup$ @SteveV.: Because as temperature decreases in the night the temperature gets slightly below dew point, light fog forms and starts to condense on things as dew and freeze. True, the fog only needs to be very light for this, so the conditions could still qualify as VMC. $\endgroup$ – Jan Hudec Jan 8 '15 at 13:57
  • $\begingroup$ You'll find supercooled liquid drops anywhere between the 0 C and -40 C levels. Above the -40 C level you'll still fund liquid where there are very strong updrafts. $\endgroup$ – casey Jan 8 '15 at 14:59
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    $\begingroup$ In flight, this would be known as icing and is formed in a different way (moisture deposited as a liquid which then freezes, rather than desublimating directly from water vapor to frost). $\endgroup$ – Lnafziger Aug 5 '15 at 21:50
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Frost does form if you fly into cloud with the correct temperature and moisture levels as explained elsewhere. It can bring planes down if not protected by anti ice systems or where such systems are not used correctly.

Secondly, be wary of taking off with frost on your aircraft. It could have unknown effects on the stall speed and aerodynamic qualities of your aircraft (in other words it might not fly). For example if you have frost on one wing only, or the tail, the outcome might be very bad.

Finally, faster aircraft such as jets have higher skin temperatures on the leading edges due to friction from the speed. They still need anti icing systems though. Taking to extremes the Lockheed SR-71 Blackbird had skin temperatures of 200 degrees C + when flying at Mach 3. It would also be above most of the clouds that could form ice at that point (not sure of the situation though when it was at 30,000 feet refuelling).

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A parked aircraft on a still, clear night radiates its heat into the dark sky and becomes colder than the surrounding air. If it drops below the dew point, dew forms; if it drops below that and freezing, frost forms.

In flight, the high speed air flow over the aircraft keeps its skin at essentially the same temperature as the air so it doesn't drop in temperature the same way.

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  • $\begingroup$ On a still, clear night, the plane will be in thermal equilibrium with the atmosphere, i.e., at the same temperature as it. If the plane somehow got colder, the atmosphere would conduct heat to it and warm it up again. $\endgroup$ – David Richerby Mar 30 '19 at 0:05

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