IO540
I think that you are labouring the point somewhat. I imagine, given my A level knowledge of physics, that the ice will sublimate eventually in dry air which remains below freezing, but, as 2D says, this has little practical application to flying real aeroplanes.
If you pick up ice there are three practical ways of getting rid of it from unprotected parts of the airframe.
i) Descend to below the freezing level, at which point it comes off remarkably quickly.
ii) Pop out into bright sunshine, which, even if the ambient temperature is a little below freezing can produce enough radiant energy to shift the ice eventually ("eventually" meaning 20 odd minutes, not the many hours that sublimation might take.) You can tell that it is radiant heat rather than sublimation because parts of the airframe in shadow take much longer to clear.
iii) Land and, as 2D said, put it in a warm hangar. Even in the hangar the ice can hang around an amazingly long time (hours rather than minutes) because of lack of airflow.
As this thread is turning into "Icing for Dummies"
it might be worth just elaborating the point that
drauk is making about altitude. For icing to appear, you need supercooled water, which is to say water which is below freezing temperature, but still liquid. When this meets the airframe it turns to ice. Small droplets form relatively benign rime ice and large droplet form the dreaded clear ice.
The reason that
climbing is often the best solution is that supercooled droplets do not exist below -15C, meaning that if the air is cold enough ice cannot form on the aircraft (though any ice you are carrying will persist, pace IO540.)
Very roughly, -15C will be found 7,000' above freezing level, or, if the surface is -5C, -15C will be 5,000' above the ground.
drauk's rule of thumb will reduce the ambient temperature by 8C, which will almost certainly change conditions enough to reduce or eliminate icing.
Sorry, drauk, don't want to steal your thunder, but you seemed to be assuming knowledge that wasn't self-evident in this thread.
Timothy