In article ,
(Andrew Sarangan) wrote:

The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.

You're talking about two very very different things.

When a gas undergoes adiabatic expansion, it gets cooler. There is no
doubt that this happens at the leading edges of airfoils, but at the
pressure drops we're talking about in any kind of airplane I'm likely to
fly is very small. How small is very small? I'm not sure, but I can't
imagine more than a degree or two.

Yes, you in the back? What's that? You think I'm trying to befuddle
the issue with big-sounding words like "adiabatic"? OK, all adiabatic
means is that there's no exchange of heat. We all know that gasses get
hotter when you compress them. You probably learned Boyle's Law and
Charles's Law in high school chemistry, or maybe the Ideal Gas Law.
These are all just different ways of saying that if you've got a certain
amount of gas which contains a certain amount of energy, if you know any
two of pressure, volume, and temperature, you can figure out the third.
As long as you don't add or subtract energy (i.e. heat), you can play
with those three variables to get all sorts of different combinations.

All those "no heat lost or gained" transitions are adiabatic. That's
what happens at the leading edge. The air moves from an area of high
pressure to an area of lower pressure on top of the wing. As it does,
it expands and cools, but the total amount of energy in a given parcel
of air stays the same.

This is not to say that adiabatic cooling can't cause very large
temperature drops. Anybody who has ever fired off a CO2 fire
extinguisher knows that the gas coming out is VERY cold, and that is an
adiabatic process. But it's also undergoing a pressure drop orders of
magnitude bigger than what goes on at the leading edge of a spam can.

Carb icing is a totally different fish. What's going on inside a
carburator is liquid gasoline is evaporating and turning into vapor.
There's a phase change. It takes a huge amount of energy to effect a
phase change. The air that enters the carburator is NOT undergoing an
adiabatic process; it's giving up energy to the gas to make it vaporize.
That's why you get huge temperature drops inside the carb, and can get
carb icing at ambient temperatures way above freezing.