Ram Effect, and a surprising review of lift
OK chaps, two separate corrections to the thread so far.
First, especially for Blip, one of those perhaps astonishing little revelations - on most aerofoils, the air accelerates across the top, AND across the bottom of the wing, especially at cruise angle of attack. In other words, there is low pressure both above, AND BELOW the wing. The reason that you get lift is that the pressure is rather lower above the wing than below it, so the net lift vector is upwards. For those of you who have lost your aerodynamics textbooks and are disinclined to believe me, take a ride in a fabric high wing aircraft with slightly tired coverings. Sure enough, you will see that the fabric "balloons" downwards on the bottom of the wing in the cruise - because of the low pressure below the wing.
Alternatively, just remember, in one of those witty (?) memory items, that "Lift Sucks".
And now for JTR:
RAM rise is the temperature increase caused as a direct result of compressing the air. The compression is because you are "ramming" the air against something (like a pitot tube, or a wing leading edge). Actually of course the air was stationary, and the airplane was pressing into it, but you get the idea. But that temperature rise was completely reversible (excluding effects to do with condensing out the moisture content, which for our purposes we will ignore) such that as soon as you stop compressing that parcel of air, and return it to ambient pressure, it will return instantly to ambient temperature as well - it will not "stay warm". So, in the shock wave at the wing leading edge (where compression is taking place) the temperature will be higher than ambient. Just an inch or two further back, where the air has already started to accelerate over the aerofoil, the pressure will already be lower than ambient, and so also will the air temperature.
Some further expansion on the original question:
RAT/TAT is actually only "true" at the temperature probe. That is, it is comprised of the ambient temperature (SAT or OAT), plus the ram rise caused by sticking the probe out into the airflow and thus compressing the air at the probe. Although, for example, your engine power settings might be calibrated to RAT/TAT, that doesn't actually mean that the temperature at any other point on the airframe, wing, engine inlet or whatever is actually at RAT/TAT. Local aerodynamics will mean that the compression at those points will, in general, be different, and therefore so will the local temperature rise. I agree with your suggestion however that at a few specific points on the airframe, and in particular the wing stagnation point, the temperature will also be very close to RAT/TAT.
As indicated earlier though, the air pressure surrounding the rest of the wing is not compressed, so RAT/TAT does not apply. In fact, the air surrounding the wing is at a lower pressure than ambient - that's how you are getting lift remember, because lift sucks. So not only is the air around the wing not as warm as TAT, it's actually going to be colder than SAT/OAT.
Which means your wing fuel gets very cold after a while. Unfortunately how cold is a function of not only OAT but also skin friction, insulation properties of the tank bladders (where fitted), radiation/insolation, thermal mass of the fuel, and perhaps more importantly all the heating from the fuel systems themselves.