It seems to me that the cooling effect caused by blowing on your coffee can’t be all from increased rate of evaporation, or else, the hot air setting would have cooled the coffee faster in Cecil’s experiment. The hot air would have encouraged even more evaporation - but instead, the cooler setting was faster. I believe the answer lies somewhere between - the movement of the air allows for evaporation to a certain point. But the temperature of the air also has a cooling effect on the liquid it is striking, which is why the cooler setting worked faster.
Unca Cece didn’t say that it was all from the increased rate of evaporation. He said that the contribution from the temperature differential was relatively negligible.
I guess there were some mixed messages there. First he said, “Conclusion: the temperature of the airstream (e.g., your breath) isn’t irrelevant, but it’s pretty damn close.” Later in the same article he said, “In sum, your breath does not cool the coffee because it’s cooler than the coffee…” It was this statement that caught my eye.
Just under four minutes versus five is ~20% faster. That seems fairly significant to me.
Cecil (and his advisory board) brushed him off quickly, but I think Michael Godfrey had a point. When you blow on the back of your hand with force, you feel coolness (even if your skin is dry, seeing as it’s winter). When you breath slowly, you feel warmth.
Godfrey was way off on the compression-decompression thing, but he was on the right track that blowing hard gathers up a lot of new molecules into the air current, bringing the average temperature of the stream quite near to ambient.
I think another unmentioned factor also has to do with water vapor. In the winter, your steamy breath condenses on your cold hands, and that gives a significant warming boost (although unless your skin somehow absorbs the water, this boost is illusionary).
But all this holds only if you warm your hands the right way. I’m not sure everyone knows how to do it, but you have to cup your hands tightly, make an internal chamber, and gradually inject warm air into it. I think a lot of people do it wrong and just blow aimlessly while rubbing vigorously, just cuz they’ve seen this on tv. (Obviously hollywood isn’t going to know anything about keeping hands warm)
I don’t know where you live Alex, but I’ve lived in Vermont, and take it from me, when the temp is freezing or below, simply blowing on your hands in the same manner you blow on your hot coffee will warm them up. There’s no question that the wide-mouth blow is warmer, but when your hands are numb, it doesn’t matter.