Why does it feel cooler if you blow air on your skin vs. still air?

Air’s moving, doesn’t that have high kinetic energy and should therefore feel warmer?

Forced convection. It’s a heat transfer mechanism.

It really does not, normally. It is the same temperature. Why do fans seem to cool? Hard to say. If you are sweating, then there is a small effect from evaporation.

So if you blow on a spoonful of piping hot soup it doesn’t get any cooler? It does for me.

The windchill effect, and convection (as in post #2) are the answers to the OP.

Most importantly it is evaporating moisture off your skin. This is why we sweat.

The latent heat of evaporation of water is remarkably big. The energy it takes to take liquid water to gaseous water is 2264 kJ per kg. That is huge. It is larger than the energy needed to take it from freezing (but not frozen) to boiling. Which would be 418 kJ per kg.

Even moving air that is hotter than your body will cool you if you are sweating.

Like Francis Vaughan said … the air absorbs the moisture and then is pushed off … we keep replacing the air so more moisture is absorbed … all the while removing the latent heat of evaporation.

The still air quickly become saturated and won’t absorb any more moisture.

Your body produces heat. If the air doesn’t move, that heat just hangs around there, accumulating, making you hotter. If you blow the hot air away, it is replaced by air from the rest of the room which is, on average, cooler.

This is the reason for the two important and related concepts in weather forecasting. Namely wind chill in the winter and humidity in the summer. Both effect your bodies ability to maintain its temperature. In the winter body heat is more easily lost thru the surface of the skin by air moving rapidly across it (via convection as stated above). In the summer body heat is less easily lost because humid air will absorb sweat much more slowly, therefore evaporative cooling is significantly reduced.

Why do cars have a radiator fan?
If moving air didn’t cool more effectively, you could just use the radiator by itself.

ETA: evaporation helps, but is not required.

The effect noted by the OP is certainly augmented by evaporation of sweat, but doesn’t depend on it. A piece of hot metal will always cool more quickly in a breeze than in still air.

CrafterMan had it right in post #2. It’s basic thermodynamics that a warm body in contact with cooler air transfers heat to the air at a rate that depends on the temperature difference. If the air is still, the warmed air tends to stay near the hot body*, so the temperature difference and thus the rate of energy transfer both decrease. Moving air reduces this effect, thus maintaining a larger temperature difference and better heat transfer.
*Even in still air, convection will nearly always move some warmed air away from the hot body.

And if the outside temperature is enough hotter than body temperature, then moving air will feel hotter than still air, because it’s transferring heat to you quicker.

Just try driving with the top down on hot summer day in Phoenix!
Like having a hair drier pointed at your face.

To have a cooling effect, the air moving over the body would have to be cooler than body temperature. At 118ºF in Phoenix that’s not the case, the moving air would transfer it’s heat into the body. That’s clearly not the case. The water evaporating and carrying away the energy is the primary method of cooling the body. A car’s radiator is closer to 250ºF, so blowing 118ºF air over it does cool it … most of the time …

By the numbers: It takes about 1 kJ/kg per ºC to raise the temperature of air, about 4 kJ/kg per ºC to raise the temperature of the body (or of water actually) … but 2,300 kJ/kg to evaporate water … that’s the main reason why humans sweat, it’s the primary cooling system.

That’s why I said “enough hotter than body temperature”. And how effective evaporative cooling is, and thus what’s the primary cooling mechanism, depends on the humidity.

I see it wasn’t clear why I included your post, and it was to show I agreed with you … my apologies if anyone thought different. Because it’s transferring heat to you quicker, and that’s what we don’t want.

In the case of your skin specifically, the moving air will allow the sweat to evaporate quicker. The moving air is pulling the humid air away from your skin, allowing more of your sweat to evaporate into the wind stream or drier air. You can notice this if you use the a hot hair drier on your body after a shower. Even though it is hot air, the cooling effect of the evaporation of water on your body will make you feel cooler. Once all the water has evaporated from your skin, then the air will feel hot.

In the case of a inanimate object which is hot, the moving air helps pull the hot air molecules away from the inanimate object. If the air is still, it takes a while for the hot air molecules to wiggle their way away from the hot object.

The direction the air is flowing probably makes some difference. If you have a hot iron and you blow air across the surface, it will pull a lot of heat away. If you instead blow air straight at the surface of the iron, the spot where the air hits the iron may actually be heated up from the kinetic action, but the air stream will flatten and spread across the surface of the iron, cooling the rest of the iron as it flows over the side. As long as the air is somewhat cooler than the the hot object, the moving air will cool more than the kinetic heating effect.

Dubious. “Kinetic” heating will be extremely small at normal (say, subsonic) air velocities. In the case of a hot iron, it will be overwhelmed by the cooling action of the airflow.

Wait, say this again? What is “gaseous water” if not steam, which isi “boiling,” as I understand it.

  1. Although it must be said that at high enough speeds, OP is absolutely correct, and dealing with the heat created by rapid air on a surface is a major engineering challenge of jet and rocket engines, rocket and spacecraft surfaces.

Yeah, ninja’ed.

Your understanding is incomplete. Clouds are '“gaseous water” but are not boiling, nor are they steam. Water vaporizes constantly from the liquid state, but only a fraction of it at once does this. To get the whole mass of water to turn to vapor, then it is necessary to bring that whole mass to a boiling point temperature.