how does swatting your hand towards your face in a hot day provide cold breeze instead of hot air?

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Or how any fast moving object also provides cold air

In: Other
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The air isn’t cold.

The movement of it across a wet surface (e.g. your skin, your sweat, etc.) causes water to evaporate and moves the evaporated water away. Cooling a human is about facilitating evaporation – the human sweats, air moves across, which allows the water to evaporate, which cools the body (relatively speaking). Humans sweat, dogs pant, and moving air makes the process more efficient.

On a hot day, fanning yourself helps cool you down through convection.

Blowing the air across your face allows the air to pick up some of the heat off of your face and carry it away. This is what gives you the cool feeling on your face, even if the air itself is warm. When the air gets warmer going across the surface of your face it naturally expands and rises, being replaced with some slightly cooler air, to continue the process.

Sweating, or spraying yourself with a cool mist, assist in cooling yourself down through a mixture of conduction (the cool mist, or sweat, drawing heat away from the surface of your skin) and evaporation.

Edit: spelling.

There are two reasons for this.

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The first thing to understand is that most of the time, your skin is hotter than the surrounding air, because the chemical processes in your body are always heating it up. Thus when you blow air on yourself, you accelerate the speed of heat transfers from your skin to the air (through convection), and thus you feel a “cold” breeze.

Indeed, what you feel is heat leaving your body, not the true air temperature, so if moving hot air makes your body cool down as fast as colder still air, your subjective perception of both will be the same.

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But what about when the air is actually hotter than your skin? When you get past the 30°C and get to the 40°C? Well, here comes the second – and more important – thing to understand : your most efficient way to cool your body down is not pure convection, but the evaporation of good old sweat.

To transform from a liquid to a gas, water needs a lot of energy, energy that it will then keep with itself on its merry gaseous way. Thus when your sweat evaporates, it will suck up a lot of heat from its surroundings – including your body – thus lowering your skin temperature and cooling you. And when you swat air on yourself, you accelerate the evaporation speed, thus you cool down faster, thus you feel like the air itself was cold.

This is the same reason a hot day in a humid country feels much hotter than in a dry country (something that can be measured using for example the [heat index](https://en.wikipedia.org/wiki/Heat_index)), as evaporation works faster when the air is dryer. This is also the main reason why throwing water on regular fires will extinguish them. There’s a few more things I would like to add, but for the sake of brevity, I’ll stop : however don’t hesitate to ask any further questions.

Plenty of good answers here.

Additionally, human being are actually quite bad at feeling absolute temperature. Were better at feeling changes in temperature*. Since the air close to you is slightly warmer than average because your body is emitting heat, the new air feel “cold” even though it’s a small difference.

*You can see that in action with a small at home experiment. Get 3 bowls of one water, 1 with cold water, 1 at room temperature, and 1 with warm water (Not too hot though !). You put one hand in the cold water bowl and the other in the warm water bowl. Remain there 1 or 2 minutes. Then put both hand in the room temperature bowl. The water will feel hot with one hand and cold with the other, we don’t measure absolute temperature, we feel relative temperature