High humidity on a hot day makes it feel hotter, but why is that?

Sweat is the answer. Sweating is our body’s way of cooling itself. Obviously, when we get too hot, we sweat, and then sweat evaporates off our skin. Evaporation is a cooling process, so when we sweat in dry air, our skin will cool.

The evaporation process is the same reason why, when you get out of the shower, you are cold. The dryer the air, the faster water evaporates and even greater cooling takes place.

Rubbing alcohol evaporates quicker than water, so if you spill rubbing alcohol on your skin, it quickly has a cooling effect do to the faster evaporation.

When the air is saturated with water, like on a humid day, evaporation is much slower. The air is already full of water and can’t hold much more. Your body will sweat in an attempt to cool itself, but you only end up wet and hot without the sweat evaporating.

The point at which the air will begin to feel warmer than the temperature shows is when the air temperature is 77 and the dewpoint temperature is 57. The best way to think of dewpoint is it is the temperature at which dew would form.

The closer the air temperature is to the dewpoint, the more humid the air.

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