Why Do Some Hot Days Feel Worse Than Others?

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Air can only hold so much water.

Warm air holds more vapour than cold air, but there is always a limit. Humidity is how full the air is compared with that limit.

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Sweating cools you by evaporation.

Turning liquid water into vapour takes a lot of energy, and that energy comes out of your skin. Losing it is what makes you feel cooler.

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Full air will not take any more.

When the air is near its limit, water molecules return to your skin as fast as they leave. Evaporation almost stops and so does your cooling.

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So you sweat more and cool less.

Your body reads the rising temperature and produces more sweat, which now just runs off you. You lose water and salt without getting the cooling.

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Which is why the same number feels different.

Thirty-five degrees in a dry desert is unpleasant. Thirty-five degrees at ninety per cent humidity is genuinely dangerous, because cooling has stopped.

Weather forecasts fold this into a 'feels like' number, which is why it can sit five degrees above the real temperature.