‘It’s not the heat, it’s the humidity’

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Why does the amount of humidity in the air affect comfort moreso than the temperature?

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A big part of how the body regulates temperature is by sweating. This is because when water evaporates, it takes some of the heat away with it.

When there’s more humidity in the air, there’s less opportunity for water to evaporate, so that leads to you sweating, and it taking longer to evaporate & cool you down. So on top of being just uncomfortably humid & “wet”, you’re also not losing as much heat from sweat so it feels warmer.

When relative humidity is high – meaning that the air is holding a lot of the moisture that it can hold at a given temperature – it becomes more difficult to cool yourself down. We cool ourselves down by sweating: we sweat, the sweat evaporates into the air, and (in ELI5 terms) takes some energy with it, cooling you slightly.

When the air is already holding a lot of moisture, your sweat is less likely to evaporate because it has nowhere to go – the air is full. That means you get much less of this cooling effect.

Humans remove extra heat from their bodies by sweating. Sweat is moisture expelled from the skin glands. When this moisture dries up (evaporates) it lowers the skin’s temperature, because evaporating requires heat energy which is thus removed from the body. In humid conditions sweat evaporation is slow because the air is already loaded with water vapor and does not take up moisture as easily as dry air does. Therefore sweating does not help very much in keeping the body temperature in the normal range, and we feel too hot, compared to the same outside temperature in dry conditions.

Humid air means that it’s harder for us to sweat. We cool down by sweating, and if the air is already saturated with water, the water in our sweat has no where to go.

If your sweat evaporates quickly and fully your temperature gets reduced by that a lot, maybe as much as 20 degrees Celsius in most extreme examples. So you can just drink more water to replenish it, but not going to overheat.

But if it’s high humidity sweating doesn’t cool you nearly as well – if humidity is at 100% sweat just doesn’t evaporate and doesn’t provide any cooling, only makes you wet.