Recently came across the phenomenon that heating a room apparently decreases the humidity. If it was just that heating the space makes a lot of the water condense out of the air, eg onto window panes or other surfaces, then I could accept that. But the bit my brain can’t get around is that if it’s 80% humidity but 2celcius outside, and it’s say 70% humidity and 15celcius inside, why is it helpful to open a window? The warm air inside would be cooled, but surely the water in the air outside would diffuse in and raise the humidity in the room? I feel like I’m missing something really obvious here :/
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The water doesn’t go anywhere. It’s more that the air is capable of holding up more water vapors when the temperatures are higher. Think of air like a bunch of baskets. When it’s cold, there are less baskets. When it’s hot, there are more baskets.
Humidity is percentage based. Imagine that it is cold and the air currently holds two baskets full of water. At this point, it’s 100% humidity. Heat the air up and now, the air has 10 baskets. There are still only two baskets of water so now we’re looking at 20% humidity.
Per Britannica:
>At 30 °C (86 °F), for example, a volume of air can contain up to 4 percent water vapour. At -40 °C (-40 °F), however, it can hold no more than 0.2 percent.
Source: https://www.britannica.com/science/humidity
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