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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We measure humidity by percentage of the maximum, not by absolute water content in the air. If the maximum is higher, then the same amount of water will be a smaller percentage.
When you heat a space, the air inside becomes less dense and rises. This causes a flow of air from the warmer space to the cooler surroundings, which can carry moisture away from the space.
As the temperature increases, the maximum amount of moisture that the air can hold also increases, so the relative humidity decreases. This is why heating a space can decrease the humidity.
Relative humidity measures the amount of water vapour currently in air divided by the maximum air can hold before saturating and starting to condense. This saturation concentration increases with temperature. So, what’s happening is that the air can hold more water the hotter it gets, so the relative humidity falls
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
It doesn’t. Heating decreases the relative humidity, because the hot air can hold more water without it condensing.
The absolute humidity remains the same if you just heat the air.
Absolute humidity is not. Relative humidity is. Relative to what, you ask? Temperature!
Temperature is how much the atoms are vibrating.
High temperature allows more mixing between air and water.
Imagine you are water and want to get in a room full of people that are air. If the people in the room are standing still shoulder to shoulder, you won’t get in. If those people start to dance song move around, you will find a gap. The more they dance the more gaps you have available. If they stop to dance you will be squeezed out of the room.
That’s how air absorbs water. If you take cold air it has very little water capacity, you heat it, it will have more room to let water in. That’s why to dry things we use hot air. The same air when cooled will release the water it can’t hold anymore.
The way we practically measure it is by relative humidity, the % of water relative to the max capacity at that temperature. It tells you how that air mass will behave, if it will have the tendency to dry things or damp things.
Absolute humidity is instead water mass vs air mass ratio. That’s used for technical calculations as it’s not related to temperature. For example, how much rain you can get from a storm is linked to absolute humidity. From absolute humidity it’s easier to calculate what pressure and temperature is needed to get to the dew point for example. “Today we have X absolute humidity, therefore we will have fog as soon as the temperature gets down to Y degrees”. Or “we have X absolute humidity so my compressor will get Y kg of condensation water to get rid of per hour of operation”.
It works the same for all the solutions. Dissolving sugar in water works the same way as dissolving water in air. Hotter water, more sugar can be dissolved. That’s why cooking pies involves a lot of temperature control, to ease or deter things from mixing or dissolving one in the other.
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