The air on my landing is at 69% humidity. Everywhere advises that indoor humidity should be 40-60% to avoid mouldy walls, the kids needing inhalers, and slugs coming in for a wander around. All sources say to increase ventilation and ensure I open the windows to let the damp air out.
The BBC weather app says outdoor humidity is currently 71%. So how does opening a window help? I feel like I’m missing something.
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The amount of water that can exist as a gas in the air depends on the temperature. The warmer the air the more water it can contain.
The number you use is relative humidity so a percentage of max at that temperature. If you change the temperature of air without adding or removing water the relative humidity will change. So if the 71% humidity is for cooler air then you have inside then the relative humidity in it will drop when you let it in and heat it up.
You can ensure absolute humidity too that is in units like grams per cubic meter. It is independent of temperature.
If we look at https://en.wikipedia.org/wiki/Humidity#Relationship_between_absolute-,_relative-humidity,_and_temperature and look up the absolute humidity. Assume 20C indoot and 15C outdoor and round the humidity to multiple of 10.
20C indoor at 70% relative humidity you have an absolute humidity of 12.1g/m^3
15C outdoor at 70% relative humidity the absolute humidity is 9g/m^3
If we now look at 20C air 50% relative humidity is 8.7g/m^3 and 60% is 10.4g/m^3 so if you let in the 15C outdoor air and heat it up you get somewhere around 55% relative humidity.
We can also look at 10C at 70% relative humidity and we get only 6.6g/m^3 which will be in the 30-40% range if you heat it up to 20C
So you can only compare relative humidity directly if the temperature is the same. If you let in air with another temperature and heat or cool it you need to go via absolute humidity and to get the relative humidity at the new temperature.
The table on Wikipedia have quite large steps so you can use https://www.omnicalculator.com/physics/absolute-humidity and input any values you like You can enter relative or absolute humidity and then change the temperature to so see what the result is
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