How does opening a window decrease humidity?

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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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4 Answers

Anonymous 0 Comments

What’s the temperature outside and what’s the temperature inside?

Relative humidity (expressed as a %) will change based on the temperature of the air. If it’s cool outside and warm inside then your 69% humidity inside probably has more water in it than the 71% outside.

Edit: Swapped hot and cool because I had them backwards.

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