Why does my digital hygrometer change when window is open?

[Hotmad]

Hi everyone,
wonder if anyone can help me on this.....

experimenting with new digital hygrometer,
in my flat we have a small condensation problem so we bought the hygrometer to chek the humidity levels,
in my room over night it is about 60 - 70 % and then in the morning when i open the windows up its about 45 - 55 %
why does this happen as i thought it had nothing to do with outside humidity?
also would a dehumidifier help? if so when and how long do you put it on for?

thank you!

If you open the windows - the outside air comes in!

Our indoors levels are up to 90% humidity overnight, so I'm very jealous of your 60-70%.

And that's with a dehumidifier running all day.

I have been since we got the hygrometers, but it sure explains the mould...

Well, air moves, so when you open the window some of the water vapour inside your house goes out of the window and your humidity decreases.