I know a closed (not perfectly insulated) space (like a house) can change in temperature if the outside has a different temperature (by diffusion, unless I'm much mistaken). From observation and asking around I've gathered the same sort of thing can happen with humidity, and I don't fully understand how that happens:
Is the change only in relative humidity, caused by a change in temperature?
Is there a way for the actual water in the air to travel through the walls?
Or is there necessarily an opening in the wall of the enclosed space?
(With actual houses there is almost certainly an opening at least part of the time, but I'm wondering what the central cause is.)
[Physics] How to humidity change in a closed room
everyday-lifehumidity
Related Solutions
The answer is a combination of physics and physiology.
The warm water in the shower very quickly heats up the air in the shower, and warms up your skin. It also drives up the humidity of the air in the shower. You acclimate very quickly to the temperature/humidity conditions in the shower as being "normal".
With the door left open a crack, you allow convection to occur... the warmer, more humid air in the shower escapes out the space at the top, replaced with cooler, drier air from the bathroom. Convection is driven by the difference in density between the warm humid air and the cooler drier air; it's not just the difference in temperature which alters density... a molecule of water (H2O) is lighter than a molecule of either of the principle components of air (N2 and O2), so humidity lowers density for equal temperature and pressure.
The draft feels unusually cool because it is somewhat cooler than what you've acclimated to (stronger subjective experience), and because the drier air in the draft is able to draw much more heat from your skin through evaporation than the warmer, more humid air.
The practical answer is here: https://www.ncbi.nlm.nih.gov/books/NBK143947/
The example of usage : point A $-8^\circ$ C, 100% rel. humidity outside will end up with 15% humidity inside when reheated to $20^\circ$C.
Best Answer
A domestic humidity meter like this one currently sitting on my desk:
measures relative humidity. Relative humidity is the water concentration as a percentage of the maximum concentration. But the maximum concentration is a function of temperature, so for a given amount of water in the air the relative humidity is also a function of temperature.
In my case this means that every morning when I turn the heating on I see the relative humidity fall as the room warms up. This change is obviously just related to the temperature because it's strongly correlated with me turning the heating on.
I do see longer term changes as the weather changes, but I've never attempted to measure the rate the humidity in my room changes when the weather outside changes. To do this I'd need to two humidity meters, one inside and one outside, with some form of long term data logging. It might be fun to do one day ...