Does relative humidity rise faster in cooler or warmer air?

Question

A bit of background: I live in an old house in Austria and have been following the changes of temperature and relative humidity in our basement (which has two small but permanently open windows).

The temperature has dropped by about 2 degrees Celsius, reflecting the drop in outside temperature. At the same time, the relative humidity has also dropped considerably, by almost 20 percentage points (from 83% to 65%).

My best explanation so far is that a very warm fall weather gave us humid air which cooled in the basement, leading to high relative humidity. But now, with temperatures dropping sharply we get cool air which warms in the basement, leading to a rather lower relative humidity. This would be reassuring to me as it means that the previously high humidity could be prevented by installing airtight windows and I don't have to worry much about humidity coming from the basement walls.

BUT, I was wondering: Obviously, warm air accepts water more readily than cooler air, so the absolute humidity rises faster in warm air than cool air - but is that effect maybe so strong that even the relative humidity rises faster in warm air than in cooler air? Because in that case, my measurements could still be consistent with humid basement walls - the warmer air would just be "drawing out" the moisture more quickly, to the point that the relative humidity is higher than it would be with cooler air.

Is hope my question makes sense and I would appreciate any comments!

Answer 1

There is no clear answer to that question unfortunately.

What's really going to affect the rate of increase of relative humidity is the availability of water vapor. If you take a hot shower, for example, relative humidity will rise faster in cold air than hot air. But that's sort of a contrived situation because the temperature of the available water vapor is constant in both cases.

If you are indoors you might think the absolute humidity would remain constant if windows are sealed, but in reality there are many possible sources of evaporation and condensation inside the house.

In the scenario you describe, where the windows are open, it's extremely common for cold/warm fronts to have different associated absolute humidities depending on where the air is coming from. Air that has to travel over mountains and then down them, for example, gets dried out considerably in the process (related to the rain shadow effect). Air that comes from over a body of water warmer than air will be quite humid, but from over a cold body of water can still be quite dry (example: atacama desert).

Answer 2

Cellars have a strong temperature regulation effect because of the high thermal mass of the surrounding earth.

Of course, the humidity of outside air varies widely, but as a general rule of thumb we can expect warm outside air to carry more absolute humidity, simply because in contrast to cold air, it has that capability.

When this warm outside air flows into the cellar, it will cool down because of the high thermal mass, which results in a higher relative humidity.

Cold outside air instead heats up upon flowing into your cellar, which will give you lower relative humidity values.

Sadly, I don't think you can infer from your observation weather and if how much your cellar walls are permeable to the water contained in the surrounding earth.