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?

Answer 1

The relative humidity of air changes with temperature because warmer air can hold more water vapor. In your basement, a drop in outside temperature leading to cooler air that warms up inside can result in a lower relative humidity, assuming the addition of moisture is constant or reduced. Monitoring and controlling humidity with measures like airtight windows or dehumidifiers can help maintain desirable humidity levels for both comfort and health.

Step-by-step explanation:

Relative humidity is a key concept in understanding how air temperature affects the amount of water vapor air can contain. When we talk about relative humidity, we're referring to the amount of water vapor present in the air relative to the maximum amount the air can hold at that particular temperature. Warmer air can hold more water vapor, hence the relative humidity will be lower for the same amount of moisture as compared to cooler air.

The phenomena you've observed in your basement are related to these principles. When warm, humid air enters a cooler environment, such as your basement, the air cools and its capacity to hold water vapor decreases, potentially raising the relative humidity if moisture is added or remains constant. As the outside air becomes cooler in autumn and enters your basement, if it warms up inside, the capacity to hold moisture increases and thus could lead to a lower relative humidity, assuming no additional moisture is added into the basement space.

If you want to maintain a lower humidity level indoors, you can use a humidity gauge to monitor the levels and take action accordingly, like installing airtight windows to prevent moist outside air from increasing indoor humidity, or using a dehumidifier if necessary.

Keeping indoor humidity levels between 30% and 50% is generally advised for comfort and health, as excessive humidity or dryness can lead to discomfort and health issues. Lastly, remember that absolute humidity is a measure of the total amount of water vapor present, irrespective of temperature, while relative humidity varies with temperature.