Understanding Relative Humidity
Relative humidity is an often misunderstood term, even among some building industry professionals. Relative humidity is how much moisture the air can hold. As the temperature increases, the more moisture air can hold; in other words, as the temperature goes up, the "bucket" increases in size and can hold more moisture. However, if for example, relative humidity is 50 percent at 32°F (0°C) and the air temperature increases to 50°F (10°C), the size of the bucket roughly doubles and the RH drops by half to 25 percent. This doubling in the bucket's size for every 18°F increase in temperature is an approximation that serves as a "soft" rule of thumb between freezing and room temperature. The potential moisture warmer air can hold may double, but the existing amount of moisture in the air often stays the same, and the relative humidity drops by half.