Dear Tom,
Why is the relative humidity not 100 percent when it rains? I have noticed that it rarely is. Can this be correct?
Thomas Morgan, Chicago
Dear Thomas,
Most of the time when it is raining the relative humidity is 90-99 percent, but not often is it 100 percent.
Clouds form when invisible water vapor gas condenses into visible water droplets, and rain occurs when droplets grow too large to be supported in the air and they fall. Condensation and raindrop formation occur within clouds where the air is saturated and the relative humidity is, indeed, 100 percent.
However, most of the time clouds and saturated air lie above drier air at the ground (where we live). Measurements of relative humidity that you hear reflect the actual moisture content of the air at ground level — in drier air that is simply being invaded by rain from above.
