One basic measure of humidity is the dew point. That’s “dew” as in “d-e-w” or the water drops that appear on your lawn at night. The dew point is a temperature -- the point to which the air would have to cool for its water vapor to reach saturation -- so long as the barometric pressure and such don’t change. Unlike relative humidity, the dew point temperature doesn’t change when the air temperature does. If the dew point and air temperature come close to the same, though, something has to give. Having reached saturation, the water vapor starts to condense -- forming water drops as fog, dew or frost. Because the dew point can’t be lower than air temperature, the dew point in the afternoon -- in the absence of a frontal system -- is a good predictor of how low the night temperature can get.
Mary Knapp, Weather Data Library