Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent. Relative humidity is also approximately the ratio of the actual to the saturation vapor pressure. The dew point is the temperature at which air is saturated with water vapor, which is the gaseous state of water. When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating.
Question from the professor:
Isn't it the ratio of the water vapor pressure to the saturation water vapor pressure?