This is a function of phase relations between the liquid and vapor forms of water. Warm air overlying cooler water will warm the water. The bonds between the water molecules break as the velocity of the molecules increases and the liquid is converted to a gas phase. The addition of water molecules to the air increases the vapor density, and thus the absolute humidity, of the air mass. Relative humidity measures the amount of moisture in air in comparison to the maximum volume of moisture the air would contain when saturated.
Saturation represents the point where increasing vapor density results in condensation.
Adding water vapor increases the actual water vapor density. Reducing temperature decreases the saturated vapor density as colder air holds less moisture than warm air. Warm air that contains as much water as cold air has a lower relative humidity. As temperature falls the relative humidity of the air rises. The temperature at which condensation begins is termed the dew point .