What happens to the rate of evaporation as relative humidity increases?

As relative humidity increases, the rate of evaporation decreases. This relationship can be understood through the concept of vapor pressure. When relative humidity is high, the air already contains a significant amount of water vapor, which means the difference in concentration between the liquid state (water) and the gaseous state (water vapor) is diminished.

Evaporation is the process where molecules at the surface of a liquid gain enough energy to enter the gas phase. If the surrounding air is already saturated with water vapor (high relative humidity), fewer water molecules from the liquid can escape into the air because the driving force, which is the gradient of vapor pressure, is reduced. Thus, the ability of the air to absorb more water vapor is lessened, resulting in a decreased evaporation rate.

In contrast, low relative humidity signifies that the air can hold more water vapor, which generally leads to a higher rate of evaporation. Hence, as relative humidity rises, the process of evaporation slows down due to the reduced capacity of the air to accept additional moisture.