Understanding Evaporation and Relative Humidity
To understand the effect of relative humidity on evaporation, it is important to understand the two conditions first.
Evaporation – The process of conversion of liquid water into vapor form is called evaporation. It occurs constantly in nature as water from water bodies like lakes and rivers vaporize and become part of the atmosphere. There are many conditions that govern the amount of evaporation that occurs in a given location. It is dependent on the amount of water present, wind, the temperature, and the amount of water vapor that is already present in the air.
Relative Humidity – The amount of water vapor in a given volume of air divided by the amount of air in that volume is called humidity. It depends on the temperature as higher temperature can increase the rate of evaporation and thus the amount of water vapor in the air. At a given temperature, there is a maximum amount of water vapor that air can hold and when the maximum capacity is reached the process of condensation begins.
Relative humidity is the ratio of the current humidity of the air to the maximum amount of vapor air can hold at a given temperature. It is related to the temperature. This ratio is given in percentage and therefore, if it is said that the relative humidity or RH is 40 percent, it means that the air contains 40 percent of the maximum vapor it could possibly hold at a temperature.
Because cold air has less maximum capacity to hold water vapor than hot air, a cold region with RH 70 percent will have less water vapor than a hot region with the same RH 70 percent.