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.
Relative Humidity Affecting Evaporation
The rate of evaporation at a given place is always dependent on the humidity of that place because if the air is already filled with water vapor, it will not have any place to hold excess vapor and therefore, evaporation will occur at an extremely slow rate.
Let us consider an example. If you take two wet cloths and hang one cloth in ample sunlight while the other in a shaded area, which one of the two will dry faster? It will be the one that is in the sunlight because the air in that area is hot and the humidity is low. It will have more capacity to hold water vapor and thus the rate of evaporation will be faster. On the other hand, the cloth in the shaded area is surrounded by air that is cooler. As cold air has less capacity to hold water vapor than hot air, the rate of evaporation of water from the wet cloth will be slower too.
Now consider if both the hot and cold areas have the same relative humidity of 70 percent. In this case, when the two wet cloths are spread in these two different temperature regions, what will be the rate of evaporation? It will still be more in the hot region as hot air has more capacity to hold water vapor than the cold air.
This experiment gives a fair idea of how does relative humidity affect evaporation. It can be easily interpreted that if the humidity of a place is low, the rate of evaporation will be faster and if the humidity is high, the rate of evaporation will be low. Also two different regions may have the same relative humidity but the rate of evaporation of the two regions can be totally different depending upon the holding capacity of the air.
Vapor Pressure and Relative Humidity
Relative humidity is also sometimes defined as the ratio of actual vapor pressure of an area to the maximum vapor pressure. Vapor pressure is measured by the amount of water vapor in a given volume of air, and it increases as the vapor increases.
When the vapor pressure increases and reaches the maximum limit in a given volume of air, a state of equilibrium arises. In this state, an amount of water is getting evaporated and the same amount of vapor is getting condensed to form water again.
Thus, evaporation never really ceases to occur on any given surface of water. Depending on the relative humidity of the surrounding air, the evaporation rate increases or decreases or reaches an equilibrium state where evaporation and condensation are occurring at the same pace.
As humidity is dependent on temperature, wind and other factors, these factors affect evaporation too. There are a multitude of factors, including relative humidity, that govern the process of evaporation whether it is occurring at a small laboratory or over the surface of a huge water body like a lake or ocean.