relation between temperature relative humidity and evaporation
Answers
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.
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.