Factors Influencing The Rate of Evaporation
- Concentration of the substance evaporating in the air
- If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
- Concentration of other substances in the air
- If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
- Flow rate of air
- This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
- Inter-molecular forces
- The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the enthalpy of vaporization.
- Pressure
- Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.
- Surface area
- A substance that has a larger surface area will evaporate faster, as there are more surface molecules that are able to escape.
- Temperature of the substance
- If the substance is hotter, then its molecules have a higher average kinetic energy, and evaporation will be faster.
- Density
- The higher the density the slower a liquid evaporates.
In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches (3,000 mm) per year.
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