Evaporation Mechanism at Night
Date: Fall 2012
We understand that the sun powers the water cycle, but that other factors, like wind, can also cause water to evaporate. My students wanted to know about evaporation at night. Is this driven by wind? Temperature differential between water and air? Any insights into the mechanism of nighttime evaporation would be greatly appreciated.
As long as the humidity is less than 100%, water can evaporate. Wind allows more evaporation to take place, because it introduces new unsaturated air next to the water. As to what "drives" it, well, entropy is what drives everything.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Wind is one factor that explains the evaporation of water at night. The other factor is that there is still heat remaining in the surface water due to the daylight sun. This residual heat drives the evaporation. The technical reason for this is that the heat capacity of water is large (4.184 J/g deg. C) compared to other liquids. Yes, there is a temperature differential between the air and the water.
The mechanisms for evaporation at night are basically the same as during the daytime.
Evaporation tends to be greater during the day because sunlight warms the surface - it takes energy to evaporate water (to change it from the liquid to the gaseous state). There is more available energy for evaporation during the day than at night when there is no sunlight.
During nighttime the energy for evaporating water comes from the air itself or from a warm surface on which the water sits (soil, plants etc.). Evaporation can only occur if the vapor pressure of water at the surface from which the water is evaporating is greater than the vapor pressure in the air. If the vapor pressure of the air is greater, condensation occurs instead, in the form of dew or frost.
During both day and night, higher wind speeds enhance evaporation by producing turbulence that can carry evaporated water from the surface into the air, thereby creating an increase in the difference between the vapor pressure at the surface and that in the air.
David R. Cook
Atmospheric and Climate Research Program
Environmental Science Division
Argonne National Laboratory
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Update: November 2011