Editing Water vapor (section)

=== Evaporation ===
Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have [[Evaporation|evaporated]]. Each individual water molecule which transitions between a more associated (liquid) and a less associated (vapor/gas) state does so through the absorption or release of [[kinetic energy]]. The aggregate measurement of this kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the temperature of the water molecules. Liquid water that becomes water vapor takes a parcel of heat with it, in a process called [[evaporative cooling]].<ref>{{harvp|Schroeder|2000|p=36}}</ref> The amount of water vapor in the air determines how frequently molecules will return to the surface. When a net evaporation occurs, the body of water will undergo a net cooling directly related to the loss of water.

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.<ref>{{cite web|url=http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId%3D208 |access-date=April 7, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080412215652/http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208 |archive-date=April 12, 2008 |title=Geotechnical, Rock and Water Resources Library - Grow Resource - Evaporation }}</ref> The measurements range from under 30 to over 120&nbsp;inches per year. Formulas can be used for calculating the rate of evaporation from a water surface such as a swimming pool.<ref>{{cite web|url=http://www.thermexcel.com/english/program/pool.htm|title=swimming, pool, calculation, evaporation, water, thermal, temperature, humidity, vapor, excel|access-date=February 26, 2016}}</ref><ref>{{cite web |url=http://www.rlmartin.com/rspec/whatis/equations.htm |archive-url=https://web.archive.org/web/20080324232701/http://www.rlmartin.com/rspec/whatis/equations.htm |archive-date=March 24, 2008 |title=Summary of Results of all Pool Evaporation Rate Studies |publisher=R. L. Martin & Associates}}</ref> In some countries, the evaporation rate far exceeds the [[Precipitation (meteorology)|precipitation]] rate.

Evaporative cooling is restricted by [[Standard temperature and pressure|atmospheric conditions]]. [[Humidity]] is the amount of water vapor in the air. The vapor content of air is measured with devices known as [[hygrometer]]s. The measurements are usually expressed as [[specific humidity]] or percent [[relative humidity]]. The temperatures of the atmosphere and the water surface determine the equilibrium vapor pressure; 100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium vapor pressure. This condition is often referred to as complete saturation. Humidity ranges from 0&nbsp;grams per cubic metre in dry air to 30&nbsp;grams per cubic metre (0.03 ounce per cubic foot) when the vapor is saturated at 30&nbsp;°C.<ref>{{cite web|url=http://www.britannica.com/eb/article-53259/climate#292984.hook|title=climate - meteorology|website=Encyclopædia Britannica|access-date=February 26, 2016}}</ref>

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