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How Wind and Other Variables Affect Evaporation

written by: Dawn Marcotte•edited by: Sarah Malburg•updated: 3/18/2011

Students studying weather or the water cycle may ask how does wind affect evaporation and why is this important? When learning about water conservation students will also need to understand what other environmental factors effects evaporation.

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    Evaporation is the process where water is converted from liquid into gas. Understanding what effects evaporation is important to a better understanding of weather cycles, water cycles and the importance of water conservation. This better understanding should include how wind affects evaporation as well as how temperature and humidity impact the rate of evaporation.

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    How Does Wind Affect Evaporation?

    Commercial Windmill Approximately 80% of all water that falls as precipitation comes from water that evaporated from the ocean. The speed of wind is an important factor in the rate of this evaporation because the wind moves humid air away and pulls in drier air, thus increasing the rate of evaporation. The walls inside a cave where there is water are an example of the affect of the absence of wind. The walls in a cave that hold a stream or pool of water will be wet all the time due to condensation of water out of the air. Wind doesn’t move the wet air out of the cave so it continues to absorb water until it can’t hold any more. Once the air reaches saturation, the moisture condenses on the walls and eventually runs back into the source of water. A more familiar example occurs in a hot shower. The water evaporates into the air and creates steam. If there is no movement of air created by a fan or open window the water will quickly condense on the mirror. If there is an exhaust fan on, this wet air will be moved out of the room and the mirror will not fog up as quickly.

    The higher the speed of the wind the faster it moves drier air over the water.In the shower example, if the exhaust fan is powerful enough to pull the moist air out of the room quickly, the mirror may never fog up. However, if the air is already at 100% relative humidity, the wind speed won’t impact the rate of evaporation, as the air is already full. In the example of the shower, if the surrounding air is already at 100% humidity, the mirror will fog up immediately, regardless of how strong the exhaust fan is running. Humidity is one of the other variables that can affect evaporation.

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    Variables that Affect Evaporation

    The two other variables that impact the rate of evaporation are relative humidity and temperature. Relative humidity describes how much water is in the air relative to how much water it can hold. For example, if the air is currently holding half as much air as it can the relative humidity is 50%. The temperature of the air determines the amount of water the air can hold. Warmer air can hold more water than colder air. This is illustrated by a foggy morning in spring. If the ground is wet and the temperature rises, there may be a fog of water molecules until the air warms enough to be able to hold additional water molecules.

    Solar energy affects water evaporation by changing the temperature of the air and the water. Warmer water will evaporate faster as the molecules move faster. This is illustrated by boiling water. The steam that emerges is hot and will condense into water droplets if an object is held above the steam. This combination of evaporation, condensation and precipitation is also known as the water cycle. Solar energy also heats the air and warm air can 'hold' more moisture, which increases the rate of evaporation.

    Wind speed is one example of how wind affects evaporation. Air temperature, water temperature and relative humidity are the other factors affecting the rate of evaporation. Understanding these concepts will provide a background to better understand weather, global warming and the water cycle.

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    Resources and Photo Credits

    http://techalive.mtu.edu/meec/module01/EvaporationandTranspiration.htm

    http://van.physics.illinois.edu/qa/listing.php?id=1471

    http://atoc.colorado.edu/~englishj/Humidity.htm

    Photo Credit

    http://www.flickr.com/photos/warrenski/2529220364/ - by warrenski