Evaporation and the Water Cycle

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For the water cycle to work, water has to get from the Earth's surface back up into the skies so it can rain back down and ruin your parade or water your crops or yard. It is the invisible process of evaporation that changes liquid and frozen water into water-vapor gas, which then floats up into the skies to become clouds.

Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

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Water cycle components  »  Atmosphere  ·  Condensation  ·  Evaporation  ·  Evapotranspiration  ·  Freshwater lakes and rivers  ·  Groundwater flow  ·  Groundwater storage  ·  Ice and snow  ·  Infiltration  ·  Oceans  ·  Precipitation  ·  Snowmelt  ·  Springs  ·  Streamflow  ·  Sublimation  ·  Surface runoff

 

Water evaporating from a power-plant cooling tower.

Cooling towers at a power-production facilityPower-generation plants, including Plant Bowen in Georgia, produce power by using heat (in this case, from burning coal) to convert water into steam. (Credit: Alan Cressler, USGS)

One very significant by-product of thermoelectric facilities is heat from the power-production equipment. Plants withdraw lots of waste, use it to cool the equipment, and then need to release used water back into the environment. Releasing hot water back into rivers would harm the ecology, so many power plants have tremendous cooling towers, where hot water is sprayed inside and evaporation is used to cool the release water before it goes back into the environment.

Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration.

A very small amount of water vapor enters the atmosphere through sublimation, the process by which water changes from a solid (ice or snow) to a gas, bypassing the liquid phase. This often happens in the Rocky Mountains as dry and warm Chinook winds blow in from the Pacific in late winter and early spring. When a Chinook takes effect local temperatures rise dramatically in a matter of hours. When the dry air hits the snow, it changes the snow directly into water vapor, bypassing the liquid phase. Sublimation is a common way for snow to disappear quickly in arid climates. (Source: Mount Washington Observatory)

 

Why evaporation occurs

Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212° F, 100° C) but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. A state of saturation exists when these two process rates are equal, at which point the relative humidity of the air is 100 percent. Condensation, the opposite of evaporation, occurs when saturated air is cooled below the dew point (the temperature to which air must be cooled at a constant pressure for it to become fully saturated with water), such as on the outside of a glass of ice water. In fact, the process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you.

 

Evaporation drives the water cycle

Evaporation from the oceans is the primary mechanism supporting the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70 percent of the Earth's surface is covered by the oceans) provides the opportunity for large-scale evaporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air. The process of evaporation is so great that without precipitation runoff, and groundwater discharge from aquifers, oceans would become nearly empty.

 

People make use of evaporation

Salt obtained from evaporation ponds in India

Your table salt might have come from an evaporation pond. (Credit: Wikipedia, Creative Commons)

If you ever find yourself stranded on an island in need of some salt, just grab a bowl, add some seawater, and wait for the sun to evaporate the water. In fact,one way to produce table salt is to evaporate saline water in evaporation ponds, a technique used by people for thousands of years.

Seawater contains other valuable minerals that are easily obtained by evaporation. The Dead Sea is located in the Middle East within a closed watershed and without any means of outflow, which is abnormal for most lakes. The primary mechanism for water to leave the lake is by evaporation, which can be quite high in a desert—upwards of 1,300 - 1,600 millimeters per year. The result is that the waters of the Dead Sea have the highest salinity and density (which is why you float "higher"; when you lay in saline water) of any sea in the world, too high to support life. The water is ideal for locating evaporation ponds for the extraction of not only table salt, but also magnesium, potash, and bromine. (Source: Overview of Middle East Water Resources, Middle East Water Data Banks Project).

 

Evaporative cooling: Cheap air conditioning!

Map of U.S. showing how evaporative coolers work best in the dry areas of the United States (western U.S.)

(Credit: California Energy Commission)

We said earlier that heat is removed from the environment during evaporation, leading to a net cooling; notice how cold your arm gets when a physician rubs it with alcohol before pulling out a syringe with that scary-looking needle attached. In climates where the humidity is low and the temperatures are hot, an evaporator cooler, such as a "swamp cooler" can lower the air temperature by 20 degrees F., while it increases humidity. As this map shows, evaporative coolers work best in the dry areas of the United States (red areas marked A) and can work somewhat in the blue areas marked B. In the humid eastern U.S., normal air conditioners must be used.

 

 

 

A dog wearing a cooling vest.

Here is Fido, looking both sharp and cool as he sports the latest in fashionable dog apparel that also keeps him cool on a hot day. Fido is wearing a "cooling vest", where the owner wets it down, places it on the dog, and the properties of the evaporation process help the dog stay comfortable. (Credit: Wikimedia, Creative Commons)

Fido supports evaporative cooling

Yes, swamp coolers aren't just for homes anymore. After all, the evaporative process is just as happy keeping a dog cool as it is keeping a house cool. Evaporative coolers are really quite simple devices, at least compared to air conditioners. Swamp coolers pull in the dry, hot outdoor air and pass it through an evaporative pad that is kept wet by a supply of water. In a home device, a fan draws the air through the pad, the water in the pad evaporates, resulting in cooler air which is pumped through the house. Much less energy is used as compared to an air conditioner. (Source: California Energy Commission)