What is the water cycle?

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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The water cycle is the process that links the movement of water through the environment.  At it's simplest, the water cycle consists of three steps: evaporation, condensation, and precipitation.  

Let's start with water that is on the ground (or in a body of water).  As light energy heats that water, the individual water molecules absorb enough energy to break away from each other and begin rising into the atmosphere.  That is evaporation. Those water molecules rise into the atmosphere and begin to cool.  Once the temperature reaches the dew point, the air is said to be saturated (100% humidity), and condensation will occur.  The water vapor will condense into either light weight water droplets or lightweight ice crystals.  Most people call those things clouds.  Condensation will continue until the condensed droplets are heavy enough to fall toward earth.  Falling water is called precipitation, and it falls in a variety of forms.  The most common would be rain, sleet, snow, and hail.  The water cycle will then start over again.  

Earlier I said that a three step water cycle was the simple version.  A five step version is much more accurate in describing the water cycle in its entirety.  The fourth step is runoff.  Water that is on earth's surface is not always stationary.  It moves.  It flows down hills and streets.  It moves along streams and rivers.  There are underground aquifers.   If the water cycle is defined as a process that moves water through the environment, runoff is a key piece.  

The fifth piece of the water cycle is called transpiration.  A lot of water is held within plants.  They need it to perform photosynthesis.  The plants also need carbon dioxide.  Plants get the gas by opening up little doors on their leaves called stoma.  It's a great system for gas exchange, but water vapor is also released through the stoma.  Transpiration is a special kind of evaporation.  It's water entering the atmosphere from plant life.  

Together transpiration, runoff, precipitation, evaporation, and condensation make up the water cycle. 

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mkoren | Middle School Teacher | (Level 3) Senior Educator

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The water cycle represents the movement of water from the earth, back to the atmosphere, and then back to the earth again.  Since the earth is about 70% water, there is constant evaporation of water from the earth into the atmosphere.  When this happens, water is in the form of water vapor.  As the water evaporates into the atmosphere, it eventually will become cooled.  Then, the water vapor will turn into liquid.  As more and more water vapor turns into liquid, this will cause clouds to form. When the clouds can't hold the water any longer, we get precipitation.  The liquid falls back to the ground in the form of rain, snow, sleet, or hail.  Eventually the cycle will repeat itself, and the process begins all over again.

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beateach | Elementary School Teacher | (Level 1) Educator

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In order to understand the water cycle there are a few main terms that you need to know. These include evaporation and transpiration, condensation, precipitation, and collection. The water cycle or hydrologic cycle is the sequence through which water evaporates into the atmosphere in the form of water vapor; it condenses forming liquid or solid precipitation that falls to the ground where it is collected in water bodies or underground aquifers to start the cycle over again.

Clouds form as the atmosphere becomes water-laden from the evaporation from water bodies and transpiration from plants. The water returns to the Earth’s surface as precipitation when the atmosphere becomes so water-laden that it cannot contain anymore. Precipitation in various solid or liquid forms such as rain, sleet, and snow occurs. The precipitation creates runs off which fills water bodies where it repeats the cycle by evaporating back into the atmosphere. The precipitation also percolates through openings in the soil that refills underground aquifers. These aquifers are usually positioned near large bodies of water thus allowing for more evaporation and cloud formation.

Image courtesy of an authorized educational download from USGS, Science for a Changing World.

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gsenviro | College Teacher | (Level 1) Educator Emeritus

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Water cycle is a biogeochemical cycle that details the movement of water through the biotic and abiotic components of our planet. It includes the various stages and processes which are responsible for water's movement on planet earth. 

The key processes included in water cycle are evaporation (from surface water bodies) and evapotranspiration (from plant leaves) which results in condensation (in atmosphere) and formation of clouds, precipitation (in the form of rain water, snow, etc.) on the surface of earth (including mountains), surface runoff (flow of water over surface), infiltration of water (into sub-surface) and interaction between surface and ground water bodies. 

And this cycle continuously moves the water (in its various states and forms) around on our planet. This continuous motion or cycling is responsible for maintenance of life and geological shaping of the planet.

Hope this helps.

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megamind-616 | Middle School Teacher | (Level 1) Associate Educator

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The water cycle is the recycling of water on Earth. It is the continued process of evaporation, condensation, and precipitation. 

As water is heated, molecules at the surface of the water gain enough kinetic energy to overcome the intermolecular forces (hydrogen bonds) of the body of water and become a gas, commonly known as water vapor. When evaporation occurs through the leaves of plants it is called transpiration. 

As the water vapor moves to higher altitudes, it cools and condenses onto dust particles. This is how clouds form! When the water droplets become large enough, they are pulled back to Earth as precipitation- rain, sleet, hail, snow, etc. The type of precipitation that falls depends on the temperature. 

After water falls to earth, it becomes runoff, groundwater, or a part of a larger body of water. 

Then the cycle begins again! 

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hafsa8244 | Student, Grade 9 | (Level 1) eNoter

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the earth has a definite amount of water which is 70% of it. The water is found in all three states of matter and is recycled through percipation evaporation etc. The process of the reusing of water is known as the water cycle

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sciencesolve | Teacher | (Level 3) Educator Emeritus

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Water cycle can be depicted as the process in which water takes a trip, changing its state during the journey it travels, on, above and below the Earth. Hence, since there is no established beginning of the water cycle, it could be considered that the cycle begins at the place where there exists the largest amount of water, the oceans. As the Sun heats the water, the water begins to evaporate and the water vapors are transported into atmosphere, by the currents of the air. Since the water vapors encounter cooler temperatures, high up, into atmosphere, they condense and they generate the clouds. Clouds particles, as they grow larger, they fall down on the Earth, as precipitations. The water fallen down on the Earth, returns, in a way or the other, into the oceans, where the water cycle begins.

Hence, the water cycle is a continuous process, that never stops. Without the existence of water cycle, the life on the Earth would not be possible.

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kcaputo277 | eNotes Newbie

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The water cycle is the process that water takes when it circulates from the land, to the sky, and back to the land again. There are a few major parts of the water cycle including evaporation, condensation and precipitation. 

Heat from the sun causes water on the Earth to turn from a liquid to a gas which is called evaporation. Once the water evaporates, it starts to rise into the sky and turns into water vapor. This water vapor forms the clouds that we see in the sky. As the water vapor starts to cool, it then turns from a gas into a liquid, as it becomes water again. When the water vapor turns into water as it cools, this is called condensation. Then the water falls from the sky through precipitation which includes snow, rain, and sleet. Then the oceans and lakes collect this water, and the process starts all over again as the water rises through evaporation. 

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