WATER CYCLE The Water CycleBy Moira Whitehouse PhD
Before talking about the water cycle thereare some things we need to review. Remember that water is made up of molecules of oxygen and hydrogen atoms, H2O that are always moving. And how fast these molecules are moving determines whether the water will be a solid, a liquid, or a gas. And just how fast these molecules move is determined by heat energy, how hot or how cold they are.
•Of course, molecules are way too small tobe seen. But if they were as big as BB’s,molecules of water in the three states ofmatter might appear something like this: Solid Liquid Gas
•We will first look at our model of solidwater called “ice.” The molecules aretightly packed and vibrating in place.•When there is only a small amount of heatenergy the molecules are packed so tightlytogether that they form ice.
•Next, we have our model of liquid watermolecules.•When there is more heat energy, themolecules move about more vigorouslywithin their container.
•Finally, we have our model of molecules ofgaseous water called “water vapor.”•There is even more heat energy now, andthe molecules are zinging about wildlybanging on the walls of whatever containerthey are in, escaping where ever there is anopening.
•The important thing is that the amount ofheat energy determines whether our H2Omolecules are a solid, liquid or gas.•Adding or subtracting heat energy cancause water to change from one phase, likeice or liquid or gas, to another.•And this is exactly what happens in thewater cycle.•But before going into the water cycle, let’slook more closely at water vapor, water as agas.
Important facts about water vapor• Water vapor is water in the form of a gas.• Like most gases, water vapor is invisible.• Even though we cannot see them, airalways contains molecules of water vapormixed in with molecules ofnitrogen, oxygen, argon and carbon dioxide.
• Having learned that there is alwayssomewater vapor found in air, we should alsounderstand that: • The warmer the air the more water vapor molecules there are. • The cooler the air the fewer water vapor molecules there are.
Water molecules in air do not remain in onestate of matter or another; they are constantlychanging back and forth between stages.Stage changes between vapor, liquid and solidhappen because of temperature changes.When liquid water molecules gain heat energythey become water vapor (evaporation): whenwater vapor molecules lose heat energy theybecome a liquid (condensation). evaporation http://www.expla inthatstuff.com/st ates-of- matter.html condensation creative commons licence
Now let’s look at the water cycle.Although the water cycle is like a circlethat has no beginning or end, we willstart our study with: evaporation.Water on the Earth’s surface is continuallyevaporating—changing from a liquid to thegas called water vapor.
Even on a cool, dull day the energy from the Sunheats the surface of the sea causing some of thewater to change from a liquid into water vapor.This process is called EVAPORATION.
Here we see a diagram of some liquid water molecules gaining enough heatHowever, remember: energy from1. Water molecules are too the sun to small to see without a change into microscope molecules of2. Water vapor is invisible water vapor.
Nearly 90% of the the water vapor in ouratmosphere comes from the oceans, seas andother bodies of water (lakes, rivers, streams)through evaporation. Free photos for educational use http://www.oceansart.us/We know that water is evaporating from thewater shown above but we cannot see it.Water vapor is invisible.
The remaining 10% of the moisture found in the atmosphere is released by plants through transpiration—release of water vapor from leaves. http://ga.water.usgs.gov/eduWhile most of the water vapor for the water cyclecomes from evaporation, transpiration adds a lot ofwater vapor. For example, a cornfield 1 acre in sizecan transpire as much as 4000 gallons of water every
Now that we understand that there iswater vapor in the air, the next step tobe examined in the water cycle is: CondensationClouds form as the result ofcondensation—water vapor that is cooledchanges to liquid water.
This process of condensation is constantlyoccurring all around us as water vapormolecules in the air bump into cool surfaces.Because when water vapor moleculesstrike a cooler surface, they slow down andsometimes change into tiny droplets ofliquid water that look like “fog”.Over time, these tiny droplets of water jointogether forming larger and larger drops.
Water vapor hitting the sidesof a cold glass or waterbottle condenses (changesfrom a gas to a liquid). condensation
Flicker.com creative commons with attribution SophieG Flicker.com creative commons with attribution jessicafmAs time passes the water dropletsincrease in size.
More condensation http://www.epa.gov/ Water vapor condensing on a cold window.Flickr.com creative commons with attribtuion House of Sims
More condensation Water vapor condensing on a cold mirror.
More condensationWater vapor condensing on the cold car windshield.
More condensationWater vapor condensing on grass or a spider web--dew.
More condensation Like the “fog” on condensation the window or mirror, steam is made up of tiny water droplets. Steam is not water vapor.When the water vapor from the boiling waterhits the cooler air outside the tea kettle, itcondenses as tiny water droplets.
More condensationAt night sometimes, water vapor condensesas it comes in contact with the cool groundcreating fog—a cloud on the ground.
However, unlike fog, most clouds do not form on theground; they form high up in the sky where the air is is much cooler. As you move up in the atmosphere it becomes colder and colder.
Clouds—another product of condensation.As water vapor is cooled high in the atmosphere itcondenses into tiny water droplets—a cloud.Flickr.com creative commons Anosmia Here we see pictures of clouds high up in the sky.
Now we have to ask what causes this warm airthat is loaded with water vapor to rise ?Usually this “lifting” of warm air is caused byanother of our “cycles.” ConvectionThe tendency of hotter and thereforelighter (molecules further apart) liquids andgases to rise, and for colder, heavier(molecules closer together) liquids andgases to sink.
under the influence of gravity.colder, de colder,nser denser(heavier) (heavier)material materialsinks sinks hotter and therefore less dense (lighter) material rises
Here is convection in action.http://www.explainthatstuff.com/states-of-matter.htmlcreative commonslicence
Another demonstration of cool dry air sinking and pushing up warm moist air.Smoke from burning stick. Cool air (more dense) being pulled Warm air by gravity. (less dense) pulled less hard by gravity and therefore being pushed up by cold air. Burning candle warming air.
Now let’s look at how convectioncurrents on Earth are important inthe formation of clouds.
The Sun heats the ground.The ground then warmsthe air above it.Warm air, with lots of watervapor, rises and cools.As a result of thecooling, the water vaporin the air changes intotiny water droplets ofliquid water—a cloud isformed.
The tiny waterdroplets making upclouds are bigenough to be seenin large groups (likea cloud) but toosmall for gravity toovercome the risingwarm air. warm air gravity
Small, yes. But just how small? Thedroplets created from condensingwater molecules are so small thatmillions of cloud droplets are neededto produce a single rain drop. Theyare kept aloft by the slowly risingconvection currents.
Here you can see the rising warm air building up a thundercloud.
However, as the tiny droplets insidethe cloud collide, they join anddroplets grow in size. Finally water drops form.
Which brings us to the nextpart of the water cycle, Precipitation.
When thesedrops becometoo large forthe rising air tohold themup, they fall asrain(precipitation).
Getting this fallen water (precipitation)back to the rivers, lakes and oceans is alsopart of the water cycle. Water can taketwo paths back to larger bodies, one is onthe surface and the other is underground.Some precipitation that falls on theground flows over the land to streamsand rivers and finally to the ocean.This is called surface run off.
You can easily observe this surface run-off if itis water running down your driveway, alongthe curb of your street and into a storm sewer.
During a heavy rain, water flows over the soil filling creekswhich then flow into streams and finally into rivers. Thesurface runoff that flows into a creek is beginning its journeyback to the ocean. Surface run-off from a farm field in Iowa Wikipedia Creative Commons Attribution
http://www.cotf.edu/ete/modules/msese/earthsysflr/water.html permission for classroom noncommercial use
Some of the water from precipitationinfiltrates--soaks into--the ground. TINY DROPLETS OF LIQUID WATER http://www.dwenvironmental.com
Some of the water that soaks into the ground istaken up by plant roots. The rest flowsunderground into what is called groundwater. TINY DROPLETS OF LIQUID The WATER groundwater then very slowly moves toward the sea. http://www.dwenvironmental.com
A common demonstration to model the watercycle in shown in the diagram below--a teakettle with hot water and a spoon with icecubes held over a glass. Can you Ice cubes explain how this demonstration models the water cycle?
A good animated water cycle.Choose Water Cycle.Then choose view on line. http://www.dwi.gov.uk/children/teachers/dow nloads/downloads.htm
Other animations of the water cycle. http://epa.gov/climatechange/kids/water_cycl e_version2.html http://earthguide.ucsd.edu/earthguide/diagra ms/watercycle/ Reading passage on clouds http://www.weatherwizkids.com/
So far in our water cycle we have seen:• liquid water being evaporated (changedinto water vapor) from the surface, •water vapor being lifted by convection,• water vapor changed into water dropletsand forming clouds, •water droplets in clouds coalescing into larger and larger drops that finally fall back to earth as precipitation.