13. To find the absolute age of a rock, scientists use .A radioactive uranium C the principle of superpositionB relative dating D crosscutting relationships

14. To find the relative age of a rock, scientists use .A carbon-14 C the principle of superpositionB radioactive uranium D half-lives

15. During the Era, dinosaurs and other reptileswere the major forms of life on the earth.A Mesozoic C PrecambrianB Paleozoic D Cenozoic

16. A radioactive element decays to form .A an index fossil C carbon-14B petrified wood D another element

17. According to the principle of crosscutting relationships, arock feature that cuts across other rock layers isthan the rock layers.A older B younger C harder D sharper

18. During the Cenozoic Era, the Great Lakes basins werecarved out by .A glaciers B rivers C wind D mudslides

Critical ThinkingWrite the answer to each of the following questions.

19. In the diagram, which lettered rocks are younger than rockD? Explain your answer in terms of two scientific principles.

20. An organism dies and becomes a fossil. Later scientistsdiscover the fossil and find it contains 100 units of carbon-14 and 300 units of nitrogen-14. How many half-lives haveoccurred? How many years ago did the organism die?

Before you begin a test, look it over quickly. Try to set asideenough time to complete each section.

A Record of the Earth's History Chapter 13 313

Chapter

The Earth's Water

Earth is called the water planet for a good reason. Morethan 70 percent of the earth's surface is covered withwater. Water is also in the atmosphere and under the

ground. All of this water is continuously moving. For example,ocean water evaporates into the air. Clouds gather and carry thismoisture. Eventually, water droplets in clouds fall back to theearth. In Chapter 14, you will learn how water moves andchanges. You will also learn about different bodies of water,such as rivers, lakes, and oceans.

Organize Your Thoughts

Water cycle

Locations

AtmosphereGround water

Rivers and lakesOceans

Processes

EvaporationCondensationPrecipitation

Runoff

Goals for Learning

To explain the water cycle

To compare fresh water and salt water

To describe the sources and movement of fresh water

To explain the water table

To describe ocean water, waves, currents, and theocean floor

To identify three major groups of ocean life

314

Lesson The Water Cycle

Objectives

After reading thislesson, you shouldbe able to

+ describe themovement ofwater throughthe water cycle.

+ explain howgroundwatermoves and formsthe water table.

+ describe springs,geysers, and caves.

+ describe howrunoff createsrivers, drainagebasins, and lakes.

+ identify threepurposes ofreservoirs.

Water cycleMovement of waterbetween the atmosphereand the earth's surface

GroundwaterWater that sinks intothe ground

RunoffWater that runs overthe earth's surface andflows into streams

Water is everywhere. Almost all of the earth's water is in theoceans. But it is also in rivers and lakes, under the ground, inthe air, and even in your own body.

Earth's water is in continuous motion. It moves from theatmosphere to the earth's surface and back to the atmosphere.This movement of water is called the water cycle. Study thediagram below and notice the different forms that water takesas it goes through a complete cycle.

The water cycle is powered by the sun. Heat from the sunevaporates surface water, and the water vapor rises into theatmosphere. The rising water vapor cools and condenses intoclouds. Water droplets or ice crystals in the clouds grow larger,then fall to the earth as precipitation.

What happens after precipitation falls? Some of it sinks intothe ground and becomes groundwater. This water collectsin the spaces between rocks and moves slowly underground;Precipitation that does not sink into the ground is called surfacewater. Some surface water evaporates. But most of it becomesrunoff—surface water that flows over the land and into streamsand rivers.

The Earth's Water Chapter 14 315

As water travelsthrough the watercycle, it carriesdissolved gasesand minerals.For example,lake and oceanwater containsdissolved oxygengas. That is whatfish "breathe"through their gills.

Why doesn't all precipitation sink into the ground? There arethree main reasons.

1. The ground may be saturated, or completely soaked,and unable to hold any more water. It is like pouringwater on a sponge. Eventually, the sponge fills and waterruns off it.

2. On a slope, the water may run off too quickly to sink in.

3. The ground may not have enough vegetation to stopthe water from flowing elsewhere. Plants and theirroots soak up water.

Eventually, surface water evaporates or rivers carry it to theoceans. If you have ever tasted ocean water, you know it is muchtoo salty to drink. Salt water also cannot be used for farms andindustry. Salt water kills most land plants and ruins machinery.In Lesson 3, you will learn more about the properties of salt water.

Like the water on land, ocean water evaporates and moves backinto the atmosphere. Dissolved salts are left behind, however.So the water that condenses in the atmosphere and falls ontothe land contains no salt. It is fresh water.

Science In Your Life

How much water do you use?Probably more than you think. The table lists the average amount of water used fordifferent tasks. Estimate how much water you have used so far today. Think about waysto cut down on the water you use. Then make a water budget by planning the amountof water you will "spend" each week.

Water Uses

Task

drinking water

flushing a toilet

taking a shower

taking a bath

washing hands

watering a lawn

Average Amount Used

2 liters per person per day

11 to 19 liters per flush

19 liters per minute

133 to 152 liters per bath

1 Il lci

91 2 liters per half hour

316 Unit 2 Enrth Science

PorousContaining many spacesthrough which air andwater can move

Water tableTop of the groundwater

layer

SpringPlace wheregroundwater flowsnaturally out of theground

GeyserPlace where hotgroundwater and steam

blast into the air

SinkholeFunnel-shaped

depression thatresults when theroof of a cave collapses

Sources of Fresh WaterFresh water is an important resource. Think of the many waysyou use it every day, such as for drinking, washing, and cooking.Farms and industry, however, use 90 percent of the fresh waterconsumed in the United States. Fresh water can be found inmany places, both above and below the ground.

GroundwaterGroundwater starts as precipitation or runoff that soaks intothe earth. The water can sink into the ground because most soilis porous, or has spaces between its particles. Loose soil, suchas sandy soil or soil with a lot of decayed plant material, is veryporous. The rocks beneath the soil may also be porous. Watertrickles around broken rock pieces and through cracks.

As water continues downward, it comes to a solid rock layerthrough which it cannot move. Groundwater collects on top ofthe rock layer, filling the spaces above it. The top of this wetearth layer is the water table. If you drill a well down past thewater table, water flows into the well and can be pumped to thesurface. About half the dr inking water in the United Statescomes from groundwater.

Springs, Geysers, and CavesThe water under the ground is moving. Groundwater flowsout of the ground as a natural spring. Certain springs, calledgeysers, shoot water and steam into the air. Geysers occurwhere groundwater lies close to hot rock or magma. Pocketsof groundwater are heated and turned to steam. The steamrises, pushing the hot water above it. The steam and watererupt as a geyser.

Moving groundwater may seep through cracks in limestone,dissolving the rock and forming caves. If the roof of a cavecollapses, a funnel-shaped sinkhole forms. Sinkholes may fillwith groundwater and rain to become ponds.

The Earth's Water Chapter 14 317

TributaryRiver that joins another

river of equal or greatersize

Drainage basinLand area that isdrained by a riverand its tributaries

DivideRidge thai separatesdrainage basins

Rivers and Drainage BasinsRivers are important sources of fresh water. They provide muchof the water that people use every day. Yet rivers make up atiny percent of the earth's water. Only 3 percent of the earth'swater is fresh water. Only 1 percent of all fresh water is notfrozen or underground. Of this available fresh water, less than 1percent is in rivers.

Rivers begin as runoff that moves over the land, carving smallpaths in the ground. These paths get wider and deeper as watercontinues to flow through them. The paths become streams.They always flow downhill because of gravity. The streams joinand become rivers. These rivers then join and form even largerrivers. Rivers that join other rivers are called tributaries. Noticethe rivers on the map below. You can see how water andsediment in the most distant tributaries end up in the main river.

The land area in which runoff drains into a river and itstributaries is a drainage basin. The map shows five drainagebasins. The Mississippi-Missouri River basin covers about40 percent of the United States. Notice how rain that falls inMontana can eventually reach the Gulf of Mexico. Ridges thatseparate drainage basins are called divides. One divide runsalong the Rocky Mountains. Rivers east of this divide flowinto the Gulf of Mexico. Rivers west of this divide flow intothe Pacific Ocean. What other divide is shown?

318 Unit 2 Earth Science

ReservoirArtificial lake created

by placing a dam acrossa river

TechnologyNote

Electricity isproduced bygenerators atpower plants.Most powerplants burn coalor another fuelto run thegenerators. Buthydroelectricpower plants ordams use waterpower. About10 percent of U.S.electricitycomes fromhydroelectricdams. Canada,the United States,and Brazil are thetop hydroelectricpower producersin the world.

LakesSurface water does not always flow along a path. Some of itcollects in depressions, or low areas. Water eventually fills thedepressions, forming lakes. Even though some of the waterevaporates, lakes continue to be fed by precipitation, runoff,springs, and rivers. Many lakes also lose water throughoutflowing streams or moving groundwater.

Lakes arc many different sixes. For example, some lakes inWisconsin are only a few meters deep. You can hear peopletalking from the opposite shore. The Great Lakes, on the otherhand, are so wide that you cannot see across them. Lake Superioris the largest freshwater lake in the world. Its deepest point isabout 400 meters. Many of the lakes in the northern UnitedStates and Canada formed when huge sheets of moving icecarved out depressions.

ReservoirsMany cities store large supplies of fresh water in artificial lakescalled reservoirs. Reservoirs are made by constructing damsalong rivers. Water backs up behind the dam, turning part ofthe river into a lake. Reservoirs serve several purposes.

Reservoirs store water for home use, fanning, and industry. Thiswater can be piped to dry areas. Much of southern California'swater, for example, comes through a canal from Lake Havasu, areservoir behind Parker Dam on the Colorado River.

Reservoirs control flooding. During periods of heavy rain andrunoff , a reservoir may fill up. This water can be released slowlyand safely downstream through gates in the dam.

Reservoirs produce electricity. In a hydroelectric dam, thewater moves through generators near the bottom of the dam.The rushing water turns the blades of a turbine, which makeselectricity.

The Earth's Water Chapter 14 319

Lesson 1 R E V I E W

Write your answers to these questions on a sheet of paper.

1. How does water move between the atmosphere,the land, and bodies of water?

2. What is the difference between groundwater andsurface water?

3. What is a water table?

4. How can runoff on a mountain end up in an ocean2,000 kilometers away?

5. How are reservoirs useful?

A Y-* A A Y •< A Y •« A Y •« A Y •* A Y •« A Y •« A Y •< A Y ^

Science at Work

Hydroelectric Power Plant OperatorHydroelectric power plant operators manage an entire plant and supervise manypeople. They control the water flow in the dam and the machinery that generateselectricity. This includes gates, valves, turbines, and generators.

Plant operators adjust the plant'spower output to meet changingelectricity demands. They check water,voltage, and electricity flows. Plantoperators maintain and repairequipment. They prepare reportsabout equipment or performance.

Hydroelectric power plant operatorsmust have at least a high-schooldiploma. College-level courses inmechanical or technical fields arehelpful. Computer, math, and scienceskills are important. A good plantoperator has mechanical ability, isresponsible, and understandsequipment and safety procedures.

Plant operators must be willing to work under tiring conditions. Their job requiresconstant attention. They may spend hours sitting or standing at control stations. Theyalso may maintain buildings, grounds, and access roads.

320 Unit 2 Earth Science

14 I N V E S T I G A T I O N

Materials• 3 plastic

petri dishes

• plastic eyedropper

• smallcontainerof water

• petri dishcover

• lamp withat least a60-watt bulb

• clock orstopwatch

• paper towels

Exploring Evaporation

PurposeHow does heat affect evaporation? In this investigation, youwill discover factors that cause evaporation.

Procedure1. Copy the data table on your paper.

Time

start

time ofevaporation

UncoveredDish

CoveredDish

Uncovered Dishwith Lamp

2. Place the petri dishes on a tabletop. Use the dropperto place one drop of water in the center of each dish.

3. Place a cover over one of the dishes.

4. Move one of the uncovereddishes at least 50 centimetersaway from the other two.Position the lamp directlyover this dish, as shown.

The Earth's Water Chapter 14 321

5. Turn on the lamp. If you have a clock, record the time.If you have a stopwatch, start the watch. Safety Alert:The lamp will become very hot. Do not touch the bulbor the lamp.

6. Observe the three dishes every two minutes. Record thetime when each drop evaporates.

7. Clean your work area and return the equipment.

Questions and Conclusions1. Which drop of water took the longest time to evaporate?

2. Which drop took the shortest time to evaporate?

3. What conclusions can you make about the factors thataffect evaporation?

4. What predictions can you make about the evaporationrate on a hot, sunny day and on a cool, cloudy day?

Explore FurtherDesign an experiment to find out how wind affectsevaporation. How could you model wind blowing overa petri dish? How could you vary the amount of windmodeled?

322 Unit 2 Earth Science

Lesson Oceans

Objectives

After reading thislesson, you shouldbe able to

+ identify twoproperties ofocean water.

^ explain whatcauses oceanwaves andcurrents.

+ describe severalfeatures of theocean floor.

+ identify threemajor groups ofocean life.

SalinitySaltiness of water

On average,one cubic meterof ocean watercontains about35 kilogramsof salt.

Salt 3.5%

Properties of Ocean WaterThe water in the oceans is saltwater. The circle graph showswhy. Notice that 96.5 percent

of ocean water is pure water.But 3.5 percent is dissolvedsalt. That amount of salt makesa mouthful of ocean water saltierthan a mouthful of potato chips. Most of the salt is sodiumchloride—common table salt. This salt comes from rocks inthe ocean floor. Salt also washes into oceans from rivers.

Not all parts of oceans are equally salty. The saltiness, or salinity,of ocean water varies. In warm, dry climates, ocean waterevaporates quickly. Since salt doesn't evaporate, the salt thatremains makes the salinity greater than average. In some oceans,the salinity is less than average. This happens in rainy climatesor where rivers andmelting ice add freshwater to oceans.

Ocean water iswarmest at thesurface where thesun heats it. Near theequator, the surfacetemperature canreach 30°C. Nearthe poles, the oceansurface is frozen.The diagram showsaverage oceantemperatures in atropical or temperatezone. Notice how thewater temperaturedecreases with depth.

5OO -

1,OOO

0-OJQ

2,OOO

3,OOO

4,OOO

Warm surface water(25 to 20 C)

Thermocline(20 to 4!)C)

The Earth's Water Chapter 14 323

ThermoclineOcean layer betweenabout 300 and1,000 meters belowthe surface, wherethe temperaturedrops sharply

WaveUp-and-down motionof water earned by windenergy moving throughthe water

Along the edgesof oceans aresmaller bodies ofsalt water. They arecalled gulfs, seas,or bays. The Gulfof Alaska is part ofthe Pacific Ocean.The MediterraneanSea is part of theAtlantic Ocean.The word sea canalso be used in ageneral sense tomean "ocean."

The temperature is fairly constant near the surface becausewinds and waves keep the water well-mixed. However, in thethermocline, between 300 and 1,000 meters below the surface,the temperature drops sharply. Below the thermocline, thetemperature decreases slowly. The bottoms of oceans are nearfreezing.

Ocean WavesWhen you think of oceans, you probably picture waves. A waveis the regular up-and-down motion of water caused by energytraveling through the water. A wave gets its energy from wind.When the wind blows, it pushes up the water to start smallwaves. The waves become larger as the wind blows longer andharder. Most ocean waves are less than three meters high.However, storms can produce waves as high as 30 meters—theheight of a 10-story building. No matter what the size, all waveshave the parts shown below.

CrestCrest

(highest point of the wave)Wavelength

(distance between two crests)

Trough ^=(lowest point of the wave)

Wave heightnee from trough to crest)

Have you ever seen a leaf bob up and down on passing waves?The waves move forward, but the leaf does not. Although itlooks like waves constantly push water forward, the watergenerally stays in the same place. Only the waves move forward.

As a wave approaches shallow water and a shore, the wave rubsagainst the ocean floor. Friction slows the bottom of the wave,but the crest keeps moving at the same speed. Therefore, thecrest moves ahead of the rest of the wave. The wave tilts forwardand tumbles over, or breaks. After a wave breaks on a shore, thewater can actually move quite a distance. It may be hurled againstrocks or pushed up the slope of a beach.

324 Unit 2 Earth Science

CurrentLarge stream of waterflowing in oceans, inrivers, and in some

large lakes

Ocean CurrentsAlthough waves do not move water, currents do. Ocean currentsare large streams of water flowing in oceans. Winds cause currentsnear the ocean surface. Therefore, currents tend to follow themajor wind belts. On the map below, trade winds and prevailingwesterlies are shown as wide arrows. Major ocean currents areshown as thin arrows. Compare the trade winds with the currentsnear the equator. Both move westward.

Currents carry warm water from the equator toward the polesand bring cold water back toward the equator. In so doing,currents affect climates on land by warming or cooling thecoasts of continents. Both wind and land absorb heat fromwarm ocean currents.

The Gulf Stream and the North Atlantic Drift are currents thathave a warming effect. Find these currents on the map. TheGulf Stream carries warm water from the tropics up along theeast coast of North America. The North Atlantic Drift carriesthis warm water across the Atlantic. It gives western Europemild summers and winters.

Now locate the California Current on the map. This currentcarries cold water from high latitudes. It has an "air-conditioned"effect along the west coast of the United States and Mexico.

The Earth's Water Chapter 14 325

Continental shelfPart of a continentthat extends froma shoreline out intoan ocean

ContinentalslopeSteep slope between thecontinental shelf andthe deep ocean floor

Mid-ocean ridgeMountain chain on theocean floor

SeamountUnderwater mountainthat is usually a volcano

TrenchDeep valley on theocean floor

The Ocean FloorUntil the late 1800s, the ocean floor was a great mystery. Todayoceanographers use complex technology to measure distancesand take pictures underwater. They also travel to the oceanfloor. The main features of the ocean floor are listed below andshown in the diagram.

^ A continental shelf is the part of a continent thatextends underwater. A continental shelf slopes gently.The average water depth is 130 meters. The averagewidth is 75 kilometers.

+ A continental slope dips sharply from a continentalshelf to the deeper ocean floor.

^ Plains are wide, flat areas where sediment constantlysettles. About half of the ocean floor consists of plains.Their average depth is about 4,000 meters.

+ A mid-ocean ridge is an underwater mountain chain.Such a chain may extend for thousands of kilometersalong the ocean floor.

+ A seamount is an underwater mountain. Many of theseare active or extinct volcanoes. A seamount that risesabove sea level forms an island.

^ A trench is a long, deep valley. Trenches are the deepestplaces on the earth. Some are 10 kilometers deep.

Island Seamount

326 Unit 2 Earth Science

PlanktonTiny organisms that liveat or near the oceansurface

NektonFree-swimming oceananimals

BenthosOrganisms that live onthe ocean floor

The ocean floor iscompletely flat.

Fact: The oceanfloor includes highmountain rangesthat are longer thanthose on the earth'ssurface. The oceanfloor also includescanyons that arethousands of metersdeep.

Ocean LifeOcean environments support a rich variety of living things.Scientists divide these forms of life into three groups, based onhow and where they live. Look at the cross section of ocean lifebelow. Which group provides most of the seafood people eat?

Plankton are one form of life in oceans. This group includestiny plants and animals that float at or near the ocean surface.Plankton are a source of food for larger animals.

Animals that swim freely are classified as nekton. This groupincludes the widest variety of sea creatures, from the tiniestfish to the largest whale.

Organisms that live on the ocean floor are called benthos.They do not swim. Some, such as corals, remain in one placetheir whole lives. Others, such as snailsand crabs, crawl along the ocean floor.

Plankton

Nekton

Benthos

The Earth's Water Chapter 14 327

Lesson 2 R E V I E W

Write your answers to these questions on a sheet of paper.

1. What are two sources of salt in ocean water?

2. Describe how water temperature changes with ocean depth.

3. What causes most ocean waves and currents?

4. How do currents affect climate?

5. What are several features of the ocean floor?

Protecting the Environment

Today most people know the importance of clean water,land, and air. However, protecting the environment hasnot always been a popular concern. Before 1960, there werefew laws about what people could dump in rivers, on land,or into the air. Thousands of people died from smog, orpolluted air, given off by factories in London and in U.S.cities. The Cuyahoga River in Cleveland often caught on firebecause of the oil floating in it. Many fish in Lake Erie diedbecause of pollution.

By 1960 the public began to understand how chemicals andother wastes could affect their health and the environment.Rachel Carson was a biologist who helped promote thisawareness. In 1962 she wrote a book that warned aboutthe dangers of pest-killing chemicals, or pesticides, on theenvironment. Her book, Silent Spring, had a strong impacton its readers. In fact it led to the banning of DDT and otherpesticides.

In 1970 the U.S. Environmental Protection Agency wascreated to protect people and the environment. Now federaland state laws exist to preserve and improve the quality ofour water, land, and air.

328 Unit 2 Earth Science

Chapter 1 4 S U M M A R Y

Water moves between the land,the atmosphere, and bodies ofwater in the water cycle.

The earth's water includes saltwater, which is too salty todrink, and fresh water, whichdoes not contain salt. Most ofthe earth's water is salt water.

Water under the earth's surfaceis called groundwater.

Springs, geysers, and caves areevidence of moving groundwater.

Groundwater moves downwardin the ground and collects toform a soaked layer, the top ofwhich is called the water table.

Rivers and their tributariesdrain runoff from large areasof land called drainage basins.

Lakes form when water collectsin a depression on land.

Reservoirs are lakes made whenpeople dam a river. Reservoirsstore water, control flooding,and produce electricity.

Ocean water is salt water becauseit contains dissolved salt

The temperature of ocean waterdecreases with depth.

A wave is the up-and-downmotion of water caused byenergy from the wind.

Currents move ocean water.Currents are caused by windsand follow the same generalpattern as global winds.

Features of the ocean floorinclude continental shelvesand slopes, mid-ocean ridges,trenches, seamounts, and plains.

Ocean life includes floatingplankton, free-swimmingnekton, and ocean floor-dwelling benthos.

Science Words

benthos, 327continental

shelf, 326continental

slope, 326current, 325divide, 318

drainage basin, 318geyser, 31 7groundwater, 315mid-ocean

ridge, 326nekton, 327plankton, 327

porous, 31 7reservoir, 31 9runoff, 315salinity, 323seamount, 326sinkhole, 31 7spring, 31 7

thermocline, 324trench, 326tributary, 318water cycle, 31 5water table, 31 7wave, 324

The Earth's Water Chapter 14 329

Word Bankcontinental shelf

drainage basin

geyser

mid-ocean ridge

salinity

trench

tributary

water table

Chapter 1 4 R E V I E W

Vocabulary ReviewChoose the word or phrase from the Word Bank that bestcompletes each sentence. Write the answer on your paper.

1. A river that flows into another river is a .

2. The land area in which runoff flows into a river and itstributaries is a ..

3. Underground water forms a soaked layer, the top of whichis the .

4. Heated groundwater blasts out of the ground at a .

5. A deep valley on the ocean floor is called a __.

6. A mountain chain on the ocean floor is called a

7. Water with more salt has greater than waterwith less salt.

8. A extends from a shoreline out into an ocean.

Concept ReviewChoose the word or phrase that best completes each sentence.Write the letter of the answer on your paper.

9. In the water cycle, water moves from oceans to theatmosphere by __,A evaporation C precipitationB condensation D runoff

10. The water cycle is powered by .A oceans and rivers C the water tableB precipitation D the sun's heat

11. Precipitation that doesn't evaporate or sink into thegro u n d .A is not part of the water cycleB becomes groundwaterC flows out of the ground as a springD flows into streams as runoff

330 Unit 2 Earth Science

12. As ocean water deepens, its temperature .A gets warmer C stays the sameB gets colder D varies

13. Artificial lakes that supply fresh water, control flooding,and produce electricity are called __ .A water tables C drainage basinsB reservoirs D tributaries

14. The top of a wave is the _,A trough B wave height C crest D seamount

15. Ocean currents are caused by __ .A waves B tides C winds D runoff

16. Underwater mountains, or , may rise abovethe ocean's surface to form islands.A seamounts B trenches C ridges D plains

17. Moisture that sinks into the ground is called .A plankton C surface waterB groundwater D a reservoir

Critical ThinkingWrite the answer to each of the following questions.

E 18. Refer to the diagram. Nameeach lettered feature of theocean floor.

19. After a dry summer, waterno longer comes up througha well. What has happened?

20. One way to make salt water fit to drink is to heat it andcollect the water vapor. How is this like the water cycle?

Before writing out an answer on a test, read the questiontwice to make sure you understand what it is asking.

The Earth's Wafer Chapter 14 331

Chapter

The Earth'sAtmosphere

The earth's atmosphere is all around us. The air webreathe is part of the atmosphere. The clouds in the skyare part of the atmosphere. Rainbows remind us that

the atmosphere contains moisture. When sunlight passesthrough water droplets in the air, the different colors that makeup the light separate. We see this separation of light as arainbow. In Chapter 15, you will learn about the gases andlayers that make up our atmosphere. You will also learn aboutclouds, precipitation, and wind patterns.

Organize Your Thoughts

Clouds

Precipitation

Layers Wind cells

Goals for Learning

To explain what the earth's atmosphere is

To explain how gases in the air cycle through theenvironment

To describe the structure of the atmosphere

To classify clouds

To explain how precipitation forms

To describe the earth's wind patterns

332