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Moisture, Clouds, and Precipitation 517

18.3 Cloud Typesand Precipitation

Reading StrategyBuilding Vocabulary Copy the table. Asyou read, add definitions.

Key ConceptsHow are clouds classified?

How are clouds and fogssimilar and different?

What must happen inorder for precipitation toform?

What controls the type ofprecipitation that reachesEarth’s surface?

Vocabulary◆ cirrus◆ cumulus◆ stratus◆ Bergeron process◆ supercooled water◆ supersaturated air◆ collision-coalescence

process

Clouds are among the most striking and noticeable effects of theatmosphere and its weather. Clouds are a form of condensation bestdescribed as visible mixtures of tiny droplets of water or tiny crystalsof ice. Clouds are of interest to meteorologists because clouds showwhat is going on in the atmosphere. If you try to recognize differenttypes of clouds, you might find it hard to do. But, if you learn the basicclassification scheme for clouds, recognizing cloud types will be easy.

Types of CloudsClouds are classified on the basis of their form

and height. The three basic forms are: cirrus,cumulus, and stratus. All other clouds reflect one ofthese three basic forms or are combinations ormodifications of them.

Cirrus (cirrus � a curl of hair) clouds are high,white, and thin. They can occur as patches or as del-icate veil-like sheets or extended wispy fibers thatoften have a feathery appearance. An example ofcirrus clouds is shown in Figure 11.

Cumulus (cumulus � a pile) clouds consist ofrounded individual cloud masses. Refer to Figure10 on page 515. Normally, they have a flat base andthe appearance of rising domes or towers. Theseclouds are frequently described as having a cauli-flower structure.

Figure 11 Cirrus Clouds

Vocabulary Term Definition

Cirrus a.

Cumulus b.

Stratus c.

Coalescence d. ?

?

?

?

FOCUS

Section Objectives18.10 Describe how clouds are

classified.18.11 Compare and contrast clouds

and fogs.18.12 Explain what must happen for

precipitation to form.18.13 Identify what controls the type

of precipitation that reachesEarth’s surface.

Build VocabularyConcept Map Have students createconcept maps of the types of clouds.Have them start with Clouds at the topand put the three families of cloudsclassified by height on the next level.Below that, they should put the types ofclouds within each family, and on thelast row, some details about each one.

Reading Strategya. clouds that are high, white, and thinb. clouds that consist of roundedindividual cloud massesc. clouds that occur as sheets or layersthat cover much or all of the skyd. the process in which larger dropsfall through clouds, collide, and jointogether with smaller, slower droplets

INSTRUCT

Types of Clouds

Students may have many misconceptionsabout what clouds are made of and howthey form. They may think that cloudsare made of smoke or water vapor. Theymay think that clouds form when bodiesof water boil. Have students makeflowcharts with diagrams to show thatclouds are made of tiny water droplets,and form when air containing watervapor is cooled.Logical, Visual

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Reading Focus

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Section 18.3

HSES_1eTE_C18.qxd 5/16/04 1:43 PM Page 517

518 Chapter 18

Figure 12 Cloud ClassificationClouds are classified according toform and height. Interpreting Diagrams Whichcloud types are the chiefprecipitation makers?

High clouds

6000 mCirrocumulus

AltocumulusMiddle clouds

4000 m

NimbostratusStratus

Low clouds

2000 m

Stratus (stratum � a layer) clouds are best described as sheets orlayers that cover much or all of the sky. While there may be minorbreaks, there are no distinct individual cloud units.

There are three levels of cloud heights: high, middle, and low, asshown in Figure 12. High clouds normally have bases above 6000 meters.Middle clouds generally occupy heights from 2000 to 6000 meters. Lowclouds form below 2000 meters. The altitudes listed for each height cat-egory are not hard and fast. There is some seasonal and latitudinalvariation. For example, at high latitudes or during cold winter monthsin the mid-latitudes, high clouds often are found at lower altitudes.

High Clouds Three cloud types make up the family of high clouds:cirrus, cirrostratus, and cirrocumulus. Look at Figure 12. Cirrocumulusclouds consist of fluffy masses, while cirrostratus clouds are flat layers.All high clouds are thin and white and are often made up of ice crystals.This is because of the low temperatures and small quantities of watervapor present at high altitudes. These clouds are not considered pre-cipitation makers. However, when cirrus clouds are followed bycirrocumulus or cirrostratus clouds and increased sky coverage, theymay warn of approaching stormy weather.

Middle Clouds Clouds that appear in the middle range, fromabout 2000 to 6000 meters, have the prefix alto- as part of their name.Altocumulus clouds are composed of rounded masses that differ from

518 Chapter 18

Build Reading LiteracyRefer to p. 278D in Chapter 10, whichprovides the guidelines for identifyingmain ideas and details.

Identify Main Idea/Details Havestudents read Types of Clouds onpp. 517–520. Ask them to identify themain idea of each subheading. Point outthat the main idea is usually in the firstor second sentence of a subheading.Encourage students to use this exercisein the notes they use to study.Verbal

Use VisualsFigure 12 Use this diagram to explainthe different types of clouds and theirnames. Ask: To what does “stratus”refer? (clouds that cover much of the sky)What can you tell about altocumulusclouds from their name? (They are mid-level clouds made up of individual cloudmasses.) What can you tell about cirro-cumulus clouds from their name?(They are high-level clouds made upof individual cloud masses.) Are youlikely to see all of these kinds ofclouds in the sky at the same time?Explain your answer. (No, becausemany of the clouds form under differentatmospheric conditions.)Visual, Verbal

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Section 18.3 (continued)

Customize for Inclusion Students

Behaviorally Disordered Have studentswork in groups to create a set of flashcards.Each card should contain information abouta different type of cloud. One side of thecard should have the name of the cloud (forexample, altostratus) and three categories of

color-keyed questions: Form of cloud, Heightof cloud, Typical weather. Color key theanswers on the other side of the card. (flatlayers, middle height, infrequent light snow ordrizzle) Students can use the cards to supportone another in small study groups.



CirrusCirrostratus

(Anvil head)

Altostratus

Stratocumulus

Earth’s surface

Cumulus(fair weather)

Clouds with verticaldevelopment

Cumulonimbus

Cumulus

cirrocumulus clouds in that altocumulus clouds are larger and denser,as shown in Figure 12. Altostratus clouds create a uniform white tograyish sheet covering the sky with the sun or moon visible as a brightspot. Infrequent light snow or drizzle may accompany these clouds.

Low Clouds There are three members in the family of low clouds:stratus, stratocumulus, and nimbostratus. As illustrated in Figure 12,stratus clouds are a uniform, fog-like layer of clouds that frequentlycovers much of the sky. Occasionally, these clouds may produce lightprecipitation. When stratus clouds develop a scalloped bottom thatappears as long parallel rolls or broken rounded patches, they are calledstratocumulus clouds.

Nimbostratus clouds derive their name from the Latin wordnimbus, which means “rainy cloud,” and stratus, which means “to coverwith a layer.” As the name suggests, nimbostratus clouds are one of themain precipitation makers. Nimbostratus clouds form during stableconditions. You might not expect clouds to develop in stable air. Butcloud growth of this type is common when air is forced upward, asoccurs along a mountain range, a front, or where converging windscause air to rise. Such a forced upward movement of stable air canresult in a cloud layer that is largely horizontal compared to its depth.

What does the Latin word stratus mean?

Build Science SkillsClassifying Show studentsphotographs of a variety of clouds.Have students classify the cloudsaccording to Figure 12.Visual

Build Science SkillsObserving Askstudents if they haveever wondered howsome people can lookout the window and predict theweather. Explain that learning toobserve and identify clouds will helpdevelop the ability to predict weather.Have students keep a log of cloud andweather observations for a least onemonth. For each page of their logs, havestudents record the date and time ofobservation; a description and drawingof the cloud cover, including the generalaltitude (high, middle, low), percent ofsky coverage, and the thickness orrelative height of clouds; a generaldescription of any weather phenomena,such as wind or precipitation; and thetemperature (either specific readingsor general terms). After a period oftime, students should analyze theirobservations and look for patternsbetween clouds and weather.Visual, Logical

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Answer to . . .

Figure 12 The chief precipitationmakers are nimbostratus and cumu-lonimbus clouds.

Stratus means “to coverwith a layer.”


520 Chapter 18

Clouds of Vertical Development Some clouds do not fit intoany one of the three height categories mentioned. Such clouds have theirbases in the low height range but often extend upward into the middleor high altitudes. They all are related to one another and are associatedwith unstable air. Although cumulus clouds are often connected withfair weather, they may grow dramatically under the proper circum-stances. Once upward movement is triggered, acceleration is powerful,and clouds with great vertical range form. The end result often is acumulonimbus cloud that may produce rain showers or a thunderstorm.

FogPhysically, there is no difference between a fog and a cloud. Theirappearance and structure are the same. The difference is the methodand place of formation. Clouds result when air rises and cools adia-batically. Most fogs are the result of radiation cooling or the movementof air over a cold surface. Fogs also can form when enough water vaporis added to the air to bring about saturation. Fog is defined as acloud with its base at or very near the ground. When fog is dense, vis-ibility may be only a few dozen meters or less, making travel not onlydifficult but often dangerous.

Fogs Caused by Cooling A blanket of fog is produced in someWest Coast locations when warm, moist air from the Pacific Oceanmoves over the cold California Current and then is carried onshore byprevailing winds. Fogs also can form on cool, clear, calm nights whenEarth’s surface cools rapidly by radiation. As the night progresses, athin layer of air in contact with the ground is cooled below its dewpoint. As the air cools, it becomes denser and drains into low areassuch as river valleys, where thick fog accumulations may occur.

Fogs Caused by Evaporation When cool air moves overwarm water, enough moisture may evaporate from the water surface toproduce saturation. As the rising water vapor meets the cold air, itimmediately condenses and rises with the air that is being warmedfrom below. This type of fog over water has a steaming appearance, asshown in Figure 13. It is fairly common over lakes and rivers in the falland early winter, when the water may still be relatively warm and theair is rather crisp.

How Precipitation FormsCloud droplets are very tiny, averaging less than 20 micrometers indiameter. Because of their small size, the rate at which cloud droplets fallis incredibly slow. Most cloud droplets would evaporate before fallinga few meters into unsaturated air below. For precipitation to form,cloud droplets must grow in volume by roughly one million times.

Figure 13 This steam fog rosefrom upper St. Regis Lake,Adirondack Mountains, NewYork.

For: Links on clouds and fog

Visit: www.SciLinks.org

Web Code: cjn-6183

520 Chapter 18

FogBuild Science SkillsComparing and ContrastingMake sure that students understand thesimilarities and differences between fogand other types of clouds. Ask: What isa fog? (It is a cloud with its base at orvery near the ground.) How are fogand other clouds similar? (They arethe same physically.) How are fog andother clouds different? (Clouds resultfrom air rising and cooling adiabatically.Fogs form as a result of radiation cooling,movement of air over a cold surface, orevaporation of water vapor.)Logical

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Download a worksheet on cloudsand fog for students to complete,and find additional teacher supportfrom NSTA SciLinks.



Cold Cloud PrecipitationThe Bergeron process, shown inFigure 14, relies on two physicalprocesses: supercooling and supersat-uration. Cloud droplets do not freezeat 0°C as expected. In fact, pure watersuspended in air does not freeze untilit reaches a temperature of nearly�40°C. Water in the liquid statebelow 0°C is said to be supercooled.Supercooled water will readily freezeif it impacts a solid object. Freezingnuclei are materials that have a crystalform that closely matches that of ice.Freezing nuclei can cause supercooledwater to freeze.

When air is saturated (100% rela-tive humidity) with respect to water,it is supersaturated with respect toice (greater than 100% humidity). Icecrystals cannot coexist with waterdroplets in the air because the air “appears” supersaturated to the icecrystals. Any excess water vapor becomes ice that lowers the relativehumidity near the surrounding droplets. Water droplets then evapo-rate to provide a continual source of water vapor for the growth of icecrystals.

Because the level of supersaturation with respect to ice can be quitehigh, the growth of ice crystals is rapid enough to produce crystals thatare large enough to fall. As they fall the ice crystals contact cloud dropscausing them to freeze. A chain reaction can occur and large crystals,called snowflakes form. When the surface temperature is above 4°C,snowflakes usually melt before they reach the ground. Even on a hotsummer day, a heavy downpour may have started as a snowstorm highin the clouds.

Warm Cloud Precipitation Much rainfall can be associatedwith clouds located well below the freezing level, especially in the trop-ics. In warm clouds, the mechanism that forms raindrops is thecollision-coalescence process. Some water-absorbing particles, such assalt, can remove water vapor from the air at relative humidities lessthan 100 percent, forming drops that are quite large. As these largedroplets move through the cloud, they collide and coalesce (jointogether) with smaller, slower droplets.

Figure 14 The BergeronProcess Ice crystals grow at theexpense of cloud droplets untilthey are large enough to fall. Thesize of these particles has beengreatly exaggerated.

Water molecule

Ice crystal Clouddroplet

Snowcrystal

For: Links on precipitation

Visit: www.SciLinks.org

Web Code: cjn-6184

How PrecipitationFormsUse VisualsFigure 14 Use this diagram to explainone way ice crystals form. Ask: How doice crystals grow? (They use water vaporfrom the atmosphere.) How is the watervapor replenished? (It evaporates fromwater droplets.) What happens to thewater droplets? (They get smaller andsmaller.)Visual, Logical

Many students think that raindrops havea teardrop shape because that is howthey are usually shown in artists’drawings. Explain that small waterdroplets, which are less than 1 millimeterin diameter, are shaped like spheres, asshown in Figure 14. Larger dropletsform a shape more like that of ahamburger bun as they fall through theair. Very large droplets, which are morethan 5 millimeters in diameter, willbreak up into smaller droplets.Logical, Visual

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The highest average annual rainfall on Earthwas recorded at a station on Mount Waialeule,Hawaii. The station is on a windward moun-tain slope and gets an average of 1234 cm ofrain a year. The record for most rainfall in a

12-month period occurred at Cherrapunji,India, where 2647 cm fell. Over one third ofthis rainfall—930 cm—fell in the month of Julyalone. This is 10 times more rain than Chicagoreceives in an average year.

Facts and Figures

Download a worksheet onprecipitation for students tocomplete, and find additionalteacher support from NSTASciLinks.


522 Chapter 18

Section 18.3 Assessment

Reviewing Concepts1. How are clouds classified?

2. Compare and contrast clouds and fogs.

3. What must happen in order forprecipitation to form?

4. Describe how the temperature profile ofair near Earth’s surface controls the type ofprecipitation that falls to the ground.

Critical Thinking5. Predicting What type of precipitation would

fall to Earth’s surface if a thick layer of air nearthe ground was �8°C?

6. Classifying Identify the following cloudtypes as producers of heavy, light, or generallyno precipitation.

a. cirrocumulus b. cumulonimbusc. stratus d. nimbostratus

Forms of PrecipitationThe type of precipitation that reaches Earth’s surface depends

on the temperature profile in the lowest few kilometers of the atmos-phere. Temperature profile is the way the air temperature changes withaltitude. Even on a hot summer day, a heavy downpour may havebegun as a snowstorm high in the clouds overhead.

Rain and Snow In meteorology, the term rain means drops ofwater that fall from a cloud and have a diameter of at least 0.5 mm.When the surface temperature is above 4°C, snowflakes usually meltand continue their descent as rain before they reach the ground. At verylow temperatures (when the moisture content of air is small) light, fluffysnow made up of individual six-sided ice crystals forms. At tempera-tures warmer than �5°C, ice crystals join into larger clumps. Snowfallsof these snowflakes are heavy and have high moisture contents.

Sleet, Glaze, and Hail Sleet is the fall of small particles of clear-to-translucent ice. For sleet to form, a layer of air with temperaturesabove freezing must overlie a subfreezing layer near the ground. Glaze,also known as freezing rain, results when raindrops become super-cooled (below 0°C) as they fall through subfreezing air near the groundand turn to ice when they impact objects.

Hail is produced in cumulonimbus clouds. Hailstones begin assmall ice pellets that grow by collecting supercooled water droplets asthey fall through a cloud. If the ice pellets encounter a strong updraft,they may be carried upward and begin the downward journey oncemore. Each trip through the supercooled portion of the cloud may berepresented by another layer of ice, as shown in Figure 15.

Figure 15 This largest recordedhailstone fell over Kansas in 1970and weighed 766 grams.

Compare-Contrast Paragraph Write aparagraph comparing the Bergeron andcollision-coalescence processes. Relate eachto the type(s) of precipitation that can result.

522 Chapter 18

Forms of Precipitation

Making HailPurpose Students will observe theprocess by which hailstones form.

Materials salt, water, 600-mL beaker,stirring rod, large test tube, ice chip

Procedure Put 15 g of salt and 50 mLof water into a 600-mL beaker. Stir thewater until most of the salt dissolves.Clean a large test tube thoroughly andput 15 mL of cold water in it. Place thetest tube in the beaker. Add crushed iceto the beaker until it is almost full. Tellstudents to watch the next step closely.Take the test tube out of the beaker anddrop a small chip of ice into it. Askstudents what they observed. (A layer ofice formed on the ice chip.) Tell studentsthat hailstones form by a similar process.

Expected Outcome A layer of ice willquickly form around the ice chip.Visual

ASSESSEvaluateUnderstandingAsk students to draw flowchartsshowing the major steps in the Bergeronprocess of precipitation formation. Theircharts should show at least three stepsand be consistent with the text andFigure 14.

ReteachUse Figure 12 to review different typesof clouds. Ask students to explain thename of a cloud and describe the typeof weather associated with it.

Both processes are mechanisms thatproduce precipitation. The Bergeronprocess occurs in cold clouds and canresult in any form of precipitation. Thecollision-coalescence process occurs inwarm clouds and generally producesrain. Paragraphs should include a cleartopic statement and several minorsentences that provide detail.

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ground. The thickness of layers at certaintemperatures must also be thick enough toimpart a change in the type of precipitation.5. Light, fluffy snow would fall.6. a. generally no precipitation b. heavyprecipitation c. light or no precipitationd. light-to-heavy precipitation

Section 18.3 Assessment

1. Clouds are classified according to form andheight.2. Clouds and fogs are physically the same.Fogs are clouds with their bases at or verynear the ground.3. Cloud droplets must increase in volume byabout one million times.4. Ice crystals formed at higher layers in theatmosphere become liquid and fall as rain iftemperatures are warm enough near the



Atmospheric Stabilityand Air Pollution

Air quality is closely linked to the atmosphere’s abil-ity to scatter pollutants. Perhaps you’ve heard“Dilution is the solution to pollution.” To a largedegree, this is true. If the air into which pollution isreleased is not dispersed, the air will become moretoxic. Two of the most important atmospheric con-ditions affecting the distribution of pollutants arewind strength and air stability.

When winds are weak or calm, the concentration ofpollutants is higher than when winds are strong.High wind speeds mix polluted air into a largervolume of surrounding air, causing the pollution tobe more diluted. When winds are light, there is lessturbulence and mixing, so the concentration of pol-lutants is higher.

Atmospheric stability affects vertical movements ofair. In general, the larger the extent of verticalmixing, the better the air quality is. During a tem-perature inversion, the atmosphere is very stable andit does not move much vertically. Warm air overly-ing cooler air acts as a lid and prevents upward

movement, which leaves pollutants trapped near theground, as shown in Figure 16.

Some inversions form near the ground, while othersform higher above the ground. A surface inversiondevelops close to the ground on clear and relativelycalm nights because the ground is a better radiatorof heat than the air above it. Radiation from theground to the clear night sky causes more rapidcooling at the surface than higher in the atmos-phere. The result is that the air close to the groundis cooled more than the air above, yielding atemperature profile similar to the one shown inFigure 17. After sunrise, the ground is heated andthe inversion disappears.

Although surface inversions usually are shallow, theymay be thick in regions where the land surface isuneven. Because cold air is denser than warm air,the chilled air near the surface gradually drains fromslopes into adjacent lowlands and valleys. As mightbe expected, these thicker surface inversions will notspread out as quickly after sunrise.

Incr

easi

ng a

ltitu

de

Increasing temperature

Temperatureprofileat night

Warm inversion layer

Cold

Figure 16 Air Pollution inDowntown Los AngelesTemperatures inversions act aslids to trap pollutants below.

Figure 17 GeneralTemperature Profile for a Surface Inversion

AtmosphericStability andAir PollutionBackground• The strength of the wind and the

stability of the air are critical becausethey determine how rapidly pollutantsare diluted by mixing with thesurrounding air after leaving thesource.

• The distance between Earth’s surfaceand the height to which vertical airmovements extend is termed themixing depth. Generally, the greaterthe mixing depth, the better the airquality. When the mixing depth isseveral kilometers, pollutants aremixed through a large volume ofcleaner air and dilute rapidly. Whenthe mixing depth is shallow, pollutantsare confined to a much smaller volumeof air and concentrations can reachunhealthy levels.

• Because heating of Earth’s surface bythe sun enhances convectional move-ments, mixing depths are usuallygreater during the afternoon hours.For the same reason, mixing depthsduring the summer months aretypically greater than during thewinter months.

• Many extensive and long-livedair-pollution episodes are linked totemperature inversions that developin association with the sinking air thatcharacterizes slow-moving centers ofhigh pressure. As the air sinks to loweraltitudes, it is compressed and so itstemperature rises. Because turbulenceis almost always present near theground, this lowermost portion of theatmosphere is generally preventedfrom participating in the generalsubsidence. Thus, an inversiondevelops aloft between the lowerturbulent zone and the subsidingwarmed layers above.

Teaching TipAsk students to research variousmethods that meteorologists use todefine atmospheric stability. Have themexplain each method and cite the prosand cons of each use.

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18.3 Cloud Types and Precipitation 1 - VM Earth & Space ...vmearthscience.weebly.com/uploads/5/8/8/5/58854521/18.3.pdf · 18.3 Cloud Types and Precipitation ... what is going on in