Weather - A Golden Science Guide - Paul E. Lehr

GOLDEN NATURE GUIDES •

Birds

Flowers

Trees I n sects

•

Stars

Rept i l es and A m p h i b i a n s Mammals Seashores

•

F i shes

Weather Rocks and M i nerals P h otography

( A GOLDEN HANDBOOK)

Zoo logy

•

Foss i l s

G a m e b irds Sea Shells of the World IN PREPARATION:

Moths a n d B utterflies N o n -flowering P l a nts REGIONAL GUIDES OF AMERICA

T h e South west The Southeast The Pacific Northwest Evergl a des N ati o n a l P ark The Rocky Mounta i ns

1 00100

De

WEATHER

AIR MASSES-CLOUDS-RAINFALL S TORMS-WEATHER MAPS-CLIMATE by PAUL E. LEHR Meteorologist, Scien tific Serv i ce s D i v i s i o n of U . S. A i r W e a t h e r Service

R. WILL BURNETT

Professor of Science Education, University of Illinois HERBERT S. ZIM I

l

I l l ustrated by HARRY McNAUGHT

r

).

{ A GOLDEN

NATURE GUIDE

GOLDEN PRESS

•

NEW YORK

FOREWORD Of a l l aspects of the natura l world, weather is outsta nd­ i n g i n its bea uty, its majesty, its terrors, a n d its conti n u a l d i rect effect o n us a l l . Because weather invo lves, f o r t h e m ost part, massive movements o f invisible a i r a n d i s con­ cerned with the temperature a n d pressure c h a nges of this a l most intangible substa nce, most of u s h ave o n l y a l im ited u n derstanding of what weather is a l l about. This book wi l l h e l p you to u n dersta n d it a n d a lso to u n der­ sta nd, i n some deg ree, how weather changes a re pre­ d icted. I n the difficult a ttempt to portray the weathe r simply, acc u rately, and graph ica l ly we have had inva l u a b l e assist­ ance from co l leag ues, experts, and many orga n izations whom we g ratefu l ly thank. The U.S. Weather B u reau (in­ c l u d i n g the libra ry) helped us l ibera l l y with information and photos. H e l pf u l materia l was supplie d a lso hy the Smithsonian I n stitution, the American Meteorological So­ c iety and its secreta ry, Ken neth F. Speng ler, the Nationa l Safety Counc il , Dr. David M. Ludlam of the F ra n k l i n I n sti­ tute, Lt. J o h n H. Boone ( USAF), and Lt. J o h n F. M a n n , J r . ( USAF). Bernice Burnett a -n d A d e l e F . Lehr r e a d a n d criti­ c ized the m a n uscript at various stages. Dr. Vincent J . Schaefer o f t h e Munita l p Foun dation exa mined both text a n d i l l ustrations a n d offered m uch h e l pfu l a dvice. P.E.L. R.W.B.

H . S.Z. Twelfth Printing, 1 963

Library of Congress Catalog Card Number: 61-8327 ®Copyright 1957 by Golden Press, Inc. All Rights Reserved. Including the Right of Reproduction in Whole or in Part in Any Form. Designed and Produced by Artists and Writers Press, Inc. Printed in the U.S.A. by Western Printing and Lithographing Company. Published by Golden Press, Inc., New York 22 N. Y. Published Simultaneously in Canada by The Musson Book Company, Ltd., Toronto

CONTENTS The effects of heat, p ressu re, wind, a n d moisture . . . Clouds

5-20

Their na­ tu re, types, a n d origins . . . Rainmaking . . . .

21-33

WHAT MAKES THE WEATHER?

RAIN, SNOW, DEW, AND FROST

THE ATMOSPHERE-RESTLESS OCEAN OF AIR .

.

THE

.

Its structure a n d weather function . . . . . EARTH'S

MOTIONS

AND

34-47

WEATHER

Seaso n a l cha nges a n d the ea rth's rotation a s they affect winds . . . . . . . . . . . . . . .

48-59

Pressure cells, their winds, and a ssociated weather . . . . . .

60-67

Major a i r masses of the world, their identification, a n d their role as a sou rce ·of o u r weather . . . . . . . . . . . . . .

68-76

AND FRONTAL WEATHER How fronts form, move, a n d change . . . The kinds of weather a ssociated with each type . . . .

77-95

T h e origin, developme nt, a n d effects of t h u n derstorms, to rnadoes, a n d h u rrica nes

96-111

HIGHS AND LOWS

AIR MASSES

FRONTS

STORMS

FORECASTING Weather i n struments and how they a re used i n forecasti n g 112-129

WEATHER

How data are plotted a n d cha rted • . . H o w to r e a d maps a n d make 130-146 y o u r own forecast . . • . • . . . . .

WEATHER MAPS

Average weather conditions worth knowing a n d using day by . . • . . . . 147-156 day .

WEATHER AND CLIMATE

BOOKS FOR MORE INFORMATION

and magazines to read . . . . . . .

INDEX

Books

157 158-160

EVERYBODY TALKS ABOUT T H E W EAT H E R C h a rles Dudley Warner said, " Everybody ta l ks a bout the weather, but nobody does anyt h i n g about it." Everyone, at times, feels as Warner did. The spoiled fa mily picn ic, the withered crops, a l l rem ind us how dependent we are on the weather. That is why weather is o u r m ost common topic of conversation, a factor i n much of o u r agricu ltura l , ind ustria l , a n d civic p l a n n ing, and a consta n t concern of everyone. Warner was wrong. Someth ing is being done. Today the science of weather-meteorology-is used to make our l ives safer and better. Some types of forecasts are 95 per cent accu rate. Storms are tracked and warnings a re g iven . C l o uds a re being seeded to ca use rainfa l l where i t is n eeded. A n etwork o f weather stations e n a b les p l a nes to fly safe ly. A continued program of research reveals more and more about the weather. This introduc­ tion to weather will help you understa n d it.

4

What M akes the Weather? Weather is the cond ition of the atmosphere in terms of h eat, pressure, wind, and moisture. These are the elements of which the weather is made. Where the atmosphere thins to noth i n g n ess, there is no weather. There is n o weather on the moon, for it has n o atmosphere. But near the sur­ face of the earth the atmosphere is dense and heavy. Here, in the lower atmosphere, you conti n ua l ly see the everchanging, d ra m atic, often violent weather show. But it takes more than a i r to make weather. If the earth's atmosphere were never heated, m ixed, o r moved about, there would be no weather-or, more properly, there would be n o cha nges in the weather. There would be n o w i nds, no chan ges in a i r pressure, no storms, r a i n , o r snow. Heat is the spoon that m ixes the atmosphere to make weather. All weather cha n ges are brought about by tem­ perature c h a n ges i n d ifferent pa rts of the atmosphere. 5

T H E S U N, sou rce of m ost of the earth's heat, is a ba l l

o f glowing gases, 9 3 m i l l ion m i les away. This giga ntic atomic fu rnace bom bards the earth with 1 26 tri l l ion horse­ power every seco n d . Yet this vast energy is but a half of o n e b i l l ionth of the s u n 's tota l output. Most of this solar energy is lost i n space; traces reach other p l a n ets. The s u n 's energy is transm itted as waves that a re s i m i l a r to radio waves. Some of these are visi b l e light waves; others a re invisible. Some, although not heat waves, change to heat when a bsorbed by ob jects such as soil or our bod ies. About 43 per cent of the radiation reaching our pla net h its the earth's s u rface and is changed to heat. The rest stays in the atmosphere or is reflected into space.

6

,�

W H AT H A P P E N S TO T H E SU N'S H EAT is shown in

the diagram a bove. This is for average weather-that is, 52 per cent c l o u d i n ess in the sky. A typical c l o u d refl ects back into space 75 per cent of the s u n l ig h t striking it. O n overcast days, o n ly about 25 per cent of the s u n's energy h its the g roun d. Energy that does reach the ground is a bsorbed a n d refl ected in varying deg rees. Snow reflects about 75 per cent, a bsorbs o n ly 2 5 per cent; this partly accou nts for the co l d of pol ar reg ions. Dark forests absorb about 95 per cent of solar energy a n d change it to heat. Such d ifferences i n a bsorptio- n a n d reflection acco u n t, i n pa rt, f o r regiona I d ifferences i n temperature a n d c l imate. ABSORPT I O N O F S U N L I G H T BY D I F F ERENT S U RFACES

dense forests 95%

plowed field 75 to 95%

7

'•' '• .

,

if9"' ' So l a r rays g o through g lass-

heat rays cannot

A g reenhouse "traps" solar radiation when ..short" sol a r rays cha n g e to "long" heat rays.

Earth's atmosphere is like g l a ss. It lets solar rays through but keeps most heat rays from· escaping.

8

EARTH

AS

A

G R E E N H O USE

The g lass of a g reenho use lets the s hort solar rays pass through. These a re a bsorbed by o bjects inside a n d a r e re-rad iated as l o n g heat rays. But these long heat rays c a n not get through the g lass. The heat rays a re conti n u a l l y re-absorbed a n d re­ rad iated inside. This h e l ps keep the g reenhouse warm on cold days. Some heat is lost by conduction through the g lass. Like a gree n h ouse, the earth's atmosphere admits most of the solar radiation. When this is a bsorbed by the earth's surface, it is re-ra di­ ated as heat waves, m ost of which a re trapped by water vapor in the atmosphere. Thus the e a rt h is kept warm.

T H E ATMO S P H E R E AS A T H E R­ MOSTAT controls the earth 's heat

a s a utomatica l ly a s i n any heati n g system . It p rotects the earth from too much so l a r radiation d u ring the day, a n d screens out d a ngerous rays. It acts as a n i nsulating blan­ ket which keeps m ost of the heat from esca p i n g at n ig ht. Without its thick atmosphere the earth wou l d experience tem peratu res like t h e m oo n 's. T h e m o o n 's surface tem­ perature reaches the poi l i n g poi nt of water (2 1 2 ° Fahren heit) d u ring the two-week lunar day. It d rops to 238 ° F below zero durin g the long lunar n ight.

Earth has thick atmosphere.

l

night 40 °F

1

Moon has a very thin atmosphere.

The earth cools faster o n bright clear nig hts than o n c l oudy nig hts, because an overcast sky refl ects a large a m o u n t of heat back to earth, where it is once again re-a bsorbed .

.

-

.

. .

perature a n d reta rds night heat loss.

o n a clear n i g h t more heat esca pes.

H EAT AND AIR MOVEM ENTS

The a i r is heated mainly by contact with the warm earth. W h e n air is Co nvection cu rrents warmed, it expa n d s a n d becomes in heated water lig hter. A layer of air, warmed by contact with the earth, rises a n d i s replaced by colder a i r which fl ows in a n d u n d e r it. This c o l d a ir, in turn, is warmed a n d rises, a n d it, too, is replaced by colder air. Such a circulating movement of w arm a n d c o l d fl uids is ca l led "convectio n . " You can see co nvection c ur rents if you drop sma l l b its of paper into a g lass conta iner in which water is being heated. The air at the eq uator receives much more heat than the air at the poles (p. 5 1 ) . So warm air at the eq uator rises and is replaced by colder a i r flowing i n from north and south . The warm, light a i r rises and moves poleward high above the earth. As it coo ls, it sin ks, rep lacing the cool s u rface air which has moved towa rd the eq uator. If the earth did not rotate, the air wou l d circ u l ate as shown . Because the earth does rotate, the circulation is different (p. 53).

Air movements over a n on-rotati n g earth

10

"" I ,

- ·,

' '

Differe n ces i n h11ating cause loca l winds.

CO NVECT I O N ca uses loca l winds and b reezes. Different

land and water surfaces absorb d iffere nt a m o u nts of heat. Da rk, plowed soi l absorbs much more tha n g rassy fields. Mounta ins a bsorb heat faster during daylight t h a n nearby va l l eys, and lose it faster at n ig ht. land warms faster tha n does water d u r i n g the day a n d cools faster at n i g ht. The air above such surfaces is warmed o r cooled accord i n g l y -and loca l w i n d s resu lt.

Mountain breezes i n daytime S e a breezes in d ayti me

Mountain breezes at n i g h t

Wate r is a lways pres­ ent i n the air. It eva porates from the earth, of which 70 per cent is covered with water. In the a ir, water exists in three states: solid, liquid, a n d invisib le va por. The amount of water vapor in the air is ca l l e d the " h u m idity." The "re lative humidity" is the amount of vapor the air is holding expressed as a percenta ge of the amount the a i r c o ul d h o l d at that pa rticular tem perature. Warm a i r can h o l d more water than cold. When air with a g iven amount of water vapor coo ls, its relative h u m i d ity g oes u p; when the a i r is warmed, its rel ative h u m id ity drops. As the ta b l e below shows, air at 86°F is "saturated" when it holds 30.4 grams of water vapor per cubic m eter. ( I n other words, it has a relative h u m i d ity of 1 00 per cent; it has reached its dew point.) But air at 68 ° is satu­ rated when it h o l ds o n ly 1 7. 3 grams per cubic m eter. That's a d ifference of 1 3. 1 grams per c u b ic m eter. So every cubic m eter of 86° saturated air that is coo led to. 6 8 ° w i l l lose 1 3 . 1 grams of water vapor as cloud d rop­ lets which, if cond itions are rig ht, will fa l l as rain or snow.

W AT E R I N T H E ATMOS P H E R E

R E L AT I VE H U M I D I TY 1 6%

24%

31%

45%

57%

28%

42%

54%

79%

1 00%

53%

69%

1 00%

52%

77%

1 00%

67%

1 00%

36%

I

1 00%

1 00%

4.85

7.27

9.4 1

1 3 .65

grams af water vapor per

12

1 7. 3 1

cubic meter

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H EAT

AND

ATMOS P H E R I C

WAT E R

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Heat eva porates m i l l ions of tons of wate r i nto the air d a i ly. Lakes, streams, a n d 1 1 t j oceans send up a stea dy stream of water t ' vapor. An amazing amount of water tra n +. spires from the leaves of g reen pla nts. A l s i n g l e apple tree may m ove 1 ,800 g a l l o n s j l J. of water i nto the a i r in a six-month g rowi J. i n g season. J. As mo ist warm a i r rises, it slowly cools. t t Fina l ly it cools so much that its relative l. ' 1 h u m i d ity reaches 1 00 per cent. C louds r o n i ra conditions, in certa under d, n a form .l, ; f snow comes down. This eternal process of + eva poration, condensation, a n d preci pita +. r j ion is cal led the water cyc le. J.. • +

+

)' .1'

r

When a i r is cooled be­ low its saturation point the water vapor in it condenses to form clouds. When water vapor at a teakettle s pout is cooled by the air around it, a s m a l l cloud forms. You r w a r m mo ist b reath forms a m i n iature c loud when it h its the cold wi nter a i r . The clouds you see nea rly every day form i n several ways but all form by the sa m e general p rocess-coo l i ng of a i r below its saturation point. HOW C L O U DS AR E FORMED

Earth radiates heat rapidly o n clear n i g h ts. Air in contact with cold earth may cool below its sat. u ratio n point a n d fa rm low clou ds, or fog {a cloud on the ground). � warm air

Warm a i r is often l ifted by a h eavier mass of cold a i r which p u shes u nder it l i ke a wedge. C louds form as warm air coo ls � below its saturation point.

..,. Warm a i r may move over a cold s u rface and be cooled below its satu ration poi nt. C l o u d s may form a s warm l a ke or ocea n a i r moves in over a cooler l a n d su rface.

col d air

Air m a y b e heated b y contact with the earth's warm su rface. I t expends, becomes l i g hter, a n d rises. Expansion lowers its tem· peratu re. The more it rises, the more it cools-at a rate of about 51f2 ° F for each 1 ,000 ft. of rise. This "adia batic coo l i n g " occurs whenever a i r rises. Most clouds form beca u se of adia batic coo l i n g .

Air movi n g up a slope loses heat adia batica lly as it rises. I f

Warm a i r often p u shes over a mass of cold a i r (a bove). C louds may form a s it cools adia batica lly because of i ts rise.

Someti mes rain or snow from high clouds may fall through warm a i r, cool it, a n d cause lower clouds to fo rm. These lower clouds w i l l genera lly be i n laye rs-often i n several levels.

it rises enough to cool below its saturation point, clouds will fo rm.

Cum ulus clouds

Stratus clouds

C louds are c lassified ac­ cord i n g to how they are formed. There a re two basic types: ( 1 ) C louds formed by rising air currents. These a re piled u p a n d puffy. They a re ca l led "cu m u l us," which means piled u p or acc u m u lated. (2) C louds formed when a layer of air is coo led below the saturation point with­ out vertica l movement. These a re i n sheets o r fog l ike layers. They a re cal led "stratus," meaning sheetlike or l ayered. C louds a re further classified by a l titude into four fa m i­ lies: h i g h c louds, m i d d l e clouds, low clouds, a n d towering c louds. The bases of the latter may be as low as the typi­ ca l low clouds, but the tops may be at or above 75,000 ft. C L O U D C LASS I F I CAT I O N

The na mes of c louds are descriptive of their type and form. The word " n i m b us," meaning rain cloud, is added to the na mes of c louds w h ic h typically produce rain o r snow. The prefix "fra cto-," meaning frag­ m ent, is added to na m es of wind-blown clouds that are broken into pieces. "Alto-," mea n i n g high, is used to indi­ cate m idd le-layer high c louds of either stratus or c u m u lus types. The pictures a n d captions on the next four pages will help you to identify m a jor cloud types and to u n d er­ sta n d better their relationship to the weather. C L O U D N AMES

16

H I G H C L O U DS are com posed a l m ost entirely of tiny ice crysta ls. Their bases averag e about 20,000 ft. above the earth. Three types exist: C i rrus clouds, thin, wispy, and feathery, are com posed e ntirely of ice c rysta ls. Cirrus clouds usu­ ally form at 25,000 ft. a n d above, where the tem perature is a lways far below freezing. These clouds are freq uently b lown about i nto feathery stra nds ca lled "mares' ta i ls." C i rroc u m u lus clouds, genera l ly form ing at 20,000 to 25,000 ft., are rarely see n . These thin, patchy clouds often form wavelike pat­ terns. These are the true mackerel sky, not to be confused with a lto­ c u m u l u s ro l l s . They a re often rip­ p led and a lways too thin to show shadows. C i rrostratus clouds form at the

same a ltitudes as c i rroc u m u l us. These a re t h i n sheets that look like fi ne veils or torn, wind-b lown patches of g a uze. Because they a re made of ice crysta l s, cirrostra­ tus clo uds form large h a l os, or l u m inous circ les, a round sun a n d moon. 17

M I D D L E C LO U DS a re basica l l y stratus or c u m u l us. Their

bases average about 1 0,000 ft. above the earth. Altostratus (above) are dense veils or sheets of g ray or b l ue. They often appear fi brous or l ig htly striped. The sun o r m oon does not form a halo, as with hig her, ice­ crysta l c irrostratus, b ut appears as if seen th roug h frosted g lass. Altoc u m u l u s (be low) are patches or layers of puffy or

ro l l - l i ke c l ou ds, gray or whitish. They resemble c i rrocu m u­ l us, but the p uffs or ro l l s a re larger a n d made of water dropl ets, not ice crysta ls. Through a ltoc u m u l u s the sun often produces a corona, o r d isk, genera l ly pa le b l u e or ye l low inside, reddish outside. The corona's color a n d s prea d d i sti n g u ish i t from t h e c irrostratus h a lo-a la rger ring, covering much more of the sky.

18

LOW C L O U DS have bases that range in height from near the earth's s u rface to 6,500 ft. There a re three main kinds: Stratus is a low, q u ite u n iform

sheet, like fog, with the base above the g ro u n d . Dul l-gray stra­ tus c louds ofte n make a heavy, leaden sky. O n l y fi n e drizzle can fa l l from true stratus c l o u ds, be­ cause there is l ittle o r no vertical move ment i n them.

N i m bostratus a re the true rain c louds. Darker than ord i n a ry stra­ tus, they have a wet look, a n d strea ks o f rain often exte nd to the ground. They often are ac­ compa nied by low sc ud clouds (fractostratus) when the wind is strong.

are i rreg u l a r masses o f c louds spread o u t in a ro l l ing or puffy layer. Gray with darker shading, stratoc u m u l us do not produce rain but sometimes c h a n g e into n i m bostratus, which do. The ro l l s or masses then fuse together a n d the lower surface becomes i n d istinct with rain. Stratocu m u l us

lii ll;ii ,� ••::JI�!t!l�r.::Eii;:;�:· lilllli '!'

C u m u lo n i m bus are the fam ilia r

thu nderheads. Bases may a l most touch the ground; violent up­ drafts may ca rry the tops to 75,000 ft. Winds aloft often m o l d the tops into a fl at anvil-l ike form. I n their most violent form these c louds prod uce torna does (p. 102). Cumulus a re puffy, caul iflower­

l ike. Sha pes constantly change. Over land, cumulus usua l l y form by day i n risin g warm a i r, and disappear at n ight. They mean fair weather u n less they p i l e up into c u m u l o n i m b us. C u m u lus and C u m u lo n i mbus ..

"-•

..;' '

are both clouds of vertica l devel­ opment, u n l ike the layered clouds described on previous pages. C louds of the c u m u l u s type result from stro ng vertical currents. They form at a l m ost any a ltitude, with ba ses sometimes as h i g h as 14,000 ft.

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CLOUD SYMBOLS

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20

cumulus

d

a ltostratus

stratocumulu s

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altocu m u l u s

stratus

---->

cirrus

cumulonimbus

L

cirrostratus

n i m bostratus

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cirroc u m u l u s

Ra i n, S n ow, Dew, a n d F rost P R E C I P I TAT I O N s u c h as rain, snow, sleet, a n d h a i l can occur only if there are clouds i n the sky. But not all kinds of c l ouds can produce precipitation. Temperature, the presence of tiny foreig n partic les, or of ice c rysta ls, a l l h e l p dete r m i n e whether preci p itation w i l l occur a n d what form it will take. For exa m p le, snow will n ot fo rm u n less a i r is supersaturated (cooled below its satu ration point or dew point without its water vapor condensin g ) a n d is con­ sidera bly below the freezing point of water.

21

Rain fa l l s from c louds for the same reason a nyth ing fa l ls to earth. The e a rth's g ravity pulls it. But every cloud is made of water drop­ lets o r ice c rysta ls. Why doesn 't rain or snow fa l l con­ sta ntly from a l l clou ds? The droplets or ice c rysta ls in clouds are exceeding ly sma l l . The effect of gravity on them is m i n ute. A i r currents move a n d l ift droplets so that the n et downward movement is zero, even though the droplets a re i n constant motio n . Droplets a n d i c e crysta ls behave somewhat l i ke dust in the a i r made visible in a shaft of s u n l i g ht. But dust partic les are much larger than water droplets, and they fi n a l l y fa l l . The cloud droplet of average size is only 1/2500 inch i n diameter. It is so s m a l l that it would take 1 6 hours to fa l l h a lf a mile in perfectly sti l l a i r, and it does not fa l l out of moving air at a l l . Only when the d roplet g rows to a diameter of 1/125 inch o r larger can it fa l l from the cloud. The average raindrop conta i n s a m i l lion times as much water as a tiny cloud droplet. The g rowth of a c l o u d droplet to a size large enough to fa l l out is the cause of rain and other forms of precipitation. This i m por­ tant g rowth p rocess is cal led "coa lescence." W H AT MAKES IT RAI N ?

C l o u d drop lets (enlarged 70 times).

Smal lest raindrop (enla rged 70 times).

..

Coalescence occurs c h iefly in two ways: (1) Droplets in clouds are of d ifferent sizes. Big d ro ps move more slowly i n turbulent air and in paths d ifferent from the paths of sma l l droplets. Bigger, heavier d ro ps are not whipped around so ra pidly. So drops c o l l ide, beco m e b igger, and fi n a l ly d rop as rain. This is prob­ a b ly the main cause of rainfa l l from n i m bostratus and other low c louds. (2) The most i m po rta nt type of coa lescence occurs when tiny ice crystals and water d roplets occ ur to­ gether (as near the m i d d l e of cumu­ l o n i m bus c louds). Some water drop­ lets evaporate and then condense on the crystals. The c rysta ls grow until they drop as snow or ice pe l l ets. As these drop through warm a ir, they c h a n g e i nto raindrops. (3) lig htning d ischa rges i n a thu n­ derstorm form oxides of n itrogen that a re extremely hyg roscopic (water­ absorbing). These oxides are added to the atmosphere a n d become one of the kinds of nuclei for future con­ densation and eventual coa lescence and rainfa l l . But the two processes me ntioned a bove a re the m a i n a n d perhaps t h e o n l y causes o f coa les­ cence and hence precipitation. Re­ search may show other possibil ities.

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3

23

Very sma l l partic les i n the a i r may a c t as n uclei u pon which water vapor will crysta l l ize to for m snow. Air m ust be su persaturated with water vapor and below the freezing point. Mic roscopic bits of soi l , c lay, sand, a n d ash a re c o m m o n n u c l e i . C l o u d tem peratu res m ust general­ l y be from + 1 0 ° to -4°F before snow begins to form. Va por cha nges to snow even without nuclei at high a lti­ tudes in supersaturated air at -38 ° F . S N OW

24

SNOW PELLETS (or granular

snow) a re w hite, a n d of various shapes. Although much like soft hail, pe l l ets a re too sma l l a n d soft to bounce. A single pel let gener­ a l ly forms from many su percooled cloud droplets which freeze together i nto crysta l l i n e form.

Snow pellets-magnified 3 times.

I C E P R I SMS form hexag o n a l plates, col um ns, a n d needles t h a t sometimes g l itter like d i a m o n d s as they a re b lown a b o ut. Because of their s m a l l size they fa l l very slowly. Ice need les often mdke halos a r o u n' d sun or moon. I n very cold c l i m ates ice-need le fogs form o n the ground.

Ice needles-magnified

10

times.

Halo caused by ice needles. I C E P E L L ETS (sl eet) consist of tra nsparent or tra nsl ucent

beads of ice. Sleet occurs when rain, dropping from upper warm a i r, fa l l s through a layer of freezing a i r. Rain drops fi rst be­ WARM AIR come freezing rain (supercoo led) and when striking the ground in this condition form g laze (p. 27). But further coo l i n g produces ice pe l lets, or true sleet, which freezing rain bounces o n h itting the g round.

ice pellets

Ice pellets-magnified 3 times.

25

H A I L forms as frozen ra i n d ro ps, formed h i g h in the cl ouds, m ove through a reas of supercooled water d roplets i n thun derc louds. Hai lston es were long thought to develop their o n io n l ike structure by being a lternately forced up­ ward by vertica l winds i n the thunderhead to a freezing leve l, then d ropped down to where more water was picked up. Such up-and-down trips do occur, but the growth of hai l stones resu lts mostly from ice pel lets' picking up water i n the supercooled m id d l e a n d u pper reg ions of the cloud. The layers result from d ifferences between the freezi n g rate a n d the rate at which water accumu­ lates on the pe l lets.

cross section • of a hailstone

26

I C E STORMS are c h a racterized by g l a ze. Glaze is a s destructive as it is beautifu l . It occurs w h e n ra i n o r d rizzle that has been supercooled (cooled below 32 ° F but not yet frozen) falls on cold su rfaces a n d immediately freezes. Glaze ice fo rmed from this freezing ra i n can snap branc hes, w ires, and poles and cause hazardous driving conditions.

27

D E W does n o t fa l l. I t i s water vapor that con­ denses o n so l i d s u rfaces that have cooled be low the condensation point of the a i r i n conta ct with them. This coo l i n g by radiation occurs usua l l y on c l ea r n i g hts. The "sweat" that forms on the outside o f a g l ass of cold lemonade on a hot day is a lso dew.

FROST is formed like dew but at tem peratures below freezi n g . The water vapor changes directly to sma l l, fi n e frost crysta ls without condensi n g into water drops fi rst. Frost crysta ls growing on windows devel o " p feathery pat­ terns as the primary frost me lts and recrysta l l izes.

28

Drainage pattern af the United States and Canada. Rain either is stored i n the ground, po n ds, o r la kes, o r runs off to b e stored i n the ocea n . It eventua l ly evaporates i nto the a i r again. S u rface runoff o f preci pitation a n d the u n dergro u n d fl o w o f wate r g ive us o u r brooks, streams, a n d rivers. Rivers a lways flow from higher to lower levels, c utting their own va l l eys. Streams and rivers merge and fi n a l ly they em pty i nto the ocea ns. When the supply of water is l a rger than the a m o unt n o rm a l ly h a n d l e d by strea m s a n d rive rs, flood­ ing occurs. I n areas of h a rd, baked g ro u n d o r clay soil, the runoff from a thunderstorm is a l m ost c o m p l ete, since there is l ittl e seepage i nto the soi l . Flash floods may resu lt. Water may a lso run u n derground, to em erge as springs. We l l s ta p this u n dergro u n d water, which provides many a reas with an a b u ndant supply. Large u n dergro u n d rivers exist, but these are rare. WHAT H A P P E N S TO R A I N A N D SNOW

29

Water is stored as snow and ice, as well as in lakes.

Ten inches of snow melts into one inc� of water.

Lakes a n d ponds obviously store a g reat dea l of water. Not so obvious is the i m m e nse reser­ voir of water stored in the po lar ice caps, in g l aciers, a n d i n s n o w o n m o u ntains a n d on t h e cold northern p l a i n s d u r­ i n g winter. Winte r snows in the m o u nta i n s determ i n e the water supply for irrigation and for power use. This snow me lts with the spring thaw a n d fi l ls the rivers. If spri n g is late, the melting wi l l be m o re sudden, resulting i n floods. WAT E R STO RAG E

Topsoil h o l ds a n i m mense amount of water. Some of this is tra nsferred into the a i r by g reen plants. But most of it seeps through soil a n d porous rock until it reaches non porous clay o r rock. I t forms a m assive subterra n ea n reservoir, fi l l i n g a l l the cracks a n d pores i n t h e rock a n d soi l . T h e u n derground water, l ike surface water, fl ows down h i l l and seeps out into strea m s or comes out in springs. Eventua l ly this water, too, evapo rates or reaches the sea. G RO U N D WAT E R

impervious rock

RAI N MA K I N G is an a ncient ho pe, a 1 9th-century fake, a n d a modern scientific fact. Every primi­ tive tribe has tried one way or an­ other to make it ra i n . Prim itive mag ic, r a i n dan ces, and sacrifices have a l l been used to i n d uce ra i n . B y coincidence, ra in has fo l l owed these efforts often enough to kee p a l ive the b e l ief in the efficiency of the methods. Qu ite a boom in ra i n ­ making devel o ped in the 1 9th cen­ tury. Dru m s were beaten, cannons shot, a n d explosives were set off, prod ucing g reat q u a n tities of smoke. The methods were worthless and so were many of the o perators. One of the smooth est of t hese con­ fidence m e n used U. S. Weather B u reau c l i matolog ica l data. He never a ppea red i n a drought area until it was a l m ost certa i n that rain was but a few days away. Then he wou l d put o n his act, wait for the rain, a n d c o l lect h is fee. Modern rainmaking tec h n iq ues a re based o n known facts of coa les­ cence a n d g e n u i nely influence ra i n ­ fa l l a n d snowfa l l (see pp. 32-33). A l l modern tec h n iques depend u p­ on the "seed ing" of a rtifi ci al n uclei i nto potential rain c l ouds. Si lver iodide crysta ls a re most common ly \Jsed.

Hopi I n d i a n s dance for rain.

1 9th-century rai nmakers.

Silver iodide generato r.

Dry ice d ropped from plane forms partly clea red strip in stratus clouds.

M O D E R N RAI NMAK I N G g rew out of studies of how a ircraft c o l lect ice on wings and other surfaces. Dr. Vincent J . Schaefer was attempting to prevent the formation of su percoo led clouds (which cause ice to form on a i rcraft). H e discovered that tiny bits of dry ice (frozen carbon di­ oxide) produced fa ntastic n u m bers of ice n uclei when "seeded" i nto c louds colder than 32°F.A piece of d ry ice the size of a groin of rice caused the formation of more than a tri l l ion ice crysta ls. Each g rew at the expense of supercoo led cloud droplets and formed snow (see p. 23). The technique of seeding supercoo led clouds has since been used to l essen ground fogs and, u n der certa i n con­ ditions, to induce snow o r rainfa l l from I o rge c u m u lus c l ouds. Dr. Irving Langmuir a n d Dr. Bernard Vonn egut pioneered further studies by seed ing c l ouds with water and s i lver iodide. The water i n itiates coalescence. Since the si l ver iodide has a molecular structure nearly identical with that of ice, it i n duces coa lescence i n m u c h the some m a n n e r as ice crysta ls wou ld.

32

C L O U D S E E D I N G experi ments continued with min iature clouds produced u n der laboratory condi­ tions. Va rious substa nces were sprayed into a cold chamber i n an effort to produce ice crysta l s or co­ a l escence. Dry ice and silver iodide ---� .- ,. both worked we l l . Finely ground ,.,.. -dry ice (with a tem perature of : - 1 08 ° F) ca used cloud drop lets to crysta l l ize a long its path. These Seeding a cumulus cloud with d ry ice. crysta ls grew rapidly at the expense of the water droplets around them, and soon be­ came large enough to fa l l . Si lver iodide, on the other hand, acted d i rectly as a n u c leus for ice formation. These ice c rystais, a lso, g rew until they fe l l . C louds a re n o w seeded with either dry ice o r si lver iodide. One po u n d of dry ice spread by a plane may start a shower in large c u m u l us clouds. Si lver iodide is l ess expensive to use, because it can be sent up from the ground to c l ouds from spec ia l gen ­ Ma n-mad e snow cloud i n erators. But cloud seed ing is not a cold chamber. successful u n less conditions are nearly right for natura l precip­ itation. Seed ing can i n duce ra i n under t h e right conditions. It can cause more ra i n to fa l l than wou l d occur with u n d istu rbed natura l con­ d itions. It can n ot produce rain from fa ir-weather c u m u l u s c l o u ds. Nor is it yet possible for cloud seeding, which is sti l l i n its infancy, to in­ duce rain to fa l l over a widespread area .

...

Courtesy General Electric Company

...

The Atmosphere Earth's atmosphere would be more a pparent to a traveler mi les out in space than to us on the ea rth 's s u rface. The s u n 's rays, scattering i n the atmosphere, wo u l d make the l ighte d side of the earth a brig ht, fuzzy crescent, faintly b lotched with c l o uds along the twi light zone. D iffusion wou l d extend the cresce nt of l i g ht far around the earth's c u rve. O n the dark side, l ig hts of g reat cities might show as tiny, dimly s h i n ing spots through the atmospheric vei l . 34

A f u l l u n dersta nding of the weather req uires knowledge of the atmosphere. We l ive a t the bot­ tom of a yirtu a l ocea n of a i r. Exte n d i n g u pward perhaps 1,000 m i les, this m assive, restless ocean is fa r d ifferent, and fa r m o re tem pestuous, than the watery ocea ns that cover three-fourths of the g lobe. A na rrow band of com pacted air lying j ust above the earth is the reg ion of continuous winds. Here, the risi n g and fa l l i n g air cu rrents sometimes develop i nto violent storms. Only recently have the most ad­ vanced a ircraft ventured a bove this thin layer, some 5 to 11 m i l es thick. The ocea n of air differs in one major way from a n ocea n of water. Water is nearly incom pressible. A cubic foot of water on a n ocea n bottom weighs much the same as a cubic foot near the surface. But the air of the atmospheric ocea n is h i g h l y com­ pressible: a cubic foot of air at the surface weig hs b i l l ions of times a s much as a cubic foot a t the outer edge of the atmosphere. The atmosphere thins so rapidly as one leaves the earth that, only 3Yl miles u p, over half the atmosphere by weight would lie below you. It is c h iefly in this 3Yl- m i l e b l a n ket of heavy a i r that weather cha nges are born. The at­ mosphere 500 miles out is so thin that there a re o n l y a bout 22 m i l l io n m o l e c u l e s o f air p e r cubic i n c h , com pared to b i l l ions u p o n b i l l ions at the earth's sur­ face. Sti l l farther out, the ever-th inning atmosphere b le n ds with the stray gases a n d dust of o uter space.

Composition of air at a ltitudes above 500 miles helium 50%

oxygen 2 1 % argon 0.93 % carbon d ioxide 0.03 % a l l other gases 0.04%

36

AI R CONSISTS MAI N L Y O F G ASES

that wi l l not d irectly susta in life. Oxy­ gen, which a l l l iving things need, ma kes up s l ightly less than 2 1 per cent of the a i r. I nert n itrogen m a kes up 78 per cent. The remainder of the g ases, a l l tota l i n g less t h a n 1 p e r cent, are carbon dioxide, argon, neon, radon, h e l i u m , krypton , xenon, hydrogen, m ethane, n itrous ox­ ide, and ozone (a form of oxygen). Besides these, air conta i n s u p to 4 per cent water vapor, a l so d ust and gases such as smoke, sa lt, other chemicals from sea spray or ind ustry, carbon monoxide, a n d m icro-org a n isms. If the a i r were perfectly q u iet, the heavier pa rticles a n d g ases wou l d settle c l ose to the earth a n d the l i g htest w oul d be fou n d the fa rthest out from earth's surface. But the consta nt motion of the air near the surface m ixes the gases so that the same proportions exist from the earth's surface up to about 45 m i les. Far­ ther out a re fou n d c h iefly the lig hter gases. Proba bly o n ly the lig htest ga ses, h e l i u m and hyd rogen, a re fou n d at heig hts a bove 500 m i l es. In intermed iate levels are found h i g h concentrations of ozone and ionized n itrogen, together with s m a l ler q u a ntities of othe r ion ized gases. Ozo ne, by a bsorption, plays a l a rge part in the earth 's heat b a l a n ce­ as does the increasin g a m o u n t of m a n ­ made carbon d ioxide.

P H E R E First a n d most im porta nt of the atmosphere's four l a . y ers is the troposphere, which lies c losest to earth. Next a bove is the strato­ sphere. Where the troposphere ends and the stratosphere begins is a boundary cal led the tropo­ pause. This averages 5 m i les above the earth n ear the poles, a n d 1 1 m i l es a bove at the equa­ tor. The stratosphere ·goes u p to a bout 50 m i l es. Above this is the ionosphere, extending out to about 650.m i les. Here are ion ized, or electrified, pa rticles that reflect long radio waves back to earth. F i n a l ly, a bove the ionosp here is the exosphere, about which ·l ittle is known.

TROPOSPHERE -WEAT H E R B R E E D ER In this layer are nearly all of the c l o u ds. Here is w here weather occurs. A i r, heated by co ntact with the earth, rises and is replaced by colder air. These vertica l currents create hori­ zonta l winds at or near the surface of the earth. Water, eva porated from the l a n d and seas, rises with the ascend­ ing warm a ir. As the air rises, the surrounding pressure lessens, so it steadily expa n ds. Expansion is a cooling proc­ ess (see below). If the air rises high enol!g h, it cools until condensation forms clo uds.

T HE

Expa nsion of any gas is a cooling process. Com pression creates h eat. The cylinder and hose of a tire p u m p get hot as air is p u m ped and com pressed. The sudden expan­ sion of gas rushing out of a n aeroso l bomb cools the tip. Most air conditioners work on the same principle, as shown below. A gas is compressed in the part outside the house. The heat of com pression is g iven off to the o utside air, a n d the gas condenses to a liquid. The liquid is forced through a tiny nozzle and expa nds sudden l y i n the coils inside the house. This expansion (and evaporation back to a gas) coo l s the coils and a lso the air w h ich is b l own over the coils i n to the room. H O W AN A I R C O N D I T I O N E R W O R K S

I N DOORS

house wall

OUTDOORS

exhaust air

outdoor air

7000 ft.

41.5 °F

6000 ft.

47°F

5000 ft.

52.5 °F

4000 ft.

58°F

3000 ft.

63.5 °F

2000 ft.

69°F

Adiabatic temperature cha nges as a i r travels aver a mountain.

The tro posphere has a kind of a utomatic air condition e r. The primary h eat p u m p is, of course, the s u n . It heats the earth's surface which, in turn, heats the a i r in contact with it. The air expands, becomes lig hter, a n d rises. But the higher it rises the more it expands, because the pressure aro u n d it is stead i l y lessening. A n d the more it expa nds, the more it coo l s. This is a n a utomatic cooling process which occ u rs without any loss of heat due to outside ca uses. The rising air coo ls a utomatica l ly at about 5Yz ° F f o r e a c h 1 ,000 ft. it rises. T h i s is w h a t ha ppens when a i r rises u p the side of a mountai n . When it g o e s d o w n t h e other side, it beg ins com pressin g . It w a r m s u p i n doing s o a t the same rate (5\12° F per 1 ,000 ft.) i t c o o l e d i n risin g . This a utomatic tem perature c h a n g e in risi n g or fa l l ing air is ca l led "adiabatic" warm ing or coo l i n g . Air need not be pushed u p by a mounta i n for this adia batic change to take place. Air rising over heated plains w i l l a lso cool at the same rate of a bout 5\12° F per 1 ,000 ft. rise.

39

I u ses this much heal

C O N DE NSAT I O N compl icates adia batic coo l i n g a n d

warming, a n d· ma kes the temperatures on t h e windward side of mountain ranges lower than those o n the leeward side. Ma inly because of condensation, there a re relatively cool va l l eys west of the Sierra Nevadas and hot deserts to the east. Considerab l e heat is needed to change l iquid water to water va por. Heat increases the speed of water molecu les, so that many more escape as water vapor. Changing a pan of water at the boiling point to vapor req uires six times the heat needed to ra ise the same amo unt of water from freezing to boiling. When water vapor condenses back into l iquid water, the sa m e large amount of heat is g iven off. This heat can increase the tem perature of the air considera b ly. When air is rising, two opposing influ­ ences operate as its moistu re condenses: adiabatic cooling tends to lower its tem perature; a n d heat of condensation tends to raise it. The net effect is a n avera ge coo l i n g of 3.2 ° F per 1,000 ft. of rise when condensation occurs. Ta ke an exa m ple: Air at 60°F moves up the west side of a mounta i n . It cools ad iabatica l l y 5 . 5 ° F for each 40

1 ,000 ft. of rise up to the clouds at 4,000 ft. As conden­ sation begi ns, heat is released, and the adia batic coo l i n g is t h u s partly offset. F rom 4,000 ft. to the mounta i n top, at 1 0,000 ft., the net coo l i n g is only 3.2 ° F per 1 ,000 ft. Thus the tota l coo l i n g of the wind as it sweeps u p the mounta i n is 4 x 5.5, or 22 ° plus 6 x 3.2, or 1 9. 2 ° Total 4 1 .2 °

from va l l ey floor to cloud base from cloud base to mounta in top from va l l ey to mounta i n top

The a i r that began at 60 ° F tops the mounta i n at 18 .8 ° F . A s t h e a i r pours down t h e eastern sl ope, it co mpresses and wa rms, ad ia batical ly, at the rate of 5 . 5 ° F per 1 ,000 ft. Because of the warming, no further condensation oc­ curs. Tota l warming of the a i r from mo untain top to val ley, 1 0,000 ft. below, is 1 0 x 5.5 ° F, or 55 ° F . Adding this to the mounta i n to p temperature of 1 8 . 8 ° F g ives 73.8 ° F as the temperature of the eastern va l l ey. The temperature at the eastern foot of the mountain is 1 3. 8 ° F higher than at the western foot. Condensation and precipitation o n the western slope ma de the d ifference. Chinook winds are a n exa mple of this down -slope warmi n g of air. They occ u r on the easte rn slope of the Rockies, often with dramatic effect. One C h i nook brought the temperature u p from -6 ° F to 3rF i n 1 5 min utes. Condensation mod ifies adia batic coo l i n g .

41

T R O P O P A U S E A N D JET

The tropopa use-the zone that ma rks the end of the troposphere and the beg i n n i n g o f t h e nearly weatherless strato­ sphere-was once thought to be continuous from poles to eq uator. We now know that the tropopause has breaks, giving it a n overlap­ ping, leaf-like structu re. These breaks are i m porta nt i n connec­ tion with the jet strea ms. A jet strea m is a tubular ribbon of h igh­ speed winds, genera l ly from the west a n d some 20,000 to 40,000 feet up. Jet streams form at the overla ps, pa rticu l a r l y of the a rctic tropopause and the extratro pica l tropopause. They are at l east par­ tia l ly the result of the strong tem­ perature contrast there. STR EAM

Typical jet stream paths.

A jet stream was d iscovered by American B-29 pilots flying to J a p a n from the Ma rianas i n World War I I . They consistently reported westerly winds with speeds far i n excess of those expected. A jet stream is usua l ly a bout 300 m i les wide a n d 4 m i les high. At its core it averages 100 m i les per hour i n winter and 50 m i les per hour in sum mer; these speeds may rise to 250 m i les per hour or more. Forming a wavy path at the top of the tropo­ s phere, the jet stream assists high-fl ying a irplanes travel ­ i n g east; p l a nes g o i n g west try t o avoid these stro n g headwinds. T h e strength o f the jet winds decreases o ut­ ward from the core, a n d from place to place i n the stream. The n u mber of jet streams a n d their paths varies from day to day and season to seaso n . A typica l jet stream has a reas of maxi m u m winds a l o n g it t�at tend to travel eastwa rd. Two p l aces where th ese wind max­ imums occ u r with g reat freq uency are across Japan a n d over the N e w E n g l a n d states. I n winter, over the North American Continent, there are th ree major jet strea ms: one over northern Canada, one over the U n ited States, a n d one over the su btro pics. Structure of a jet stream

20,000 ft.

43

T H E STRATOSP H E R E is the a l most weatherless pa rt of the atmosphere. It extends from the tropopause u pward a bout 50 mi les. The name suggests its nature, for it usua l l y has very l ittle vertical air movement; it is a u niform l ayer, or stratum . Temperature drops much more slowly with height than i n the troposphere. I n fact, the tem perature begins to rise again near the top of this layer. F lying in the stratosphere is genera l ly smooth, a n d the visibil ity is a lways exce l lent. The air is thin and offers very l ittle resis­ tance to a pla ne; hence a gallon of fuel ca rries a plane m uc h farther through the stratosphere. A region of no weather, the stratosphere is preferred by jet pil ots, for here they can fly at top speeds with l ittle fear of turbu­ lence. Often too far above us to be seen, the jets c ha l k their path across the sky a s moisture from their engines · forms condensation trails-strea ks of fi n e ice crysta ls in an otherwise weatherless atmosphere.

44

T H E I O N O S P H E R E lies a bove the stratosphere, extend­

ing outward some 650 mi les. Here the air is extremely thin. The scattered a i r particles are ion ized; that is, electrified by the remova l of a n electron, o r negative charge of electricity. The relentless bombardment of cosmic rays from o uter space ca uses this ion ization . Were it not for this ionized air we could not receive radio broadcasts beyond the horizon, for radio waves go in a stra ight line. Layers of ion ized air reflect the radio waves back to earth. The severa l m irrorl ike rad io-wave-refl ecti ng layers are the Ken n e l ly-Heaviside or E Layer, 50 to 80 m i les up; the F Layer, 1 50 to 200 mi les u p; and a varia b l e n u m ber of other laye rs that resu lt from spl itting of the E a n d F Layers. The air particles in the ionosphere are hot. Estim ates put them at some 1 ,000 ° F to l ,500 ° F during the day a n d 300 ° F at n i g ht. These extreme temperatures a re proba bly ca used by cosm ic-ray bomba rdment. But when space travel becomes a rea lity, we need not fea r being broiled a l ive. The a i r partic les a re so fa r apart and so tiny that we will proba b l y not notice them at a l l . Much more dan­ gerous wi l l be the cosmic rays.

Radio waves reflected by the ionosphere can be received beyond the horizon.

45

-

-- --�----

T H E EXOS P H E R E is the fina l a n d hig hest layer of the

atmosphere. Hotter,. even, than the stratosp here's a i r particl es, t h e ions o f g a s in t h e exosphere a r e possibly as hot as 4,500 ° F . They are bombarded so fi ercely by cosmic rays that they exist o n l y in atomic fo rm, instead of mo l ec u l es. At night, shielded from the sun's d irect rays, the temperature of the partic les drops nea r ly to absol ute zero -about -460 ° F . HIGH

ALT I T U D E

P H ENOMENA

A u roras resu l t from the action of streams of solar particles on the ionosphere, 50 to 600 m i l es up. What happens is much l ike events in a neon tube. Beca use the earth is a magnet, ion ization is stro ngest near the poles, a n d the a u roras are seen ma inly at h ig h l atitudes. Mother - of - pearl Clouds may

C u rta i n a u rora

consist of water droplets. These rare c l ouds a p pear as bands of paste l colors, 1 4 to 1 9 miles hi gh. The sky is otherwise clear at the a ltitudes where they a re seen.

Nocti l ucent C l ouds, the hig h est clouds known, a re

proba b l y formed from meteor d ust. Appearing in the western sky shortly after sunset at a height of some 50 mi les, they have a gold edge near the horizon and a re b l uish w h ite a bove. Both nocti l ucent a n d mother-of- pea r l c l o u d s travel a t terrifi c speeds-394 miles per hour was once observed . Meteors are visitors from outer space. They h it our atmosphere at tremendous speeds- perhaps 90,000 m i les per h o u r. Friction with the a i r of the upper atmosphere h eats them to incan descence, a n d most of them va porize into gases o r disinteg rate into h a r m l ess d ust before they come to within 30 m i les of the earth's surface. Thus our atmosphere protects us. Mi l l io n s of meteors, most of them sma l l e r than grains of sand, hit our atmosphere every day. Very few ever reach the ground.

The E a rth's M oti o n s a n d Weather The earth has five motions i n space. It rotates on its axis once each 24 hours, with a slow wobble ( l ike that of a top) which takes 26,000 years to com p l ete. It revolves around the sun at 1 8 Vz m i l es per second, making the circuit i n 365 1;4 days. It speeds with the rest of o u r so lar system at 12 m i les per second towa rd the sta r Veg a . F i n a l ly, o u r entire ga laxy, with its b i l lions of stars, is rotat­ ing in space-our pa rt of it at a speed of 1 70 m i les per second. Only two of these motions affect the weather. But their effect is profou n d. Earth 's a n n u a l trip around the sun g ives us o u r seasons and their typica l weather. Earth's d a i ly rotation not on ly resu lts in n i g ht and day; it produces the ma jor wind belts of our earth, and each has its typica l pattern of weather. 48

CAUSE OF THE SEASO NS Seaso ns result from the fact that the axis on which the earth spins is sla nted 2 3 Y2 ° to the plane of its orbit. When the North Pole is tipped to­ ward the sun, the northern hem isphere has sum mer. The s u n 's rays beat more directly down o n the northern hem­ isphere a n d the days are longer. The sun is farthest north at the summer so lstice, about J u n e 22. Then the s u n is directly over the Tropic of C a ncer, and daylight hours a re lo ngest in the n orthern hem isphere and nights are the shortest of any time in the yea r. The North Pole is i n the middle of its a n n u a l period of six months of s u n l i g ht, a n d the South Pole i n the middle of its six months of relative darkness. At the w inter solstice ( a bout Dec. 22) the North Pole is tipped farthest away from the sun, which is now di­ rectly over the Tropic of Capricorn. The southern hem isphere has sum­ mer; the n orthern hem isphere has winter. C o n ditions are just the re­ verse of those at the summer solstice. The Anta rctic is now the " l a n d of the midnight s u n " and the Arctic is sun­ less. At the fa l l and spring equinoxes (about Sept. 23 a n d March 2 1 , re­ spective ly) the earth's tilt is side­ wise with respect to the sun. light fa l ls equa l ly on northern a n d south­ ern hemispheres. Day and night are of equa l d uration everywhere on the earth. The equi noxes mark the be­ ginn ings of spring and fa l l .

SUMMER IS WARMER tha n winter for two reasons: the

days a re longer (more time for the sun to heat the earth) and the sun's rays, striking our part of the ea rt h more d i rectly, a re therefore more concentrated. Days a n d n i g hts at the eq uator a re a lways 1 2 h o u rs long. The farther north you go i n summ e r, the l o n g e r the days a n d the s horter the nig hts. T h i s is so be­ cause the . sun in summer shi nes across the pole. The nearer one g ets to the pole, the longer the day be­ comes, until it fi n a l ly becomes 24 hours long. To a n observer i n the "land of the m id n ig ht sun" the sun a ppea rs to move a l o n g the hori­ zon i n stead of risi n g and setting. But a s winter comes, the s u n s e e m s t o m o v e south. The farther 4-- north you go, the shorter the win­ ter days a n d the longer the nig hts. summer The sun 's a pparent movement relative to the e a rth in w inter and in summer is shown below. O n Dec. 22, at the latitude of Was h i n gton, D.C., the s u n a t noon is o n ly about 27° a bove the south­ ern horizon , The day is short because the sun's path from eastern to western horizon is short. But on J u n e 22, at the sa m e l atitude, the s u n rises farther i n the north and at noon reaches about 74° a ltitude. Its path is longer a n d so daylig ht lasts about 1 5 h o u rs.

sun at noon December 22

sun at noon June 22

'W s

s

To see the effect of the d i rectness of t h e s u n 's rays, set a flashlight 1 ft. a bove a large sheet of pa per. Mark the outline of the circle of light it ma kes. Now h o l d the flash­ light at a n angle to the paper-but sti l l a foot away. Mark the outline of the e l l ,i pse of light it m a kes. The sa m e a m o u n t o f l i g h t h it the pa per both times. But the fi rst time it was concentrated, the second time more spread out. When the rays were direct the s u m mer concentration for a u n it area was g reatest. sun The summer sun fo l l ows a path more nearly overhead, so its rays are more con­ centrated. The winter sunlight h its the earth at a g reater sla nt. The sa m e amount of sun­ light now spreads out over a larger area. I n addition , winter s unl ight m ust pass through more atmosphere, because it has a more sla nted path . More energy is dif­ fus � d by the atmosphere a n d less reaches the earth to warm it. winter sun

solar radiation under a cloudless sky

temperature average

SEASONAL LAG Aug ust is hotter than J u n e even though the sun is more n early overhead and the day is longest on June 22. I n terms of so lar radiation reaching the earth, May, J une, a n d J u ly shou l d be our warm est months. But J une, Ju ly, and August actu a l l y a re. Why? During the yea r the earth, as a whole, l oses precisely the same amount of heat it receives from the sun. But as the sun moves north in spring, our part of the earth gains heat faster than heat is l ost. On J u ne 22 it is receiving maximum solar radiation. The heat gain continues to ex­ ceed heat loss until maximum warmth is reached, usu a l l y i n late J u ly. Heat gain continues t o exceed h e a t l oss, at a diminishing rate, until about Aug ust 3 1. Then our part of the earth sta rts to lose heat faster tha n it receives it, a n d b e g i n s t o c o o l down. T h e process is l i k e sta rti n g a fi re in a stove: the roa ring fi re heats the room s l owly, but the room will stay warm for a while after the fi re has died down. The same heat lag accounts for the fact that the warmest time of day is usua l l y about 3 p . m .-not noon, when the sun's rays are most intense.

52

90°

. 0 mph

•

80° . 70° . 60° . 50� 40° .

1 75 340 500 640 770

30°.

865

20° .

940

1 00 .

985 . 1 000

A p p roximate speeds of the earth's rotatio n at various l atitudes.

If the earth were not rotating, h eated air wo u l d rise over the eq uator a n d move n orth a n d south h i g h a bove t h e earth's surface. I t wou l d c o o l a n d si n k down at the poles. The s i n k i n g a i r wou l d force air at t h e earth's surface toward t h e eq uator. So surface wi n ds o n a smooth non-rotating ea rth would a l l move directly towa rd the eq uator with equal speed (see p. 1 0). The earth's rotation cha nges th is. The earth at the equator is about 25,000 m i les around. The ea rth ma kes one