The Climate of Cities

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Established 1845

SCIENTIFIC AMERICAN 1961 August

Va.

The Cllinate of Cities
The variables of climate are profoundly affected by the physical characteristics and human activities of a city. Knowledge of such effects may make it possible to predict and even to control them
by William P. Lowry

t is widely recognized that cities tend basic inSuences that set a citv's climate to be wanner than the surrounding apart from that of the surrounding area. countryside, and one is reminded aJmost daily by weather forecasts such as The first inJIuence is the difference between surface materials in the city "low tonight 75 in the city and 65 to 70 in the suburbs. - Exactly what accounts and in the countryside. The predOmifor the diHerence? ~teteorological stUd- nantly rocklike materials of the city's ies designed to answer such questions buildings and streets can conduct heat have now been made in a number of about three times as fast as it is concities. :\luch worL:remains to be done, ducted by wet, sandy soil. This means but one thing is clear. C~tiesdiffer from that the city's materials can ac:cept more the countrYside not onlv in their tem- heat energy in less time, even though it perature b~t also in alJ ~ther aspects of takes roughJy a third more energy to climate. heat a given amount of rock, brick or By climate is meant the net result of concrete to a certain temperature than several interacting variables, including to beat an equaJ amount of soil. The temperature, the :amount of water vapor temperatw"e of soil at the wannest time in the air, the speed of the wind, the of the day may be higher than that of a amount of solar radiation and the south-facing rock wall, but the temperaamount of precipitation. The fact that ture three or four inches below the SUlthe variables do not usually change in face will probably be higher in the wall. the same wav in a city as thev do in the At the end of a day the rocky material open countrY nearby"can oft~n be mea- will have stored more heat than an equal sured directly in diHerences of tempera- volume of soil. Second, the city's structures have a ture, humidity, precipitation, fog and wind speed between a city and its en- E3r greater variety of shapes and orien"irons. It is also apparent in such urban tations than the features of the natural phenomena as persistent smog, the ear- landscape. The walls, roofs and streets lier blooming of flowering plants and of a city function lilce a mue of reSeclonger periods free of frost. tors, absorbing some of the enngy they The city itself is the cause of these receive and directing mucb of the rest to differe~. Its compact mass of build- other absorbing surfaces [ue top ill.".. ings and pavement obviowly constitutes tration on page 11]. In this way almost a profound aheration.of the natural land. the entire surface of a C'ityis wed for ac$Cape, and the ac:thities of its inhabi- cepting and storing heat. whereas in a tants :u-e:1considerable source of heat. wooded or open area the beat tends to Together these factors account for five be stored in the upper parts of plants.

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Since air is heated almost entirely by contact with warmer surfaces rather than by direct radJation, a city provides a bighly efficient system for using sunlight to heat large volumes of air. In addition, the city's many structures have a braking deet on the wind, thereby increasing its turbulence and reducing the amount of heat it c::m:iesaway. Third. the city is a prodigiow generator of beat. particularly in winter, when heating systems are in operation. Even in summer, however, tbe city has many sources of heat that the countryside either ladcs or has in far smaller ~umbers..Among them are factories, vehicles and even air conditioners, which of course mwt pump out hot air in order to produce their cooling eJfect. Fourth, the city has distinctive ways of disposing of precipitation. IE the precipitation is in the form of rain. it is quickly removed from the surface by drainpipes, gutters and sewers. U it is snow, much of it is cleared from the surface by plows and shovels. and signiJi. cant amounts are carried away. In the country much precipitation remains on the surface or immemately below it; the water is thw available for evaporation, wbich is of course a cooling process pow. ered by heat energy. Because there is Jess opportunity for evaporation in the city, the beat energy that would have gone into the process is available for heating the air. Finally, the air in the city is different IS

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HEA T P.-\TTER:'i5 in Ihe lower :\Ianhlnaa area of :'iew York Cil}' on a ~ummer dlY 3re .howo b~' infrued pholo,r~ph1". In Ihe pholo,r2pb.. ,,'hi.h ,,'ere ma.le wilh " Same. Iherrao,nph. the Ii,hlest are.. are Ihe ,,'arm..t "od Ibe darl..e.1 are the coolesl. The "ie... "bove

,bow, Ihe baildin" at .boUI 11:00 .1._'1.:lDd the oriew below .1 .boat 3:30 P.:W.Tbe d.y w.. tUDD1"bal huy; Ihe lemperalUre in the .ity durin!! Ihe time .ovend by the pholo,..pb5 WI' .boat 75 de,rus F.brecheil. Tbe .Iora,e of beat by buildiD,. .Iect'. city', climale

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in that it C".1rries heavy load of solid, a liquid and gaseous contaminants, About 80 percent of the solid c:ontaminantsare in the form ol particles that are small enough to rem.un suspended lor s~\"era)
U.1 in still .1ir, Although these particles ~'s collectively tenr.! to reSect sunlight. thereby reducing the 3D'Iount of heat reaching the surfaces. they also retard the outBow of heat, The gaseous contaminants. which u~ually ha\"e a greater total m3SS than the solid ones, l-ome primarily from the :ncompl~te c')mbustion of fuels, One of the principal g:1Ses in many cities is suiIur dioxiue; when this gas is dissoh'ed under the appropriate meteorological conditions in c:loud uroplets or raindrops, it is oxidized to lonn dilute sulfuric acid.

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US consider how these fh'e inBuences act o\'er a penoJ of time on the

climate of a 1.1I"'~eity. Our hypothetk;1\ c c:t\' '-ies in an .1re3 of Bat or gentlv roll, ing cvunt,:'<iJe and h3S :10 I.~rge bodies VI water nearh\", The d.l\ is a Sunda,', so th.Jt :10 SUbst~ltiaJ arn~unts oi fuei are b<:'illgu~et! for indu~tr!.l! pU!'pO~e:;,It i~ .1 summer d.1~', with "h:ar skies 30d light winds, . :\s the sun rises it shines equ.llly on cit~. .Jnd ,'ount,:', The sunlight strikes :lIe 3.1t, O~II colmtr;' at .I lo\\' angle; much of it is rellected from dl~ surf.lce. The nun." \'ertil'a! \\:Ills of :he city. how, t'\'er, are .Ilmost p"'rpendil'ular to the sun's .:I~'S, In spite oi the f.lct that when the sun IS lo\\' UI the sic:\" ra\'s are less its intense because the:-' o;ust j'>.L~shrough t mure oi ;/;e e.lr:h's .1mlOsphere, :he walls !.IegL'1 .Ilmo~t .11ollce to .1osorb he.1t. In the 'OIJntc\' ::ule heat is !wing Jbsurbed. <,"en in th~ SUII:it .1!e:lS, L.Jte~ in the d.l\' :hc ru:.11 .areas b~g:n to re>polld ~rhm~'like the dt~.., The s-un II.lS:1sen hi::h ~nuu!:h for Its r:loi.Jtion to unpll1g'! 01; the sl~rj.ace more directly .Inu \\ Ith !ess re:ll'ction. The air outside the: dty be~ms to wann r.1pidly, The city h:.lS.I:read\' !>e-~:I arming f0r some time, w howe\'er, 'ano so it ha~ ;1 laroe lead '" llI\\'ard the Jay'> rn:.1.XlIl1Um temper.ature. The warm air in the city conl1:mtrates ne.1r the ~'Ity's cent!!'! of ~:lSS, T ,",ward o:iumIJrning the J:r III the l:e:mer be~ins to rise, Being wamler :It !!'acn I!!'\'el th:1n the ..Iirat the s;une level in tile surround. ing countr~;side. the city 3ir continues to rise in a gentle stream ffo\\ing upward from the center. The :lir thJt rises must ~ rep13ced; hence a Bow from the rural ,u~as into the city begins in the lavers .il':1r the g~ound, The ;ir from the c~un. 11'\'must ;tIso be reol.1ced. Jnd graduallv .I 's:ow circulatio~ is established, Air
RADIATIO,," I~ COl'~TR'1SIDE lomds 10 be retlerled bar.k to Ihe "k~' beeau.e Ih.. rountry. side ha! ie..'er ,'ertiral.urfare. Ihan Ihe dl~'. To,,'ard mitlda~'. howe,'er, wh..n Ih.. .un', ny' are perpendicular to Ih" ,round. dly aDd rOUDtry I..mp..r:nur ma~' be about Ihe :ame.

5H..-\PE A:iD ORiE~,TATlO:-; OF ~l'RF.\CES in a dlY ha.-e a ,Iron, be:arin, on tb~ .Iimale. "erlkal ...aU. tend to reRet'1 .013r radialion toward Ib~ ,round iD.lead 01 tbe ;ky, RodJike materials al.o .Iore heal, so thai Ihe ,'ilY ofteD be"ome~ warmer than iu "n";rODS.

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TE:\IPERATl"RE DI::TRIBt:TIO~ in San Franri~co OD a !prinll tvrDiD, is drpi't-aed by DlUM of i.other1l1f. ..hirb an liAe. 01 eqaa)

tempenture. The lbadiA, hIDpel from the mOlt deDlel! built.up arru I dark I throap. Ie.. denae lectioDA to OpeD CODDtry 'lip,).

nlJ~S)O~S OF BEAT from aD automoDile wids ill enpDe idiiDI appeal" iD aD iDlr.and photolraph made ..itb a Bamea tbermo:8

,nph. Bri~t area below the ear i. paveaeal. wbitl! "":I, ia direfl lUDIilbL Yebides are a major sOlUce01 beat productioD in a dty.

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movesinto the center of the city in the lower layers, rises in the central core, flows ourward again at a higher altitude and as it coolssettles down over the open country to complete the cycle. Near midday the sunlight strilces the open country still more directly, and the differeo<:e in temperature between city and country becomes quite small. Now the air rising over the city is not appreciably warmer than the surrounding air, so that in tbe early afternoon the cyc:1e f circulation is considerably weak. o ened. As the afternoon progresses. how. ever, a situation similar to that of the early morning develops. The sun sinks, its rays striking the open country at a lower and lower angle; aD increasing proportion of its radiation is reflected. During this time the waDsin the city are still intercepting the sun's radiation quite directly. The difference in temperature between city and country begins to in. crease again, and the circulation of air rising over the city and sinking outside it is relnyjgorated. Just before SW1Set he t circulation is fairly strong, but it we3lc. ens again as darkness falls. At about this time one would be liJceJy find the tem. to perature at a weather station outside the city (such as at an aJrport) lower than the temperature at the downtown weather office. During the night the surfaces that radiate their wannth to the sky most rapidly are the streets and the ~ftops. If much of the rooftop area of the city is al about the same height, there will be a strong tendency for a cool layer of air to be fonned at that level. With cool air at the rooftops DOW lying below wanner air just above it, a rather stable stratification of air develops, and any tendency for upward movement of wann air in the spaces between buildings is inhibited. The o,,-erall situation now is that the roraJ area is cooling rapidly and the city area is cooling slowly. Heat is being remo,'ed from the fields by light winds and by ahnost unobstructed radiation to the night sL")'.10 the city, however. pockets of air are trapped. They cannot move upward, and they are still receiving heat from the release of energy stored io the walls of the buildings during the day. Througb the night both the city and the countrYside will continue to cool, but bv da~ the citY is stilllikeiv to be four ~ five de~s warmer dun its surroundings. E:uly ~lond:1ymorning the factories in the city begin to put forth heat, smoke and gases. Automobiles, trucks and buses start to emit large quantities

ANNUAL TEMPERATtJJlE

RECORD

o. Wuhiafl6D,

D.c., and I.. eD.-ir4!'!I' :iYM ~

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enle 01 aDDU) miDimDJD '_peraton, 'or ihe period 1946-1960.n. area iDaide eloted ilotherm. eoDllitate what is DOWD.. ihe hat .bad.. Bere .. iD olber eities the ialaacl ia ...oeialed with the IDO.'deDiely huill-up p.n of the IU'b.. eompla.. nil map aDd the ODe below are b.led ODau obtaiDed by ClareDee A. WOOUIUD the u.s. WMtb.er Banaa. of

RECORD OF PRECIPITATION in the WuhiDl10D .rc. eoyft1 the ..me 15 yun U Ihe lemperature record. Both tOpcllrapby ad the esUleDce of the eil7 .flec' preripiUltioa. 19

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DUST ooalE

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pc periodicaOy y"!us. chi.. MaG" .f O

the panicl.. of du.t aad 'lIIokcWI enter tbe air .. a reed .f ac. li..lIl.. iDIbe elly. Air ICDdto riM:oYerthe w_er cellini pan.f t of heat 3nd fumes. Even sto"'es in kitchens constitute a .sourceof heat that cannot be neglected. .o\rtificialheating and air poUution thus become meteorologicaDysignificant :ISthe day begins. As before, the early sun starts to warm the dty's walls IUld streets, and heat begins to accumulate in the downtown

tho city aDd to MttJ. ...er lb. eooJer 8DYir to tbaa a elrealatory 'yMea de...&op.. Dome ill likely t. penitt, .ipi6c8DIJy a~ccUnl &be city', cUmate. aDliJ a ItrODI wiDd or a baa", nia carriea II away.

area. Today, however, there is a difference because of the heat being added to the system by the tall chimneys of factones. Ordinarily air rising to the height of the chimney tops would have had a chance to cool, but now it receives more heat at that level and wiD probably rise higher above the city than it did on Sun-

day. ~Ioreover, the column of air now c:arries a freight of particles of dust and smoke. The smallest particles will faD only after they have been carried away &om the rising column of air and out over the suburbs. Other particles will remain suspeoded over the dty aDday. Over a long period of time the coo-

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SO 100 120 140 CiTY COMPA;<EDWITH ENVIRONSiPERCENT)

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~J..UOR DIFfERENCES IN CLnI.-\TE betweeD a city Icolor) aDd iI. euyjroD' ',ray I an M:tout io ICnD:iof the per..eulale by which the city bat IIIOreor leu of neb climatic variable dunal a year tbu i. etperieuced ill the ..oantryfide. For esalllple, the city rOo 20 106

W 5 pcrceDt Jes. uhnYiolea ndialiou tb.an Ibe ".!mtryside h. ._er. 30 pereeDll... iDwiDter; frequency of fo, iu city ia 30 per. teIIt hillier ill _Iller and 100 Perceal.hieAer ia winter. findia,. were made by Hellllut E. Land.berl of the Univerlity .f 11.ryland.

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The heat island has been observed in percent more fog in summer and 100 percent more in winter. many cities, some large and some small. T. J. Chandler, director oE the Lon- some near water and some not, some don Climatic Survey, has compiled a with biDs and some with none. How, number of records for the London are3. then, can one be sure that the heat He has found that over a period of 30 island, and thus the city cUmate itself, is years the average miL'(imumtempera- really attribu~ble to the works of man? tures in the city. the suburbs and the J. MUrTayMitchell, urban climatologist surrounding counb)'side were respec- in the U.S. Weather Bureau, has contively 58.3, 57.6 and 57.2 degrees and sidered the question and found three In sum. a citY'seffect on its own climate the average minimums 45.2. 43.1 and kinds of evidence that the city climate is is comple~ and far-reaching. Helmut 41.8 degrees. His figures also show that caused by the city itself. First, cities exhibit the heat island E. Landsberg of the University of ~far:v- over the period the city had consistently land, who until recently was director of less sunshine than its environs did. whether they are Sat like Indianapolis or climatology in the U.S. Weather Bureau, Some of these broad findings merit built on hills like San Francisco. Hence has drawn up a balance sheet show- c:loser consideration. The patterns oE topography cannot explain the heat. ing the net effect of the variables [see temperature in a city can be shown on island pattern. Second, temperature rec, Ixmom fllustrGtion on opposite page). maps by drawing isotherms, or lines of ords averaged by day of the week show Among other things, he has concluded equal temperature, for variow times. marked diJrerences betweeD Sundays that cities in the middle latitudes receive Under a great variety of \vind. c:loudand and other days. Since many of the heat15 percent less sunshine on horizontal sunshine conditions isotherm maps all creating processes distinctive to cities surfaces than is received in surrounding show the highest temperatures clustered are inactive on Sundays, it is evident that rural areas and th:1t they receive 5 per- near the center of the citv, with lower those man-made processes account for cent less ultraviolet radiation in summer temperatures appearing ~dially toward the heat island. Finally, Mitchell has and 30 percent less in winter. undsthe suburbs and the countryside. The re- carefully examined the population and berg's figures also show th:at the city. sulting pattern oEisotbernu suggests the temperature records of a number of citcompared \vith the countryside, h3s a 6 term -heat island" for the wannest area ies and found that the size of the heat percent lower annual mean relative hu- [.see top illustration on page 19). The island and the diJrerence in temperolture term is wed regularly by meteorologists between it and surrounding areas in. midity. 10 percent more precipitation. 10 percent more cloudiness, 25 percent to describe this major feature oEa city's crease as population does. Another fact to be noted about tern. lower mean annual wind speed and 30 climate. -.'

tinuow introduction and movement of particles cre:ltes a dome.sh:aped layer of haze over the city. This structure, v:uiously c:alled the "dust dome" and the -haze hood," has long been cbar.1cteristic of large cities, although in recent years the general dirtiness of the air h3s made the dome harder to distinguish from its surroundings than it was several decades ago. Nonetheless, it still has a marked effect on the city's climate. At night. as the particles in the dome mol, they can become nuclei on which the moisture in the air condenses as fog. Tbe phenomenon occurs over cities in the middle latitudes when conditions are precisely right. Tbe first layers of fug will uswilly form near the top of the dome. where the particles cool most rapidly by radiation; the blanket becomes thicker by downward growth untif it reaches the ground as smog. This extra covering of water droplets over the city further retards nighttime cooling. Fog helps to perpetuate the dust dome by preventing the suspended particles from moving upward out of the system. Thus one day's contribution oE solid contaminants will remain in the air over the city to be added to the ne;'(t day's. In the absence of II strong wind or a heavy rain to clear away the dust dome. the haze becomes denser each day. In winter. since less and less sunshine penetr.Ites the dome to warm the citv naturally. more and more fuel is b~ed to make up the difference. The combustion contributes further to the processes that build up smog. It is in this gradual but inexorable way that the smog problem h3s attained serious dimension.~in many large cities.

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4 7 10 EASTERN STANDARD TIME

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7... AUGUST (SUNDAY.MONDAY)

CENTRAL PARK

10-11AUGUST (WtDNESOAY, THURSDAY)

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JOHNF.KENNEDY AIRPORT

JOHNF. KENNEDYIRPORT A

TEMPERATURE DIFFIRE~CES .ppar in rudin"'1 I .u&her "'IioD iD N_ York CiIJ .nd one .I.n .irpon in lb. enYiron. for Iwo 24-boUl' period. in Auf1I8I, 1966. Tbe Jl'8pb besiD8 .1 ~:00 A.M. for eub period. Temperalure diJrereDCCI .re oft.1I 1... Pl'OlIo_eetI OD weekead. &h,D00 weekd')'1 b«IIIK fewer of . dly'. hC81101U'U8 .ro opera1iD, ODweekend..

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MAR. APRil

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JUNE JULY AUG. TIME (MONTHS)

SEPT.

LOSS OF BRIGHT SUNSHINE in London compared with UeM lliftoaDdin. Ihe city i. esprelHd in lerml of aUuulel p8Z'day for each month. The fi,aret lbow the CilY'1 nue,e 1_
durin, the ~riod 1921 10 1950. London uea', di.triclI an npre.enaed by the derk line el lOp, the iDner luburbt by the middle line end the oater .aburbl by the hoUom line.

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tends to perpetuate itself until the climatic situation changes. Another connection between winter and the higher frequency of fog arises from the low temperatures. Alter an in cursion of cold arctic air the residents o. the city increase their rate of fuel consumption. The higher consumption of fuel produces more particuJate ponutants and more water vapor. The air above a city is usually quile stagnant following the arrival of a cold wave, and thus the stage is set for the generation of fog. Lacking ventilation, the city's atmosphere fiDs with smoke. dirt and water vapor. The particles of smoke and dirt act as nucleifor the condensationof the water vapor. Becausethe water is shared 3D1onga large number of nuclei, the air contains a large number of small water droplets. Such a size distribution of water droplets forms a persistent fog. andthefogretards warmingof the city. Retarded warming prolongs the need for extraheating.Onlyanotherchange of air mass will relieve the situation. This chain
of events has been associated with nearly

every major disaster resulting from :W pollution. pentures is that the maximum difference there. Part of the reduction of sunlight Reduction of visual range by smoke between city and countryside appears to in London :md other cities caDbe l:tid to aJone is not regularly recorded in cities. be about 10 to 1.5degrees Fahrenheit, the fact that a city tends to be more It is recorded at airports, however, and regardlessof the size of the city. Chan- cloudy than its environs. Warm air rising Landsberg bas been able to use dati' dler has foundthisto be the case in Lon- over the center of the city provides a from the Detroit City Airport. which i. don, which has a population of eight mechanism for the formation of clouds near the center of the city. and Wayne and I h:lvefound on many days when clouds fail to form County Airport, which is in a more rural million; my colleagues the same in Corvallis, Ore., which has a in the country. :area, to deduce something about clipopulation of about 20,000. The frequency of fogs during the win- mAtic differences between a city and ter has to do with the greater relative re- the nearby countryside. The reconls ductions in sunshine during the winter incfjcate that a city will have, in tbe Chandler's 6gures for the loss of sunlight in London show l:lrger losses nlonths, One caMot simply say, how- course of a year, 10 timesDlorehours in in winter, when the sun is low, than in ever, that the greater frequency of fog which smokerestrictsvisibilityto a mile summer. when sunlight t:tl.:es a short- explains the reduced total of SUDShine. or less than will be experien~ in rural er path through the atmosphere. The A. feedback process is involved. Once :areas. amount of reduction increases markedly fog forms. a weak sun has most of its enContr:1r)' to what one might thinlc.. tow',lfd the center of the city, showing ergy reSected from the top of the fog this situation may be improving someboth the greater depth of the dwt dome lilyer. Uttle of the energy penetrates the what. Robert Beebe of the u.s. Weather and the greater density of pollutants log to warm the city, and 50 the fog Bureau recentJy studied records of the visual range at the major municipal airports that did not change either their l0I I cation or their schedule of weather observation between 1945 and 1965. He CITY found that the number of times when : I i ! smoke reduces horizontal visibility at the i airports is less now than it was in 1945. SUBURB. . . I The change might be explained by efI forts to control air pollution, resulting I I in reduced concentrations of smoke and COUNTRY I . i in changes in the size and cb3racter of o 5 10 15 20 25 :JO 35 smoke particles.

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lOSS OF VISIBiliTY(PERCENTOF TIME)

FOG IN PARIS cal viaibiJity more i.. the cilY ,"'n in the l.rroUDdin. areal. Da.. ere lor winler and .bow .b~ pefl:~nt 0' lime wb~n Yilibility w.. nchu:ed 10 between OM aUJ. IDd a C(1WIeromilr y lilbt fOII'i,1II I. a qaarter-lDile 10300 reef b, moderate ro, (1Mflw.) b md Ifll lha 300 feel by denle fo..clorlci.ln Ibe 5aIDmer there wcre fu fewer day, 01 foe.

The diJlerences in moisture and precipitation between a city :md its environs :aresomewhat eantr:tdietorv. During periods without rain the ;elative

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scarcity of water for evaporation in the city results in a reduced concentration of water vapor in the air. Expressed as relative humidity. the diierence gives the city a reduction of 6 percent in the annual average of the countryside. of 2 percent in the winter average and of 8 percent in the summer. Even though the city is somewhat drier than its environs. on the days when rain or mow ,falls there is likely to be more in the city than in the countryside. The difference amounts to 10 percent in a year. It buiJds up mostly as an accumulation of small increments on drizzly days. when not much precipitation faDs anywh~re in the area. On such days the updrafts over the warm city provide enough extra lift so that the clouds there produce a slightly higher amount of precipitation. perhaps cited wiDleave the reader thinkhave the cablogue of differences I ing that the city climate offers no advantages ovec the country climate. ActuaDy there are several..i~luding lower he01tingbills, fewer d01j's ~'ith snow ~nd a longer ganlening season. Landsberg has estimated that a citY has about 14 percent fewer days with mow than the countryside. The season between the last freeze in the spring and the first freeze in the fall mav be three or four weeks longer in the' city than in the countryside. Both the advantages and the disadvantages of city climate testify to the fact that the city's climate is distinctly different from the countryside's. Every m:1jor aspect of cUmate is changed.. if only slightly. by an urban comple.'t.The differences in a small city may be only occasional; in a large city every day is different climaticallv from what it would have been if the ci~ were not there. Fuller unde~ding of the dimatic changes created by a city may make it possible to manage city growth in such a wav that the effect of troublesome changes wiD be miniuud. Perhaps the changes can even be made beneficial. Several organizations are accumulating climatological data on cities. I have ale ready mentioned the London Climatic SUlVey. Similar work is in progress in the U.S. Environmental Science Services Administration. at the University of California at Los Angeles, at New York Uni"'ersity and in the research laboratories of the Travelers Insurance Company. ~Ieteorologisls in those organiz:1tionsare driving instrumented automobiles, 8ying instrumented aircr:l.ftand operating hundreds of ground stations to obtain weath.

er data. Although the studies are aimed primarily at undeJstandiDg the meteoroIogic:a1problems of air pollution, other aspects of the loc:a1modiDcatioo of climate by cities will be better UDderstood as a result. What may be even more important is the possibility of ascertaining the potential of extensive urbanl%ation for

causing large-scale changes of climate
over entire

-contiDents.The evidenceis

not yet substantial enough to show that urbanization does cawe such changes, but it is sufficient to indicate that the possibility cannot be ignored. The acquisition of more lcnowledge about the climate of cities may in the long run be one of the keys to man's survival.
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