~LY,1921. MONTHLY WEATHER REVIEW. 891 THE EARTH’S WINDBELTS AS FACTORS OF CLIMATE.’ 5s/.$27 BY L.WBONACINA. 127 Tanza Road, Hampstesd, London. N. W. 3, July 2,1821.1 smopmn. Palestine, and the North African littoral, are character- The purpose is to show, with the aid of particular illustrations, that ized by the most obstinate drought, and even thunder- the study of causative climatic factor, No. 5, above, redly resolves storms tend for the most part in this peculiar region to itself into a discussion of all the temporary phases of wind and pre~~ure be deferred till the breaking of the drought in October. which make up the characteristic wind belts a8 Been on the niean annual and seasonal charts of the globe. These mean wind belts are to some The reason for this parching regime evident1 extent fictitious in that they hut rarely represent any close approach dynamical one, namely, that in summer t eis Atlanticfa;r1y a to actual momentary phases in the general circulation of the atmos- anticyclone ex ands so as to bring the Mediterranean phere. PO that it is necewry to study them in the light of the day-to-day within the em race of the northeast trade wind which variations of which they are the resultant effect. In other words, it is R necessary to regard climatic factor No. 5 as constituted of more or less can not bring southern Europe any rain, accentuated departures, often amountin to complete subversions of this current is flowing south and partly rtlyecause because it is a type, from the steady flow as depicted on tie mean charts. It is to these descending wind, the tendency to drought being, more departures that we really owe the changes of daily weather with their over, in these conditions further accentuated the immense consequences in the economy of the earth’s surface. The by “tradee” and “westerlies” are studied in this li@. It is shown, for steep tem erature adient from the coasts inland. example, that the theory of the “polqfront” of Bjerknes constitutes a But consit ering the %editerranean lands alone without sfendid system of reference in studying the day-to-dq‘ conditions of reference to other regions in the same latitude, it is e westerly wind belts of both Hemispheres. It is to this theory we difficult to understand why it is that an intensely heated must look to explain one of the most imprewive climatic features of the North Atlantic, namely, the midwinter concentration of cyclonic peninsula like Spain is not able to dispute the dominance storms. of the Atlantic high ressure regimen to a sufficient Emphasis is laid on the effect of the “monsoonal” or seasonal factor estent to permit the ;Pevelopment of monsoonal or con- of the mean surface circulation which in the Northern. far more than in vectional rains such as occur in China a.nd the eastern the Southern, Hemisphere distorts the eimple parallelism of wind and presaure bel& which should exist if the surface of the earth were all land art of the United States, which lie in Mediterranean or all water. Some puzzling anonlalies are ex lained on the principle ratitudes. There is. indeed, a summer lowv-pressure over of the regional com etition for air supplier. Y‘hus the ol,stiiiate sum- Spain, but it does not suffice to break the desiccating mer drought of the iediterranean Barin with its intense insolation and control of the northeast trade. The reason for thls exdveevaporation can only IJC? understood in the light of a drought- anomaly suggests itself when one remembers that in this providing wind and preaqure righie which has to fit in dynamically with different sigQirs coexisting in other regions. latitude the exigencies of the general circulation, in other words, the d namical factor referred to above, The various wind belts (climatic factor No. 5 in the pre- demand that the suQ tropical HIGHS exert their influence ceding paper) such as the ‘I trades” and “westerlies” somewhere, and that monsoonal modifications can not which appear on charts showing the average surface cir- ain the ascendant everywhere. There a pears, indeed, to culation, and which constitute very powerful climatic fe regional competition going on for trl e different wind factors, are just the resultant effect of continuously vary- systems, with the consequence that powerful rain- ing hases of the circulation, some of which roducing circulation being, for dynamicd reasons, marR edly from the average type represented Emited regionally, is most easily set u on the com act and it is essential to a true understanding of eastern sides of the Asiatic and Nor$ Anierican and of the belts that they be studied in the light masses, in China and the United St.ates, which lie tor the day variations. The circulation depicted on the mean west of the subtropical HIGHS where t,he air currents lend annual and seasonal charts of wind and pressure may, as themselves more readily to rain production through hav- is well known, be regarded as governed by three rimary ing a component of motion from south to north, mstead factors (a) the eneral thermal gradient from the Equator of north to sout,h, as on the western sides of these land to the Poles, (% ) the deflective influence of rotation, (c) masses. It is to be noted, however, that on account of the “monsoonal” or seasonal modifications due to the the unbroken compact character of the western side of irre ular distribution of land and sea. the North American Continent the Pacific Ocean HIGH Tie simple ardlelism of wind and pressure belts does not embrace the western United States to the extent which tends to fl e established b the joint effects of (a) that the Atlantic Ocean HIGH does the western Medi- and (b) is badly distorted in the horthern Hemispher,: by terranean, and the aridity of the Western States in sum- (c) , particularly in the months of extreme mean tempera- mer, or for matter of that at an season, is largely a ture, January and July, and the resulting circulation is “rain shadow” drought, the resuf t of drought-favoring really en interesting compromise between the different to ography. One has, therefore, to come to the some- influences. In January, for esample, the center of max- w% at paradoxical conclusion that the Mediterranean imum pressure in central Asia lies considerabl south of peninsulas to a large ext-ent owe their rainless summers the center of reatest cold, indicating that colB is not the to the presence of the Mediterranean Sea, whose relatively sole factor in 5eciding where the Asiatic anticyclone shall cool surface permits the enstward expansion of the be centered, there being a general dynamic factor as well. Atlantic anticyclone, and whose excessive evaporation But by far the most interesting case of compromise can only be restored in winter? where, the trade belt between the dynamical and monsoonal factors is sug- havin moved south, the dynamical situation permits of gested by a study of the summer conditions of the coun- rainfaP 1. tries bordering on the Mediterranean Sea in relation to It is now time to consider causative climatic factor other regions in the same latitude belt. Notwithstand- No. 5, “prevailing wind-belt,” in relation to departure ing the intense insolation and excessive evaporation over from the stable system of average conditions just dis- the Mediterranean Basin, the hot months, May to c.ussed. It is pointed out in another paper on the defi- September, in Spain, Italy, southern fiance, Greece, nition of climatology that it is important in the investi- gation of climatic conditions to study the articular 1 The above two eom lete ~~DCRare d ted in the library of the United States Weather Bureau, aud tle thirdis printedewith. weather conditions of which the averages are ma ae up; and

Unauthenticated | Downloaded 10/02/21 01:56 AM UTC a92 MONTHLY WEATHER REVIEW. JULY, 1921 so in the same wa factor 5, to be really understood, be on occasion quite deep, extending up to at least 15,000 will hence have to te examined in future in the li ht of feet, and perhaps to the limits of the tropos here, indi- actual day-to-day dispositions of the eneral circlli: ation. cating that the polar outflow is not such a sYl allow sur- The mean pressure chart for January sgh ows a barometric face current as used to be supposed. Moreover, it is to depression near Iceland and a HIGH in central Asia; but Bjerknes’ theory of cyclones one must look in order to on occasion one finds an anticyclone over Iceland or explain more closely rn the light of the day-to-day con- indications of a cyclone over Siberia, and it is a matter ditions of the circulation what was explained above in of fundamental importance to know the simultaneous the more general terms of the avera e conditions, namely, ressure distribution over the whole globe when such the excessive winter storminess of the North Atlantm sisplacements from the normal occur. Again, factor No. Ocean. 5, when specified for the British Isles, 19 the westerly This region of intense winter cyclonic energy extends belt; but we know very well that in consequence of the from about the middle of the United States east of the location of these islands between an ocean and a conti- Rockies, northeastward to the Arctic regions north of nent the westerly winds are far from regular, being Noma affecting the Great Lakes and the o en Atlantic liable at any time, but articularly during the spring es ai& that portion between Iceland anif the British period (March and A rile, to be interrupted by spells Isands.P Throu hout all this zone conditions are par- of east wind with mar1 -ed effects upon the weather. In ticularly f avorabfe to the inter lay of equatorial and olar other words, the “westerlies” in the Northern Hemi- air. Over the North Atlantic %etween Scotland anB Ice- sphere are very liable to be diverted by monsoonal influ- land warm equatorial currents are constantly being con- ences referred t.0 above, the term “monsoonal” being fronted with the outflow of cold air not only from the used in a eneral scnse for disturbances in the circulation Arctic regions, but also from the frost-bound continents of brou ht aB out by the distribution of land and sen. In North America and Europe which in winter enormously the 8outhern Hemisphere, on the contrary, there is no extend the reservoir of “polar” air. Similar rough con&- land in the track of the corresponding “westerlies,” tions afFect the North PacXc, but this ocean is a parentl which are ac.corclingly subject to no interruption other not quite so subject to severe winter ales as %e Nord than the temporary turbulent variat.ions associated with erhaps because of the supp5 y of “polar air” the passage of cyclonic s.ystem. In close connection from the 8orth American Continent is partly cut off from with this difference in re u1arit.y between the northern the Pacific by the trend of the great mountain ranges on and southern “westerlies‘ is a si nificant difference in that continent. In summer, on the other hand, when the the seasonal distribution of yks. In the southern air is limited to the Arctic regions pro er, ocean there does not ap ear to e that marked concen- oceans is much enfeebled; %ut tration of storms in the iY epth of winter which is such an it is also feeble over the conti- outstanding feature of the climate of the North Atlantic. season, the mean pressure is The interpretation suggested is this, that in the Southern lower than it is over the oceans. According to Bjerhes’ Hemisphere there is no land disturbance in the latitude theory c clonic activities is fundamentally a form of con- of the “westerlies,” or “roaring” forties, which are rough vection J motion in that it is chiefly due to temperature and stormy througliou t. the year without conspicuous contrasts. But over the sun-heated continents in summer seasonal variation. In the Northern Hemisphere, on the it k just this form of convection which does not take place other hand, the ver-v steep temperature gradient which to any vigorous extent. The form of convectional storm subsists in winter from the oceans (particularly the At- which does conspicuously occur is concentrated in lantic) to the cont,iiients is highly conducive to frequent loceneF zed thunderstorm systems. and severe cyclonic gales over the warm oceans. In It would appear, therefore, that in summer when the summer the reverse and less st,eep thermal gradient acts su ply of polar air necessary to intense cyclonic activity differently, reducing the tendency for oceanic gales and is Pqely cut off, the circulation due to local inequalities of Over the sun-heated continents the develop- denslty and pressure tends to be localized-short-cir- mentavorin7 o localized conrectionnl thunder systems rather cuited, as it were-in small thunder s stems character- than extensive cyclonic wind systems. ized by strong vertical motions which o not involve ex- But it is the theor\- of the “polar front” of Prof. tensive horizontal disturbance (advection).B In a climate B’erhirs which provides the most illurninsting system like England, whose climate is a good blend of oceanic and o1 reference in studying the day-to-day variations of the continental controls, there is a remarkable seasonal o po- westerly wind belts in eithcr hemisphere. The repre- sition between the oceanic cyclonic gales of winter ana the sentation of a simous oscillatiw line, often traceable continental thunderstorms of summer, the former being round the globe, where e ustoria7 air is confrontin the most frequent and severe in the four months surrounding outflow from the poles, 3ustrates t>hestructure 09 the the winter solstice (November to February), and $he “westerlies” of temperate latitudes in a way that was latter in the four months surrounding the summer solstice not available before. Thus the westerlies theoretically (May to August). blow a little south of west in the Northern Hemisphere In the Southern Hemisphere the westerly wind belt is and a little nort,h of west in the Southern, and in a region not so conspicuously more stormy in the depth of winter like the British Isles the most prevalent single direction than in the height of summer, evident1 because of the is actually southwest. But it is more correct to sa that absence of disturbing land matter. Revertheless the the revsiling westerly winds of northwestern Jurope southern westerlies of “roaring” forties, and the meeting osc‘ late betweea southmest and nort.hwest, according as place of e uatorial and polar currents, are stormy all the theJ air sup ly in t,he general westerly drift is equatorial ear roun and are accordingly riddled with cyclones. or polar. the polar air currents have been shown‘ to $hey are not the innocent-looking homo eneous winds which they seem on the mean wind charts of the globe, and ISee, for instance, Brooks. C. E. P., “The climate and weather of the FdLland Islands and south Ueor&.” (51.1 Gcnphvaieal Menoir*r No. 15. See also Ward, R. Da., aeronauts who might be tempted to use this southern belt “Climate Cmsiilerbl Es ~ecidlyin Relation to Man ” 1917. of “westerlies” as a quick west-to-east route from, sa , 4 Douglas c’. K. u.: hemperature variations in iowest iwr Lilameters. Jar. ~ov. Mettor. &e:, Jan., 1921. Abstract and discussion iu later REVIEW. South Africa to Australia, should be duly mindful of Je

Unauthenticated | Downloaded 10/02/21 01:56 AM UTC JULY, 1921. MONTHLY WEATHER REVIEW. aw rapid changes of wind and weather associated with the air between the Equator and the Poles is day by day cyclonic syst0lns. effected; and that it is owing to these momentary The most constant and steady of the earth’s surface turbulent deviations from the stable steady flow as wind belts is that of the trades, and on this account they represented on mean charts, that we get our daily weather might be looked upon as the mainspring of the circula- changes with all their momentous consequences in the tion of the atmosphere to which the other wind systems, economy of the earth. If there were no such de artures lower and upper, not so immediately related to equatorial from the steady flow, an inevitable result woul a be, for heating, adjust themselves. In relation to equatorial low example, that over large parts of the lobe it would pressure the trade winds represent a balance of cause and absolutely never rain at all, whilst over ot5 ers an almost effect, for while in the initial processes of the circulation continuous drizzle would fall; in other words there could these winds are set blowing to supply the defect of pres- sure over the heated e uatorial re ion, in the final adjust- ment they develop an% maintain t% e low pressure through the effects of the earth’s rotation. All land re ions in both Hemispheres, sub’ected more or less througa out the year to the influence o! this system of winds, are reduced to desert conditions except the windward slopes of mountain ranges which obstruct the path of the currents. But although fairly steady and regular over ext.ensive areas, the trade winds vary their limits greatly with the came from the Indian or the At.lantic Oceans. The mean seasons and with the nonpeiiodic changes connected with wind and pressure charts throw little light on the prob- day-to-day weather. In Europe, as already indicated, lem, and the very esistence of such a controversy is a the northward extension of the trade system causes the hint that both views are partly correct. But a knowledge normal summer drought of the Mediterranean Basin, and of the daily disposition of the sir currents, whose average when, as occasionally happens, the same system, as repre- structure we generalize under the term li southwest mon- sented b the Atlantic HIGH, pushes unusuallv far north, soon,” would reveal what roportions of the moisture weeks o9 very trying summer drought a.re likely to be may be attributed to each o!? the two ocean reservoirs. experienced in England. This brings out the importance It might be noted, further, that the nearly rainless of studying the average wind system in relation to the coast of Somaliland is an almost unique feature in the deviations of the momentary circulation from the normal rainy equatorid belt. The esplanation is no doubt this, standard pattern. that in consequence of the immense monsoonal disturb- Again, the July or winter mean chart for Australia ances due to the Asiatic Continent this coast is at no shows that country to be dominated by an expansion of season the place where opposing currents from the north- the oceanic HIGHS of those latitudes, and the effect is to ern and southern trade systems converge. The heavy weaken somewhat the southeast- tra.de along the east conventional equatorial rams seem to be conditioned by coast. But it has been shown that Aust,ralinn wint.er such convergence. weather is made up of the passage of a series of anticy- Enough has, perhaps, been said in this paper to show clones traveling around the globe from west to east, the that studies of climatic factor No. 5 must proceed with system kee ing south of the cont.inent during the summer the particular or actual momentary phases of the circula- months. &early, then, what looks a stationary HIGH on tion of the atmosphere, if the science of cliniatology is to the average chart is due to the passage of these traveling be fully developed on geophysical lines. anticyclones with tongues of low pressure between them. The systems travel, apparently, in the direction ss/.58 (b4#)* of the upper wester1 current. The important point, CLIMATE AND VEGETATION OF THE HIGH PAMIR.’ however, to note is t ’s: that the limits, strength, and structure of the southeastL hade wind in the Bustralian [Esceqits from a rcvifv in S~thrt(1,onrlon). April 2s. 1921, pp. 270-2274.] region must vary greatlp from day to day in accordance The term “Parnir,” when strictly used, connotes the mth the position of the controllmg ant,icyclones. The level floor of a wide-based mountain valley in the uplands fact is that the t,rades, as well as the other wind systenis. that connect the Hindu-Kush and Karakoram Ranges to which look such solid realities on charts of mean wind the south with the Alai and Timanshan Ranges to the and pressure, are apt t? become rather elusive shadows on north. On its eastern side t,his tract rises rather abruptly charts of particular circulat,ion and difficult t.0 identify from Kashgitr ; westwad it descends more gradually to escept over quite restricted areas. And this demonst,rates Fer hana. * * * the supreme Importance of investigating the part,icular Ta e climate of this region is rigorous, for the winter$ day-to-day phases of the circulation in such a manner as are long. July and August are the only months when its to identify and locate them, whether normal or abnormal, lants grow and flower. Though the days are then mostly in the general structure of the stable circulation as repre- lright, and the thermometer, an hour before sunset on sented on the average charts. It is necessary so to coor- an August afternoon, may register 75’ F., the temperature dinate momentary structures of the circulat.ion with the during the ensuing night inay be 14’ F., and even in July average stable structure that, when pronounced distor- snowstorms occur. As a rule, however, bitterly cold tions or even complete subversions of the latter occur, it winds blow day after day until sunset, and, even when may be possible to trace the steps by which the balance the days are calm, brief but violent evening gales may is upset. It shouldnever be forgotten that the momentary sweep down the mountain slopes, carrying with them circulation-that is, .the disposition of t,he air currents gavel and stones [hurricane air draina e.] At noon on with respect to part.icular center of hi h and low pres- overcast August day the water weling from a hot sure-is the actual process by which t% e interchange of an 7 1 ’- 11 “The second Dariish I’arnir expedition. CimA..;cted ltv Iieut. 4 1. I-Wsen- 6 See Blair W R on lanetary cirrulation in Mo. WEATHERREV. for AprllJ916. Sturlics in the vegetation of Pamir.” f’p. is-132. (Copenhaien: G‘!ildmdnlrke Bog. 0 Sea L&ear; J.k; &cussion of Austnli Meteorology, 1900. ha.:dil, 1326.)

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