~L Y , 1921. MONTHLY WEATHER REVIEW.
THE EARTH’S WINDBELTS AS FACTORS OF CLIMATE.’
BY L.W BONACINA.
5s/. $27
127 Tanza Road, Hampstesd, London. N. W. 3, July 2,1821.1
891
smopmn.
The purpose is to show, with the aid of particular illustrations, that the study of causative climatic factor, No. 5, above, redly resolves itself into a discussion of all the temporary phases of wind and pre~~ure 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
extent fictitious i n that they hut rarely represent any close approach to actual momentary phases in the general circulation of the atmos- phere. PO that it is necewry to study them in the light of the day-to-day variations of which they are the resultant effect. In other words, it is
necessary to regard climatic factor No. 5 as constituted of more or less accentuated departures, often amountin to complete subversions of
type, from the steady flow as depicted on t i e mean charts. I t is to these departures that we really owe the changes of daily weather with their immense consequences in the economy of the earth’s surface. The “tradee” and “westerlies” are studied in this li@. I t is shown, for
example, that the theory of the “polqfront” of Bjerknes constitutes a sfendid system of reference in studying the day-to-dq‘ conditions of I t is to this theory we
must look to explain one of the most imprewive climatic features of the North Atlantic, namely, the midwinter concentration of cyclonic storms. Emphasis is laid on the effect of the “monsoonal” or seasonal factor of the mean surface circulation which in the Northern. far more than in the Southern, Hemisphere distorts the eimple parallelism of wind and presaure bel& which should exist if the surface of the earth were all land or all water. Some puzzling anonlalies are ex lained on the principle of the regional com etition for air supplier. Y‘hus the ol,stiiiate sum- mer drought of the iediterranean Barin with its intense insolation and e x d v e evaporation can only IJC? understood in the light of a drought- providing wind and preaqure r i g h i e which has to fit in dynamically with different sigQirs coexisting in other regions.
The various wind belts (climatic factor No. 5 in the pre- ceding paper) such as the ‘I trades” and “westerlies” which appear on charts showing the average surface cir- culation, and which constitute very powerful climatic factors, are just the resultant effect of continuously vary-
e westerly wind belts of both Hemispheres.
ing hases of the circulation, some of which
and it is essential to a true understanding of of the belts that they be studied in the light
mar R edly from the average type represented
factors (a) the eneral thermal gradient from the E quator
to the Poles, ( % ) the deflective influence of rotation, (c)
which tends to fl e established b the joint effects of (a)
the center of reatest cold, indicating that col B is not the
sole factor in 5 eciding where the Asiatic anticyclone shall
day variations. The circulation depicted on the mean annual and seasonal charts of wind and pressure may, as is well known, be regarded as governed by three rimary
the “monsoonal” or seasonal modifications due to the irre ular distribution of land and sea. T i e simple ardlelism of wind and pressure belts
and (b ) is badly distorted in the horthern Hemispher,: by
(c) , particularly in the months of extreme mean tempera- ture, January and July, and the resulting circulation is really en interesting compromise between the different influences. In January, for esample, the center of max- imum pressure in central Asia lies considerabl south of
be centered, there being a general dynamic factor as well. But by far the most interesting case of compromise between the dynamical and monsoonal factors is sug- gested by a study of the summer conditions of the coun- tries bordering on the Mediterranean Sea in relation to other regions in the same latitude belt. Notwithstand- ing the intense insolation and excessive evaporation over the Mediterranean Basin, the hot months, May to September, in Spain, Italy, southern fiance, Greece,
1 The above two eom lete ~~D C R are d ted in the library of the United States Weather Bureau, aud t l e thirdis printedewith.
Palestine, and the North African littoral, are character- ized by the most obstinate drought, and even thunder- storms tend for the most part in this peculiar region to be deferred till the breaking of the drought in October. The reason for this parching regime evident1 dynamical one, namely, that in summer t e Atlantic anticyclone ex ands so as to bring the Mediterranean
can not bring southern Europe any rain, this current is flowing south and partly ecause it is a
descending wind, the tendency to drought being, more over, in these conditions further accentuated by the steep tem erature adient from the coasts inland. But consi t ering the % editerranean lands alone without reference to other regions in the same latitude, it is difficult to understand why it is that an intensely heated peninsula like Spain is not able to dispute the dominance of the Atlantic high ressure regimen to a sufficient estent to permit the ;P evelopment of monsoonal or con- vectional rains such as occur in China a.nd the eastern art of the United States, which lie in Mediterranean ratitudes. There is. indeed, a summer lowv-pressure over Spain, but it does not suffice to break the desiccating control of the northeast trade. The reason for thls anomaly suggests itself when one remembers that in this latitude the exigencies of the general circulation, in other words, the d namical factor referred to above,
somewhere, and that monsoonal modifications can not ain the ascendant everywhere. There a pears, indeed, to
systems, with the consequence that powerful rain- roducing circulation being, for dynamicd reasons, Emited regionally, is most easily set u on the com act
masses, in China and the United St.ates, which lie to the west of the subtropical HIGHS where t,he air currents lend themselves more readily to rain production through hav- ing a component of motion from south to north, mstead of north to sout,h, as on the western sides of these land masses. It is to be noted, however, that on account of the unbroken compact character of the western side of the North American Continent the Pacific Ocean HIGH
does not embrace the western United States to the extent that the Atlantic Ocean HIGH does the western Medi- terranean, and the aridity of the Western States in sum- mer, or for matter of that a t an season, is largely a
to ography. One has, therefore, to come to the some-
peninsulas to a large ext-ent owe their rainless summers to the presence of the Mediterranean Sea, whose relatively cool surface permits the enstward expansion of the Atlantic anticyclone, and whose excessive evaporation can only be restored in winter? where, the trade belt havin moved south, the dynamical situation permits of
It is now time to consider causative climatic factor No. 5, “prevailing wind-belt,” in relation to departure from the stable system of average conditions just dis- c.ussed. It is pointed out in another paper on the defi- nition of climatology that it is important in the investi- gation of climatic conditions to study the articular weather conditions of which the averages are ma a e up; and
is fa;r1y a
within the em R race of the northeast trade wind which
r t l y because
demand that the su Q tropical HIGHS exert their influence
f e regional competition going on for t rl e different wind
eastern sides of the Asiatic and Nor$ Anierican r and
“rain shadow” drought, the resu f t of drought-favoring
w % at paradoxical conclusion that the Mediterranean
rainfa P 1.
a92 MONTHLY WEATHER REVIEW. JULY, 1921
so in the same wa factor 5, to be really understood,
actual day-to-day dispositions of the eneral circ ation.
depression near Iceland and a HIGH in central Asia; but on occasion one finds an anticyclone over Iceland or indications of a cyclone over Siberia, and i t is a matter of fundamental importance to know the simultaneous ressure distribution over the whole globe when such sisplacements from the normal occur. Again, factor No.
5, when specified for the British Isles, 19 the westerly belt; but we know very well that in consequence of the location of these islands between an ocean and a conti- nent the westerly winds are f a r from regular, being liable a t any time, but articularly during the spring period (March and A rile, to be interrupted by spells
other words, the “westerlies” in the Northern Hemi- sphere are very liable to be diverted by monsoonal influ- ences referred t.0 above, the term “monsoonal” being used in a eneral scnse for disturbances in the circulation
the 8 outhern Hemisphere, on the contrary, there is no
land in the track of the corresponding “westerlies,” which are ac.corclingly subject to no interruption other than the temporary turbulent variat.ions associated with the passage of cyclonic s.ystem. In close connection with this difference in re u1arit.y between the northern and southern “westerlies‘ is a si nificant difference in the seasonal distribution of yks. In the southern ocean there does not ap ear to e that marked concen-
outstanding feature of the climate of the North Atlantic. The interpretation suggested is this, that in the Southern Hemisphere there is no land disturbance in the latitude of the “westerlies,” or “roaring” forties, which are rough and stormy througliou t. the year without conspicuous seasonal variation. In the Northern Hemisphere, on the other hand, the ver-v steep temperature gradient which subsists in winter from the oceans (particularly the At- lantic) to the cont,iiients is highly conducive to frequent and severe cyclonic gales over the warm oceans. In summer the reverse and less st,eep thermal gradient acts differently, reducing the tendency for oceanic gales and Over the sun-heated continents the develop-
than extensive cyclonic wind sys tems. But it is the theor\- of the “polar front” of Prof. B’erhirs which provides the most illurninsting system
westerly wind belts in eithcr hemisphere. The repre- sentation of a simous oscillatiw line, often traceable round the globe, where e ustoria7 air is confrontin the outflow from the poles, 3ustrates t>he structure 09 the “westerlies” of temperate latitudes in a way that was not available before. Thus the westerlies theoretically blow a little south of west in the Northern Hemisphere and a little nort,h of west in the Southern, and in a region like the British Isles the most prevalent single direction is actually southwest. But it is more correct to sa that the revsiling westerly winds of northwestern Jurope
the air sup ly in t,he general westerly drift is equatorial or polar. the polar air currents have been shown‘ to
lli:
will hence have to t e examined in future in the li h t of
The mean pressure chart for January s gh ows a barometric
of east wind with mar 1 -ed effects upon the weather. In
brou ht a B out by the distribution of land and sen. In
tration of storms in the iY epth of winter which is such an
ment avorin7 o localized conrectionnl thunder systems rather
o 1 reference in studying the day-to-day variations of the
osc‘ J late betweea southmest and nort.hwest, according as
I See, 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., “Climate Cmsiilerbl Es ~ecidly in Relation to Man ” 1917.
4 Douglas c’. K. u.: hemperature variations in iowest iwr Lilameters. Jar. ~o v . Mettor. &e:, Jan., 1921. Abstract and discussion iu later REVIEW.
be on occasion quite deep, extending up to a t least 15,000
feet, and perhaps to the limits of the tropos here, indi-
face current as used to be supposed. Moreover, it is to Bjerknes’ theory of cyclones one must look in order to explain more closely rn the light of the day-to-day con- ditions of the circulation what was explained above in the more general terms of the avera e conditions, namely,
Ocean. This region of intense winter cyclonic energy extends from about the middle of the United States east of the Rockies, northeastward to the Arctic regions north of Noma affecting the Great Lakes and the o en Atlantic
ticularly f avorabfe to the inter lay of equatorial and olar
land warm equatorial currents are constantly being con- fronted with the outflow of cold air not only from the Arctic regions, but also from the frost-bound continents of North America and Europe which in winter enormously extend the reservoir of “polar” air. Similar rough con&- tions afFect the North PacXc, but this ocean is a parentl not quite so subject to severe winter ales as %e Nord
from the 8 orth American Continent is partly cut off from the Pacific by the trend of the great mountain ranges on that continent. In summer, on the other hand, when the air is limited to the Arctic regions pro er,
it is also feeble over the conti- season, the mean pressure is lower than it is over the oceans. According to Bjerhes’ theory c clonic activities is fundamentally a form of con-
contrasts. But over the sun-heated continents in summer it k just this form of convection which does not take place to any vigorous extent. The form of convectional storm which does conspicuously occur is concentrated in
It would appear, therefore, that in summer when the su ply of polar air necessary to intense cyclonic activity
denslty and pressure tends to be localized-short-cir- cuited, as it were-in small thunder s stems character- ized by strong vertical motions which B o not involve ex- tensive horizontal disturbance (advection). In a climate like England, whose climate is a good blend of oceanic and continental controls, there is a remarkable seasonal o po- sition between the oceanic cyclonic gales of winter an a the continental thunderstorms of summer, the former being most frequent and severe in the four months surrounding the winter solstice (November to February), and $he
latter in the four months surrounding the summer solstice (May to August). In the Southern Hemisphere the westerly wind belt is not so conspicuously more stormy in the depth of winter
than in the height of summer, evident1 because of the
southern westerlies of “roaring” forties, and the meeting place of e uatorial and polar currents, are stormy all the ear roun and are accordingly riddled with cyclones. $hey are not the innocent-looking homo eneous winds
aeronauts who might be tempted to use this southern belt of “westerlies” as a quick west-to-east route from, sa , South Africa to Australia, should be duly mindful of J e
cating that the polar outflow is not such a s Yl allow sur-
the excessive winter storminess o f the North Atlantm
es ai& that portion between Iceland an if the British Isands. P
air. Over the North Atlantic % etween Scotland an B Ice-
Throu hout all this zone conditions are par-
erhaps because of the supp 5 y of “polar air”
oceans is much enfeebled; %ut
vection J motion in that it is chiefly due to temperature
loc eneF zed thunderstorm systems.
is P q e l y cut off, the circulation due to local inequalities of
absence of disturbing land matter. R evertheless the
which they seem on the mean wind charts o f the globe, and
JULY, 1921. MONTHLY WEATHER REVIEW. aw
rapid changes of wind and weather associated with the
The most constant and steady of the earth’s surface wind belts is that of the trades, and on this account they might be looked upon as the mainspring of the circula- tion of the atmosphere to which the other wind systems, lower and upper, not so immediately related to equatorial heating, adjust themselves. In relation to equatorial low pressure the trade winds represent a balance of cause and effect, for while in the initial processes of the circulation these winds are set blowing to supply the defect of pres- sure over the heated e uatorial re ion, in the final adjust-
the effects of the earth’s rotation. All land re ions in
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 seasons and with the nonpeiiodic changes connected with day-to-day weather. In Europe, as already indicated, the northward extension of the trade system causes the normal summer drought of the Mediterranean Basin, and when, as occasionally happens, the same system, as repre- sented b the Atlantic HIGH, pushes unusuallv far north,
experienced in England. This brings out the importance of studying the average wind system in relation to the deviations of the momentary circulation from the normal standard pattern. Again, the July or winter mean chart for Australia shows that country to be dominated by an expansion of the oceanic HIGHS of those latitudes, and the effect is to weaken somewhat the southeast- tra.de along the east coast. But it has been shown that Aust,ralinn wint.er weather is made up of the passage of a series of anticy- clones traveling around the globe from west to east, the system kee ing south of the cont.inent during the summer months. &early, then, what looks a stationary HIGH on
the average chart is due to the passage of these traveling anticyclones with tongues of low pressure between them. The systems travel, apparently, in the direction of the upper wester1 current. The important point,
structure of the southeast hade wind in the Bustralian region must vary greatlp from day to day in accordance mth the position of the controllmg ant,icyclones. The fact is that the t,rades, as well as the other wind systenis. which look such solid realities on charts of mean wind and pressure, are apt t? become rather elusive shadows on
charts of particular circulat,ion and difficult t.0 identify escept over quite restricted areas. And this demonst,rates the supreme Importance of investigating the part,icular day-to-day phases of the circulation in such a manner as
to identify and locate them, whether normal or abnormal, in the general structure of the stable circulation as repre- sented on the average charts. It is necessary so to coor- dinate momentary structures of the circulat.ion with the average stable structure that, when pronounced distor- tions or even complete subversions of the latter occur, it may be possible to trace the steps by which the balance is upset. It shouldnever be forgotten that the momentary circulation-that is, .the disposition of t,he air currents with respect to part.icular center of hi h and low pres-
cyclonic syst0lns.
ment they develop an % maintain t % e low pressure through
both Hemispheres, sub’ected more or less throug a out the year to the influence o ! this system of winds, are reduced
weeks o 9 very trying summer drought a.re likely to be
however, to note is t L ’s: that the limits, strength, and
sure-is the actual process by which t % e interchange of
6 See Blair W R on lanetary cirrulation in Mo. WEATHER REV. for AprllJ916.
0 Sea L&ear; J .k ; &cussion of A u s t n l i Meteorology, 1900.
air between the Equator and the Poles is day by day effected; and that it is owing to these momentary turbulent deviations from the stable steady flow as represented on mean charts, that we get our daily weather changes with all their momentous consequences in the economy of the earth. If there were no such de artures from the steady flow, an inevitable result woul a be, for example, that over large parts of the lobe it would
continuous drizzle would fall; in other words there could absolutely never rain at all, whilst over ot 5 ers an almost
came from the Indian or the At.lantic Oceans. The mean wind and pressure charts throw little light on the prob- lem, and the very esistence of such a controversy is a hint that both views are partly correct. But a knowledge of the daily disposition of the sir currents, whose average structure we generalize under the term li southwest mon- soon,” would reveal what roportions of the moisture
It might be noted, further, that the nearly rainless coast of Somaliland is an almost unique feature in the rainy equatorid belt. The esplanation is no doubt this, that in consequence of the immense monsoonal disturb- ances due to the Asiatic Continent this coast is a t no season the place where opposing currents from the north- ern and southern trade systems converge. The heavy conventional equatorial rams seem to be conditioned by such convergence. Enough has, perhaps, been said in this paper to show that studies of climatic factor No. 5 must proceed with the particular or actual momentary phases of the circula- tion of the atmosphere, if the science of cliniatology is to be fully developed on geophysical lines.
may be attributed to each o !? the two ocean reservoirs.
ss/. 58 (b4#)*
CLIMATE AND VEGETATION OF THE HIGH PAMIR.’
[Esceqits from a rcvifv in S ~t h r t (1,onrlon). April 2s. 1921, pp. 270-2274.]
The term “Parnir,” when strictly used, connotes the level floor of a wide-based mountain valley in the uplands that connect the Hindu-Kush and Karakoram Ranges to the south with the Alai and Timanshan Ranges to the north. On its eastern side t,his tract rises rather abruptly from Kashgitr ; westwad it descends more gradually to Fer hana. * * *
are long. July and August are the only months when its lants grow and flower. Though the days are then mostly lright, and the thermometer, an hour before sunset on an August afternoon, may register 75’ F., the temperature during the ensuing night inay be 14’ F., and even in July snowstorms occur. As a rule, however, bitterly cold winds blow day after day until sunset, and, even when the days are calm, brief but violent evening gales may sweep down the mountain slopes, carrying with them gavel and stones [hurricane air draina e.] At noon on from a hot
T a e climate of this region is rigorous, for the winter$
an overcast August day the water weling 7
1 ’- 11 “The second Dariish I’arnir expedition. CimA..;cted ltv Iieut. 4 1. I-Wsen- (Copenhaien: G‘!ildmdnlrke Bog. Sturlics in the vegetation of Pamir.” f’p. is-132. ha.:dil, 1326.)