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MONTHLY ‘WEATHER REVIEW AWBLD J. REITBY, Idltor.

VOL 50, No. 8. AUGUST, 1932. GLOaED OCToBEB S,lg!% W. B. No. 782. hWSD OCTOBEB 31, 1922 - - - -_ _. ------. ------____ - THE EARTH’S ATMOSPHERE AS A CIRCULAR VORTEX. By ANNELOUISE BECK. [Berkeley. Cali1ornia.l

Miss Anne Louise Beck of Berkele Calif receivedone of ths Fellowships awarded by the American-Scandinavik Foundagon ol‘heea York. This Fellowship rded the is deflected eastward by the rotation and gives in lower dPflege of8 year’s study of weather forecssting at the BerrGeophysical Institute of the antitrades, the northerly current by the bergen, Norway under Profs. V. Bjerknes and B. Helland- ansen. On her return to this count.y, bflss Beck has kindly authorhd the publication of the same force is deflected into the easterly trades. They following account of her studies at Bergen.-E~~~o~. are separated by a surface of discontinuity, “the sliding SSNOFSIS. surface of the trades,” which according to the formula The general circulation of the atmos here considered a8 a circular of Margules (and in its extended form by Bjerknes), vortex, aa postulated by Professor BjerEies is briefly discmaed; it is slants upward from the E uator. Part of the up er pointed out that surfaces of discontinuity must arise along the margins current IS sufficiently coole1 to be forced to descen 2in . of air streams moving equatorward and poleward, respectively,and that the region of the tropical highs. That the cooling under certain conditions theae surfaces of discontinuity are unstable, a wave motion seta in with the cold air pressing southward in the actually takes place is proved h the fact that the rear of warmer air moving northward; thus a series of wave motions in temperature at the of &e tropical highs is the atmosphere originate and are pro orrted eaatward. lower than at the in a hgher level. A surface The criteria sre given for fixing tepsition of that surface of diR- circulation from the tropical highs toward the Equator to which the -e “polar front” has been applied,.and the %%:?&fering conditmns whch arise in nature in connectmn with forms part of the c cle in each hemisphere inclosed the changes of position of the polar front is discussed in ~ome.detai1. between the tropical 5ighs and the Equator. The rest The occurrence of in groups or famihes of two to six, with four of the upper current below the stratosphere continues being the moat probable number according to the statistics of the to flow northeastward and eastward as the upper part Norwegian weather maps, h diacueeed and mveral families are con- sidered in detail. of the main westerly current of the Temperate Zones. The fore-of the weather of Norway by the Bjerknea theory, Part of the latter also emanates from the tropical hi hs. aa carried on at ergeqis described for re reaentative weather types At the oles, especially over the high lateau Ianclg s of and finally an attempt is made to plytL Bjerknes theory to the Greenlan and the Antarctic Continent t e polar coolin optic weather mape of the U. 8. eather Bureau for January, 1921. a 1 obaervatiOqS1 ma- pripted on theae pa lacke much of the is especially efficient, finally transforming the origin3 data available in the Norweolan Meteorol~ca~O5ce,nevertheless e uatorial air into the outflowing current of polar air. surfaces of discontinuity -%e traced and more exact and detailed Tie easterly surface at the poles, together with the seem to be pmible when more detailed observational data theoretically necessary winds of wester1 ::Z%%le. the outflowmg upper current and the diz erence m tem-in When we consider the motion of the atmosphere, in Eerature, develops a second surface of discontinuit reference to coordinates which do not partici ate in the the olar sliding surface” which according to t%’ e rotation of the earth, we may assume for t!l e sake of MarguLs-Bjerknes formula slants upward toward the simplicity that the earth is at rest and heated by a h her latitudes forming the “polar calotte” of Helm- sun in the form of a ring in t.he earth’s equatorial plane, hstz.’ The difference in temperature at a given level then we have the well-known scheme of clrculation only between an air column over the central part of the in the planes. Insolation at the Equator, polar calotte and another at its border, accord’ to radiation toward space at the poles causes the isobaiic the Bjerknes’s circulation theorem gives rise to aywed surfaces to slope from the E uator toward the poles in the circulation, descent in the central art of the polar u per atmosphere. Acco 1% ing to Professor Bjerknes’s calotte, outflow at the ground, forceB rise at the polar tt eory of circulation,’ olar air flows at the surface of sliding surface, and inflow in the hei hts. Thus there the earth toward the l3quator, and equatorial air in the are for each hemisphere three circu5 ations “running hei hts toward the poles. like cogged wheels.” #he insolation at the Equator then, overcompensatin A surface of discontinuity slo ing north-south with radiation at the same place, causes the air to ascend easterly cold winds below it on tg e northern side of its whereas radiation causes cooling and, conse uently, intersection with the earth’s surface and warm westerly descent of the air at the poles. This circulation 1evelops winds above and to the soubh is unstable. A wave until it becomes constant. Polar and equatorial air motion sets in with the cold air pressing southward as a are thus seen to be se arated by a discontinuity. “spreading cold tongue” and warm air running north- This picture is moiified by the rotation of the earth ward ascending the surface of discontinuity as a “warm and a ‘planetary vortex”--cts Professor Bjerknes has sector.” As in wave motion in any fluid with a reciable named it-is formed. The southerly upper-air current friction of viscosity, these waves hare a markeBffe cycle with characteristic stages of development. 1 V. Bjerhea: On the Dpamica of the clreular Vcatex with A pbtions to tha Atmosphere and Atma? heric Vortex and Wave Motions, Chrtstiang, 1921, Qmf#Jiskr pcrhlilationcr, V~I.11, I&. 4. All the air inclosed by the sliding surface and the surfme of the earth at the poles. l3352-2&1 383

Unauthenticated | Downloaded 09/26/21 02:04 PM UTC 394 MONTHLY WEATHER REVIEW. AUQUST,1922 The life cycle begins with a mere waver in the line of distribution of land and water, etc., esert a stron er discontinuity, becoming more and more marked as the influence upon the temperature of the air masses at 5lt e cold air forces southward, finally curving into spiral- surface. formed “breakers.” s (2) In the vicinit of the low, the polar front is almost In the course of this process the warm air is at last always accompanie8 by pressure formations in the form embedded in the cold and cut off from its supply. It of a trough. Often the troughs are well marked; some will continue poleward in the upper air, while cold air times, however, they can on1 be seen when a close net- spreads down toward the Equator. Thus this wave mo- work of stations is used. lometimes convergences of tion in the sliding surface is a necessary link in the gen- the are the only signs of these troughs. At the eral circulation. The stronger effect of the deviating slowly moving arts of the polar front between the c - force of the earth’s rotation at higher latitudes, as well as clones the trou \s for the most part are missing, and t Ke the greater contrast in temperature between the air wind may be t Ee same on either side of the front. masses of different origin to either side and the fact that (3) The trace of the barogrn h is one of the best the polar sliding surface cuts the earth’s surfwe, ought to c,riteria in fising the position of the polar front. In give the wave motion in question a more cons icuous front of the steerin line pressure drops rapidly; at the character at higher latitudes than nearer the quator. assage of the line tf e fall sucldenly ceases and the curve The investigations of the Geophysical Institute2 at Ber- gecomes nearly horizontal. When the pressure falls also gen have confirmed these general conclusions. in the warm sector this often indicates the LOW is dee - The intersection between the polar sliding surface and ening; when the pressure rises in the warm sector tR e the surface of the earth has the character of a line of low is often filling up. discontinuity in the fields of temperature, as well as in In front of the squdl line e there is often a sli ht drop ressure and wind. The investigations of J. Bjerknes, in the pre,ssure?then at the passage of the line t5l e prk- 6. Solberg, and T. Bergeron proved the existence of such sure suddenly rises. a line, for which the appropriate name of ‘*polarfront” The falling ressure in front of the squall line may be was adopted. altogether lacK -ing. Barometric tendencies in synoptic The germs‘ of the theory are contained in Dove’s representation are very useful in fixing the line of demar- theory of the struggle between equatorial and polar cur- cation. rents. In a theoretical paper Helmholtz has develo ed The ty ical warm-front assage shows a barogram with the idea that there is always a tendency toward the ror- a fair1 sR arp downward geflection. It is often accom- mation of a surface of discontinuity between the cold panieiby a strong ust of suddenly veering wind, the polar air tending toward the Equator and the warm diminlsE es rapidly, passing to fo and drizzle equatorial air tending toward the ole, while the or even to cloudy humid weather, while visibiI ity sinks to consists in a kind of “rolling up” oP this surface. Similar a low degree owing to the dust-laden tropical air. The views have been taken up in theoretical papers by t pical cold-front passage is indicated by an upward de- Brillouin and by Margules. Sir Napier Shaw has arrived Action of the barogram. at the same result from the em irical side.s (4) and (5) areas form at the boundary of the The polar front is a line of tlemarcation between two warm tongue where the warm air ascends above the cold. air masses, one of Arctic ori , the other flowino from At the center of the low, two rain belts start, one along the . As a rurthis line can be foqlowed and in front of the steerin surface, the other accompany- through all the centers of the cyclones of the circumpolar ing the squall line, gener9 ly to the rear of it. The first zone of LOWS. Between the centers it makes a longswing is usually PO0 to 400 kilometers broad, the other seldom toward the south. The movement in the front is usually more than 100 kilometers, often only 10. With a close experienced as a cold front (squall line) southward; the network of stations these rain belts are movement northward as a warm front (steering line). tions of the polar front positions. On The following facts may be regarded as indicating the tions of precipitation it should always position of the polar front: First, discontinuities of tem- that orographical conditions play an important part in perature found on the synoptic chart; second, tem era- the formation of rain. ture variations at stations assed by the polar gont; Precipitation is almost alwa s found at stations where third, trough lines in the &ld of pressure and corre- the olar front has passed. dnly in the outermost part sponding discontinuities of air motion; fourth, tenden- of t%e cold ton ue, the front may ass without causing cies in the pressure record; fifth, rain areas along the precipitation. %his may also be &e to protection by polar front; sixth, precipitation at stations passed by to ograph . the polar front; seventh, amount of and form of ; bhe codfront is followed b a belt of precipitation in eighth, and haze; ninth, discontinuities and variations the form of showers often wit thunder and hail or soft in absolute . hail-if the temperature contrastsK are strong eno There are special phenomena to note under these head- This stripe of precipitation is always narrower thanT t e ings, as follows: rain area before the warm front. (1) The difference in temperature along the olar front (6) In front of the first rain belt, there is a broad in autumn and in winter usual1 is rather marE ed, about zone, about 300 kilometers, of cloudy weather. The three or four degrees Centigradet etween adjacent stations forms in this zone are cirro-stratus and alto- on the two sides of the front. In spring and in summer, stratus, sometimes the tufted cirrus are the htsi of the discontinuity of temperature at the surface of the approaching rain, a roximately simultaneous witythe earth may have disappeared, the temperature differences beginning of the faPP in the barogram. In the warm in summer, u on the whole, bein much smaller than in sector the usual succession of cloud forms is strato- other seasons 1ecause insolation, cf oudiness, precipitation, cumulus, stratus, fracto-stratus, but often it is clear. By and by, marking the approach of the cold front, 8 FipjlBB, p. 80, V. Bjerhes: On the Dynamics of Vortex, ate. the Circuhy ~~ ~ ~~~ ~~ - 4 V. erhes On the Structure of the Atmos em when Rain Is Mug. Qartcrlv Joutvlio/t~iqai Meporolofid 8ociety, Aprflm, p. la. #Since squalls are frequent17 absent in connection wlth this so-called Ilne In the 6 Fa. 10, p. Ille. lot. at. , the expression ‘ wind shllt” 11118 Is to be ~~~.-EDIwB.

Unauthenticated | Downloaded 09/26/21 02:04 PM UTC AUOU~T,1922. MONTHLY WEATHER REVIEW. 395 lenticular alto-cumulus may occur, later changing to compass and actually have been shown to so move in the altogtratus and nimbus. In most cases, the weather is three years’ forecasting, accordin to the Bjerknes prin- overcast a hundred kilometers ahead of the rain front. ciples which have been practice din Scandinavia. This The rear of the squall line is marked by cumulo-nimbus, is due to the varying position of the polar cap during dif- cumulus, and fracto-cumulus. ferent eriods and under different conditions. The foremost discontinuity (in the direction of the The ri rst cyclone model, described by J. Bjerknes in his propagation of the c clone) the so-called warm front or paper on “The structure of moving cyclones, ” 10 refers steering line marks tK e course of the warm air traveling to a wave in the polar front in the medium stage of ita up the slope of the polar sliding surface. When this development or life cycle. It soon proved impossible, warm moist current of tropical origin’ ascends, it is however, to apply this first cyclone model in all the actual cooled adiabatically and cloud formation takes place. cases of forecasting. Further analysis led to the tracin The “optraek,” * before an advancing cyclone generally of unbroken polar fronts from cyclone to cyclone an f consists in a series of cloud forms be inning with c.irrus showed the existence of waves in all stages of develop- or cirri in tufts merging into cirronef uls wth the com- ment. mon optical phenomena followed by cirrostratus, alto- In the life cycle of a cyclone the principal stages.of stratus, and lastly the nimbus (pallio nimbus) of the rain development may be represented by Figure 3 1, page 88, V. belt just preceding the warm front. B’erknes: On the dynamics of the circular vortex, etc. As will be seen these cloud forms follow each other debirth is indicated first b a slight curving in the front from higher to lower levels, and as the sequence is a between two air currents o? 180’ difference in direction. continuous one already proves that they are the effect The pressure is falling, the winds weak and on1 indicate of dynamical cooling caused by travel along a slanting a tendency toward circulittion around a har appre- surface. It is further proved by aerological observations ciable center. The oung cyclone has the formf o a water * * * from the close net of stations in Flanders wave with a broad g ase in the second stage, though ita during the war that this surface really exists in wind, pressure field is still very flat. The pro agation of the temperature and humidity. cyclone in these two early stages is genera ly rather slow. (7) Directly behind the steering-line, especially when The third stage is the model referred to Pabove from the the wind velocitiea are small, fog is often to be found, paper by J. Bjerknes. The rate of rogress at this stage due to the presence of warm air above the cold sur- is rather great and velocities of 150K ilometers (93 miles) face. Also along the slowly moving part of the polar an hour have been experienced in Norway for cyclones front (at the southern end of the tongue of polar air) from the Atlantic Ocean. fog occurs. Haze is to be found almost exclusively The point of intersection of the two fronts lies well to on the southern side, especially marked, immediately the center of the cyclone. The area of warm tropical air preceding the SqualLline. between the two fronts is the so-called warm sector. It (8) The polar air is dry and the equatorial air quite is characterized by little precipitation, unless it be in the humid. Good visibility is a normal characteristic of form of fog or drizzle when the warm air is very moist and polar air, though it sometimes happens that the polar flows over colder water. If the warm sector comes over air is also polluted-that is, when it has traveled a long a part of the earth’s surface where the air has been heated way over northern France and England before it reaches considerably and is already warmer than the air of the the western coast of Norway. warm sector, precipitation may occur in the form of slight Finally, the followi method in fixing the position occasional showers due to the resulting instability. As of the polar front mayT e used. The front having been regards the weather experienced by different places, sure1 marked on one map, its future position may be according to their situation relative to the moving cyclone, calcuateds from the gradent wind at the rear, con- reference is again made to the paper by J. Bjerknes, “On sidering both direction and velocity. This is not quite the structure of moving cyclones.’’ true for steering lines, the air at the rear of them in The decline of the cyclone is marked by the tendency eneral moving more ra idly than the line itaelf. In of the cold front to ain on the preceding warm one on &e case of squall-lines tgi s rule works very well, how- the equatorial side oB the center. As the cold front over- ever, because the air behind the line can not possibly takes the warm the cyclone becomes a “seclusia” move faster than the line itself. Ths method is to be (Be eron)” and the warm sector is now cut off from ita used especiall when the front has arrived in dis- suppP y of warm air. A continuation of this mo- tricts where o3 y few observations are available. tion carries the air in the rear of the wave up me htinvesti ations concerned discontinuities in the the warm front. This proc- moving cyclones of the Temperate Zone, as they are of t.he warm-air experienced in northwestern , and especially sup ly causes the warm-front recipitation area of the Scandinavia, whioh is a convenient place for the study cycP one to diminish, while the criscontinuities of pressure of this phenomena. and wind may still exist at the. surface of the earth for It is no mere chance, in fact, that the existence of the some length of time. This is the case which Bergeron polar front was htproven for the western coast of calls the ’‘ up er front.” Norway, that coast being perpetually swept over by In the fin 3stage the warm front is overtaken by the waves in that front. cold throughout, and the cyclone has become an “ occlu- It should be added that cyclones generally come from sia” (BeTeron), its warm sector existing only in the upper the west, but they may come from any point of the air as an lsolated mass inclosed by polar air. Ths warm sector becomes more degenerate until the cyclone is 7 The expression “of tropical mi&&” must not be taken too Utdy. It must fm+ quently ha that the warm air iu front of a cyclone only a day or so previously thermically dead, its discontinuities no longer apparent came from %&titudes on the front ofanantieyclone.-EDmR. 8 We have no word amonrc our meteorological terms which exactly trsnslatas the * Them isaatrikiirelation between the position ofthe polar front markin&the bound- ary of the polsr calotte and the southward extension of the ice Be& which u1 the winter of 1920-21 for example were far south In the Labrador Straits while the Idand-SDIts- higher atmos here of the L&gue of Nations has in hls kee ing records froh aero bergend waspractlcall h. Hte and b&oon ascents cover1 nearly the whale perpod of the war, Fren&%> Mo. $.mrR REV.,gebebyuarp 1919.47: 95-99. Edshobservations as well 8s c$ea of material raphued from Germany. Statements arBlng from dlscussfons in the laboratory.

Unauthenticated | Downloaded 09/26/21 02:04 PM UTC 396 MONTHLY WEATHER REVIEW. AUQUST,1922 either at the ground or in the prevalent when the tropical highs over the Atlantic hsve cyclone has often a powerful a great east-westerly axpanse and lie rather far to the surface. Doctor Calwagen, at the north. s ested this may be avplained The southwesterly t pe is characterized by more %e newborn cyclone, with its broad warm sector, occluded warni sectors wK en the first of the series a pears represents a great amount of potential energy due to the on the chart, thus indicating a birth farther off t1 an in temperature contrasts between polar and equatorial air, the westerly t pe. but it has relatively little kinetic energy. During the The first of t Ie series passes often to the west of Iceland life cycle this potential energy is transformed into lunetic through Denmark Sound or over the western part of and the kinetic ener y is transformed into heat by fric- Iceland. Though nearly occluded, these first waves are tion, external as we1!? as internal, and we have the often often marked by long fads which swee over the western very stron wind field in the apparently dead c clone. art of Scandinavia giving rain an I! stroj winds in It should Ee observed in forecasting, therefore, ti at an favorable points, as ainst a prominent hea and where old cyclone, due to its kinetic eneroy, acts as a “dynaniic a distortion of the col3 front causes the so-called “corner shelter”. _^__ against new cyclones with pess strongly developed effects.” wind fields. The waves following enerally have better developed These forms of cvclones above described follow each warm sectors, as the co d air in front of the first one is swept awuy. The fourthP is often the strongest and draws after it the strongest outbreak of cold air. This gives either anticyclonic conditions confined to the North Atlantic or a wedge of high pressure extending southward to the tropical highs. The last cyclone thus be’ L deep one, generally has several cold fronts or s u&%es marking the steps of advance of the cold out reak. One of these secondary squall lines may give Brise to one or more secondary waves. The generation of these secondaries takes place over southern England or France-t FallYough if the area of general circulation is perceptibly moved, for instance, far to the north, these secondaries, too, develop farther north. An example of this is found in Series 14, D wave (April 4 and 5, 1921).15 At their fist a pearance these secondaries may seem cussing the matter fully. very innocent anB diffuse, often difficult to detect in the At an informal discussion at the Ber en Institute when charts, but may develop into dee depressions. These the fact was noted that vty~strong co5 d outbreaks were are the real rain bringers in centraP Europe and in some followed by new cyclones m higher !atitudes than the cases also in Sweden (13 D it~).1~ lest of the precedi series, the question arose as to the The third wave in series 11, 11C l5 has 8 remarkable possibilit of a colT wave closing a series and after the secondary. The third s uall line of this depression, 11C, cold outg reak the arrival of a new. The first wave of marked a heavy outbreat of cold air over Ireland (March the series was jestingly called “ Protesilaos.” The 10, 7 . m. chart 9. On that chart the primary had discussion led further and it was suggested that there move%as far north as Lafoten. ~btrans ort of rela- seemed to be a preferred number of waves in a series; tively warm air by winds at the rear, togetg er with the Mr. H. Solberg considered the number to be four accord- low temperatures as the squall line swept over Ireland in to his observations. built up a ver large uasi-warm sector, accentuated as it %he above discussion led to the earnest attempt to traveled nor ti%ward y the contrasts of air relatively count and classify these waves. The series of 14 families warm and air lying over the Arctic Odan (Spitzbergen of January, February, and March, 1921, seem to indicate -31’ C., Bjorn-oen -5O C. in the polar air, in the warm the number is reaUy four where the waves come from the sector the tem ratures average +7” C. to the north, westerly or southwesterly direction. There was one +lo0 or 11’ E in the south). That secondary con- example of a southwesterly series from March 7 to March sequently grew to a very strong cyclone, deepen4 from 12 confined to three waves. From March 12 to March 749 111111. in the center to 732 mm. by the time it had 20 B south-sou.thwesterly type occurred with as many as assed Spitzber en. Its warm sector had diminished six in the series. gut it remainedp strong enough to reach much farther The westerly type is a common winter ty e, which north, probably even to the . During the evidently origmates west of Iceland k the A E antic or, passage the temperature at S itzbergen increased to rather, one should sa , first appear on the synoptic -5’. This was at the center oP the cyclone. With the charts of Norwa in t1 at portion. They are probably passage of the warm sector at Bjorn-oen, the temperature generated by cod3 air currents over Greenland which rose to +3’. This cyclone was a good example of those come down against the warm air over the which have been experienced passing Franz Joseph’s and are characterized by broad warm sectors emphasiz- Land by different observers. ing the fact that these cyclones are born within a rel- During the formation of these secondaries the cold front atively short distance from Scandinavia. This type is in whioh they form becomea stationary in that part or ~~ ~~ ~ moves slowly as a whole, corresponding to the behavior I* W. J 8. Loelryer: Southern Hds here Surface Air Ciulatiau, Ia~don1010- PuW&ns of the Solar Physics Canmi8ee. of the polar front of themother “ calotte” in the formation ArcWmlri: ResulWW du V 1881-1899. -- Minardus: Deutsche Slidpo~~y~~i-l~,Btlnd 111, Yetmrol~gia 1, 1, 8. of new cyclones. This is of vital im ortance in forecast- ii8. ing. Northwesterly outbreaks, whic me usually accom- Bodman: Schwedische Slidpolarax edition, lWl-1903, Band 111,4 8.17. i u Lehrbuch der Meteorologm BoofII Kapital N, p. 200. ‘ 1‘ Exner, Dynsmkhe Yet&logie, ‘1916. 1’ Synoptic charts, Bergen Weather Bureau.

Unauthenticated | Downloaded 09/26/21 02:04 PM UTC ATJQUST,1922. MONTHLY WEATHER REVIEW. 397 panied by strong squally winds, absolutely re uire Secondary c clones often originate in a well-develo ed warnings, but it often ha pens that these out1 reaks fail, cold front. dis may occur after the third wave, gut due in the cases especidy reviewed to just the above- more especial1 the fourth of a series, because these swee dascribed and artly unexpected formation of a secondary so far south tx at they come up against air warm enougFl in the awaite cold front. to feed a warm sector. These secondaries may often have On the 8 a.B m. chart for February 9 (Synoptic charts, first appearance on the Bergen Weather Bureau) a squall-line was lying east of into cyclones with gradient Iceland, with a rising tendency in the ressure in some the primary cyclones. places of as much as +7 mm. in three tours. A storm sunimer, when the general warning was issued for t,he coast from Bergen north- with the regeneration of ward. The expected gale did not come, but on the 2 old “secluded” c clones or cold fronts which have been . m. chart of the next day a secondary was revealed nearly obliterate but may be revived by a renewal of forming southwest of Lofoten. This secondary gave a the warn1 air supplyB from a warm continent. It may also southwesterly prefrontal gale for quite another part of the happen that new fronts are developed in the cold polar coast. air of a recent outbreak, when the air has been sufficiently Secondaries occur in both the southwesterly and west- warmed in parts to give enou h contrast in temperatures. erly types. The series may or may not be marked by Thus, also, new fr0nt.s may fevelop along the coast of a them. Perhaps we think they are lacking only because continent in the winter time due to the contrasts in tem- they have not been detected on the chart. The question perature between the cold air masses of an is certainly open to much discussion and study. cooled by radiation and the relatively warm air over the The type termed southerly, or more properly south- adjoining sea or ocean. For instance, such a “maritime” southwesterly, is more of a spring or summer phenomenon front gave the snowstorm of 500 millimeters of precipitcc in contrast to the winter . The senes is marked tion at Bergen in November, 1919. Second? front8 in by long occluded waves, seldom by waves having a well- summer frequently develop over Denmark an northern developed warm sector. They extend from a stat,ionary Germany, and, moving ver slowly following the upper center to the south of Iceland south over England and drift, give great amounts o9 rain in Denmark and south- France. These waves are probably generated farther ern Sweden. This type of discontinuity is often very south than those of the series mentioned earlier in this intricate and ext,remely hard to trace on the map, often discussion. The general trend of the isobars is n0rt.h- lacking a marked resemblance to the life forms described south and the extended valley of relat,ively low pressure above. The ressure field often takes the form of a south of Iceland is an effective shelter against the pro- barometric valeyP or trough. The slow propagation of trusion of cold polar air under warm tro ical, until some- these “ amoeba” cyclones and the strong convergence of where rather far south of Cape Farewelf: winds make them one of the strongest rain producers in The tro ical high in this case generally is pushed east- southern Sweden. The same is the case when an origi- ward unti its center lies near Spain and the Mediterrttr nally real polar front is retarded in its movement or be nean. TheY high is accentuated. This is partly due, in comes stationary. all probability, to the descent of the outflow of air in the It may be pointed out that the average duration of u per strata from the cyclones to the west of the high. each wave in a family according to the statistics of the ‘&e length of the path of the cold air from this source to first 140 days at the beginning of 1921, is 6.5 da s for its turning point and back toward the British Isles Scandinavia in good accordance with the period of defant naturally exhausts much of its original potential energy. for average precipitation periods foipcl. for the Northern The temperature difference between polar and tropical a!r Hemisphere. The existence of a periodicity IS also known is less marked under these circumstances, and on this for the . Quite probably further account the cyclone is poorly developed in the beginnin , details could be determinedfroni a study of the phenomem and conse uently after its journey northward is stil7 of t.he “southerly burster” of Australia and the “pampero” feebly devef oped, which is seen bv the dimensions of its of Argentina, both typical cold-front phenomena. warm sector, the boundaries of which then, especially in The step to tropical cyc.lone is natural. Professor the more central arts of the cyclone, between Iceland Bjerknesl6 has advanced the theory that they have their and the British Is es, are clapped together. origin in the “sliding surfaceJJof the Trades.” Accord- Secondaries alsoY occur in the southwesterly type, ing to H. U. Sverdrup’* the general variations in the althou h of a somewhat different nature than in the features of this surface account for the various phenom- westerP y type. Apparently every ena in certain arts of the tropics and their frequent ap- secondaries which appear first over iha7 nglan and wave France has pea.rance in otR er parts. as quasi-warm sectors attached to the occluded primary. The origin of these secondaries is in reality not so far NORWEGIAN WEATHER SERVICE : FORECASTING BY different from the origin of the secondaries in the types BJERKNES THEORY. mentioned above. The Norwegian Weather Service is directed from three The occlusion has the character of a so-called cold forecast centers, Christiania, Tromso, and Bergen. Ber- front, advancing from the south-southwest although the gen ranks second to Christiania m population, and has instability of the cold air over the warm surface 1s less really a more important forecast division-that along the marked. When the cold front advances against air that western coast,,divided into 30 districts, north to a certain is warm enough in comparison with the polar air, a warm point where Troniso district begins, fishing being the prin- or quasi-warm sector is formed in that part of the cold ___ front,. This generally is the case over south England and M V. Bjerknes, On the Dynamics of th8 Circular Vortex with A plhtlons tO the AtmosphereandAtmw heric Vortex and Wsve Motions. ChristinnE,1921, Qmfgatakr western France. This is also characteristic of the sec- Prblikatfpncr,Vol.II 80 4 p 83 IT This IS referred td. in ih6 skinw- MONTHLY WEATasR REVIEW,March, 1821, Of ondaries of the first-mentioned types. Showers follow an article b C. E. P. Brooks and €I.&. Braby BS the “mobile center of action” under the passage of the occluded wave, indicating the unstable the clash o?Tradea In the Pacific. 18 H U Sverdrnp Der Nordatlantiache Passst, Vcrbflmtlichungcn dr8 Qrophgdka- iiature of the air. lischt~I&tiluts der ~?mioerait6tLdpzQ, 11, Ser.. 1917.

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