338 MONTHLY WEATHER REVIEW. JUNE, 1916
METEOBOLOQICAL ASPECTS OF OCEANOQRAPHY.
By Dr. HANS PETTERSSON.
June 12, 19lG.l [D.Lkd OcesnograRska Institutimen, Vniversity of Gotheuborg, Nay, 1916. Rrrcived
For imny ccnt.urics oceanographical invest.ig a. t.’ 1011s
mere curriecl on for )urely practical, viz, nnut.icti.1 pur- oses oiily. Practica 1 ly all wo hiow of t11e ]xi t1ipirt.rii.nl
s(.iuiiclings t.akrn for prnct.ica1 roitsons. The smie :q) ,lien to our knou*lcdge of t.icln1 ihenonienn which lias c‘11 II mi- natccl in modern t.ida.1 prec t ictiom, field wiiere sciwce ap lied t,o pmcticnl purposes 1ia.s scored (JIW of it,s iimst.
bifi r imt. succcsscs.
The vast. systems of currents which t.raverse tlis sui-fttcc
of tlie ncea.iis have become hiown to us miiinly through uiinumbered observnt,ioiis by ocean-going merchmt
vessels : observations which were for t,he first, t,iinc t,liorouglily invest,iga.t.cd and imde useful t.o man by tlie clnsaicnl work of t,lie dist,inguisliecl Ainerican scient,ist.,
Matur~, the fa t,lier of oceiinogrn.phy. During t,he last 10 or 30 years 8 new prwt.ical renson
for nisrine rcsearcli bns arisen,. miiiely, t.lie riipicllj- ill- crensing import~ance of the fislimg industq-. Since t,he introduction of st.enm trawlers the high-sea iislicries in various iniportmt area.^ have become so iiit.ensified t.hn t. discluiet.ing. rcwilt s of overiisliiiig we plninly vieihle. The necessitry c d safegunrding against, t.liis eril wts one of tlic niniii iiiotkw wiicli lei1 to 6lie est,;i.blislinicnt of cii
interiiatiunal orgmizntion for the st.udy of trhe w:i. which, for inore t.Iian 12 pe:rrs, liacl vigorously pursued spteiiiat.ic iiivest,igat ions of tlie prcqmgatxm, growth, and iiiigmt.icms of t,lie most. vnlu:~~l>l~ food fishes ns well ;IS their clrpcnil- eiice on t,ke ~tirinhleq!ipsic~i.l and chemical properties of the sea wat.er. A dcfiiiite proposnl for int.ernntiona1 legislntive measures ngainst escessive fishing of iinnia t,ure fishes had already bemi 1x1 t foiward by this orgsiiizut.ion wlien t.l:e outbreak of the present war brou@it its work to n t.enipor;iry st,anrlst.ill. Besides t,litw t.rw practical iiiotives for syst.eni:it.ic investigaticjiis of the ocean there is a t.hird reiLsoii for iiicreasmg tilid pei-fec ting our knowledge of t4lie
sea,, a rcason which will no more gencrslly scknowledgecl in tlic ncar fut,ure. I refcr t.o the 2111- important influence which the ocean esercises over t,lie cliiiiate aiicl the weat.lic-r of the surrounding continents. The object of tlie present paper is t.o give ;I brief surrey ways hi which this influeiicc
INFLUENCE OF THE OCEAN ON AIR TEMPEEATZTRES.
From it t.limiid point of view the funct,ion of the oceiiii
is twofold. It nccuiiiulates sulnr licnt uiirl redistrilmtcs it more evenly both in ii.int7 a d in spcice. 1t.s poww t.c., clo so is derived mainly from two phyaiciil propert,ies of
water, riz, its lnrge heat ca acity (1 cubic iiietcr of wi.t,er on cooling 1’ C. will raise t 1 e t,eniperat.ure of 3,000 cubic iiiet,ers of nir by the same amount,) and it.s niobilitoy. The mobility of tlie water in the vertical dimension assist.s in cltrrying out t,he first of tlie two functions before iiieii- tioncd, while its niobiii ty in the horizontal plme ennblcs the ocean t.o esert it.s second thenno-regulhng function, i. e., by means of it.s large wurni and cold surIace current.s. As regards the capcity of the ocem to clistribute heat of solar origin niore eveilly in tinie-to act us i~ lriiicl of
“savings bank” for solar energy, receiving deposits in
seasons of excessive insolation and paying them back in seasons of wa.iit--we can get a faint idea of trlie state of things which woulcl prevail were tlie earth devoid of its oceans b coni aring the diniate of central Eurasia with tlint o?Scaiic\navia or the British Isles. The fol- lowing fipres, which give the number of heat units (kg.- cal.) pa.ssing in and out, throu 11 1 square meter of differ- ent surfttces per annum, are aE.0 very illuminating.
Kimthtu qf l i d units (kg.-ctrl.) passinq i n wid out through 1 square
meter of d$twif su$ms nnniially.
S:iiidy soil in a forest. - - - . - - - -. . -. -. - - - -. . . . . -. . - -. -. . . -. . 13.000
Sandy sui1 under grma. - . . . -. . . . -. . . - -. . . . -. . -. . - -. - - - - - IS, ,500 A fresh-water lake (down to 24 meters). . . . . . . -. . . . . . . . . . . . SS0,OOO
h i t ,l i e r n Bsltic (down to 55 meters) - - - - - - . . . . -. . . . - - - -. . - $20, OOo
Not less im ortaiit is the scconcl function of the ocean, viz, that of (P istributing its storc of heat (accumulated chiefly in tlie Tropics) more evenly over the surface of the
globe. The profound manner in which the ocean modifies the tempcrature conditions of our planet is lierha s best tlenioiistratecl hy a chart showing the isotherms &early
a\-crugcs) of the North Atlantic surface wntoiu, specially h i r abiioriiinl trend to the north in its iiorthenstern part wliixe t.hc sea’s isotherm of 0°C. runs beyond tlie SOth
l):irttllel, i. e., some 10’ farther tliaii its average latitude for tlie Nortlieim Hemisphere, aiid where tlie air tempera- ture ttiioiiinl in January for certain areas off the Norwe-
T o retarn to our metaphor: The savings hank for solar heat which w-e call the oceaii has a very extensive “foreign
Yo fnr tlieso facts are of course generally ttchiowledged. Import.aiit, 11s the t,cnil)erature-reol~ting functions of the oi:imi undoubtedly are, they wouh sff ord but little actual ii! tertst were they alwn-ys, from year to year, esercised in t!sat.tly tlin sttnie niimiier and wit,h unvarying int.e!isity.
iiowiw!r, there is am le evidence to show that this is not t,lic citso. Or, nietap P ioricnlly, the nniiun.1 “ divideiids” p i t 1 1 )~- this hnnk to its “shareholdei.s,” i. e., the conti-
lil!:lt,S rouiicl its borcli?rs, are omitbed on it variahle scdt!.
iliere arc+ gctni-s wlien the heat supply runs dowi to fmiiiie vnlues, arid other years of lavish abundance. k:s:ic.t. proofs of this fact. were first produced 20 years ago
iir 11 ptper by Ot,to PetLeixson,’ which may bo said to opcii iicw perspectives 011 t.hc relationship between oceun- iigmp1i~- and meteorology. By nieaiis of curves estend- iiig ovsr niaiiy )-ears it, wtts shown that the surface tem- periit.ures of the Atlniitic off the coast of Norway in winter iniont81ily avorages for Janunry and February 1 vary con-
siclernbly from year to yea.r, aiid that these variations are reflcotcd 011 it ningnified scale in peifec.tly purallel fluctaa-
tioils of the air temperttture over central Sweden. Fur- t.iici-, it WIS ~~~~~~~~II thet tlie same p:Lrnllelisni is revealed in vi~rious important phenomena in nature, such as the clisnppentiice of thc snow cover, t,he development of
rcgtht,ion (flowering of certftin sprin herbs), aiid also in tho thnwiiig of the soil (marked by t fl e coniniencelneiit of hwiiig).
d curve (fig. 1) showing the latt,er relationship is r e I>rocluccil here from Pettersson’s paper.’ To put the niatker inore explicitly, the date on which agricultural spring ivwk can beoin, which is of verj- great importance
for tho sul>scciuent 8evelopment of the crop, is determined by ocrnnogral~hical conditions .wA.ich prevail two to three
aio,zth s ecrrlier.
h -g .4 .
gim coast s r iows positive vrtlues of not Zess thur,, +V C .
c!:ic:h~nge. ’?
,:,
. .- . . . - - -.
1 Prllcraxorr. Ollu. IkAer die Beziehmigen ewischcn hvdrogmphlschen and meteore logisohen l”h&uumemn. Yeteorolog. Ztschr., Aug., lS9G; 13: 9 5 and 316, fig. 14.
JUNE? 1916. MONTHLY WEATHER REVIEW. 339
FJG. l.-Curvrs comparing fluctuntious in JntCs (4 beginning of spring field work near Upsah, with variations in the temperntiire of the -4tlnntic 017 coast of Norm>-.
A nuniber of other investigators, such as Meinardus and Hildehrandsson, have taken up work a.lon~ the lines
pointed out by Ot.to Pettersson and have clen?ly proved the surprising ex ten t to which dominant meteorological features of Europe are wreriied 1)y tlie prevailing state
of the surface sheet of txe North Atlantic. So far, how- ever, hsrtlly any practical beiicfit has been derived from these discoveries. The efforts of inodern nicteorologists seeni t.0 be almost exclusively directed toward tlie attainnicnt of slmrt-rrnn:qc forecasts, riz, the prediction of the probable weather which will prevail :I couple of days later. The greatest living nuthoritzy on dynnniictil meteorology, V. Bjerknes, is reported to have said that the problem of foretelling the weather three days in aclvitnce from a sufficiently detailed knowleilge of the me teorologicsl elements over a
certain area is coiiipletcly solved. Only the intricnt.c
. calculations required for ti solution in the individual case will keep a staff of tminecl assistants busy for several weeks! The iniportaiice of successful short-range forecasts of course must not be nnderestiuixted. Apart from its value for other open-air ?cti\-ities, it niay become emi- nedy useful t.o ngriculturc, specidly a t harvest timr ;
While their value for the fishing and shi ping industries, in the shape of storm warnings, has E een established
beyond disput,e. This is particularly the case in Sweden where these predictions hare proved eminently successful 'owing to the excellent. work of Dr. Nils Ekholm. However, it would a >pear that still greater practical
the avera.ge character of a coming season, such predictions as Pettersson >rodaimed t,o be possible in his aper before
be more or less ruined by a cold spring, a hot and dr summer, or a wet autumn, and an early warning of suc. i
prospects would have a value which can hardly be over-
estimated. Besides trying to redict the extremely vari-
attention to the c o ~i ~e t ~~~u t .i ~~e u l ~n i m t in meteorology, viz, the surfnce sheet of the ocean where changes may be observed months before their effect on our weather be- comes manifest. Unfortunately our resent lainentable lack of hiowl-
sib 9 e now to give siiiiilitr forecasts of any value. E is therefore it most welcome symptom that the nieteoro- logical services of several countries have of Lite startcd to issue monthly charts giving t,he actual state of the surface of the North Atlantic. (or rnther of those parts which lie along the principal ship routes) and also to work up iortionu of the immense nisterial cont,ained in the log h d i s o f the ocwn@ng merchant vessels.
benefit might be reaped I rani long-range forecasts defining
mentioned. Growing crops worth millions o P dollars niay
r
able state of the fickle atmosp 7l ere one should give niore
e e of this ninrine e Y einent iiiitlies it practically in1 os-
Pioneer work of this kiud has already been coiiiplet.cd by Nansen and Helland-Hansen whose result:s, as yet un- mblished, prove that uniiiistakiible changcs in the sur-
kxce temperature of the North At,lant.ic occur from year to year and t,hat these changes m n paralld over W I R ~
awns, if not over the whole ocem.
Finally the Int,eriintional Council for t,he Study of the Sea, a t Copenhageu, nro employing the leisure due to the temporary interruption of their regular research by the war, by taking up this kind of work on a still broacler
scale, viz, the average hydrognphicctl condit.ioiis of cer- tain represenOative areas for t,he period 1900-191 3, 21s well
as the departures from thew rulues for each n1ont.h of t.he same time. In this work the escellent obsermt,ions estending over some 40 yews coinpiled by the Dmish Meteorological Office lvdve proved pnrticulidy v:xlluahle. to be hopctl t.liiit when t,he end of thc
possible, the Internntionnl Council niay be able t o es- tend its carefully worked-out iiiethods nncl esperience of organized scientific work on practical problems, to the larger field of the North Atlantic Ocean. With regard to long-range forecixsts of the kind indi- cated in t,his paper the iiiost importtuit p r t of the whulo North Atlantic are the cowhd u-sters of t.he es-t a b ern
United States, specially the Strait of Floriclu. By means of a few recording current iueters-to be described in a succeeding aper-conibined with t,lieriiiooraphs a
over this pulse of the Gulf Stream. A sensible departure from the average value of the vast aniount of stored heat carried through this strait (some 10': heat units per hour if the water were to be cooled only to +loo C.) might have profound effects on the weather of the following nionths both on the European iind the North American continents. Similar observations off the coasts of
Forniosa would no doubt be of corresponding value for predict.ing the weather of Japan.
It is earliest1 present state o ? war makes a return to snne occupations
close watch mig 1 t be kept at coiiqxirativelp pow cost
INFLUENCE OF THE OCEAN ON ATYOSPHERIC CIRCULATION.
I t is quite easy to prove t h t if the temperature were and remained uniform all o\-or the surfnco of our planet there would be no atmospheric: circulation whnt.ever; a
erpetual dead calm would rei n supreme. On the other
erature varies along tlie enrthk suifsce at a certain kcnlity thc more intense? in enerd, will be the atmos-
reat contrasts of tem )eraturu prevail within short %stances, the locality wi 1 1 be distinguished by the large nunibor of atmospheric disturbances which either or@-
iiate there or itre stiniulsted by it.
gand,it is also evident that t % e more rilpiidly the tem-
pheric disturlmnces set up t. B iere. Therefore, wherever
340 MONTHLY WEATHER REVIEW. JUNE, 1916
Now the oceaii, in discharging the se.conc1 of its teni- erature-regulating functions, Yiz, redistributing s01n.r
geat in space, is responsible for bringing tovether great contritsts of tempor'-\tme. This is s ecialf;- the case
runs along in( Iuiitiiincius ridges at, the edge of a winter- cold continent,. A striking os:unple of the first type is
the const nf the northenstriii United Stn.tcs niid Ntv- fouiidlnnd where the icc-cold Liil>rador C!iirrcii t8 runs
right iiito the Chlf Strenni drift. current niid w\.hc~c? the isot,lieiins run more cruwded tclgethr than in nny othor part. nf tohe world.? Also the 1oc:ility in questinn hns cd
old been popularly known n.s the "storni I)rc+odi~," and it.
ins to he a junction of the first order for the track9 (.If t,rans~nnt.inentii cyclones coming from the west.
If t,lie int,ensit,j- of the temperature contrmts for this tireti,
i. e.! tlic st,rength of the tenipersture gradient :dong the surface. were hiown for n nuniber of years it might show
a pronounred 1);i.rallslism with siniultniieous changes in t.he storm-frequency curve for the surrounding parts of the 0~'e?311.~
An es;i.mple of t,he secoiill type is afforded by thr region
oft' t,he cor,st. c d northwestern Scnndinovia, where in Jan- uary thr ii.foresd ten1 ,erntiire alio1idy of + 27' C'. pre-
most, striliing, as hns beeii proved through recent! st.udies by J . W. S;nndst.riini, the eminent, Swedish niet.eorologist.. Haying set! himself t.he t,nsk of discorering hy what kind
of ~.~niw;~lieric niechnniwi t.hc Norw-eginn Sen does csert.
over t,lie Scmdiii:~vim c.1inint.e t,lia.t, influence clismrerrcl
by 0. Petkersaoii, Silndst.riini iiiade prcilonged es1)lur;i-
ti0i.s in t,lic Sc:i.nclinaviaii Alps during iiiiclwin ter niid n t conaider.Me 1)eisi:mtil risks. He then found that t.hc preni.ilin~ enst,erly winds, once t.hcy get clrer the highest. ridges? mi1 clesccwd don the slo )es t.0 t.lie sen, gii:ning
cre:id by t.he iiiiiss of snow i t carries nlong w4th it) so
that tlir air finnllj- rushes like an ice-cold cntj;i.rttct :it it
t.cw i iic p c e cl~-:\i-tiw\..a:.d to t.hc m r n i sen surfnce. Thme its twiij)t*r:it1irca ::iid n~i-ioiiiit, of moist,urc are incrcvwd :uiiI
sei?:.? (it t.lw rc>-\viimirci air wt,urns t.ownrd the Europenn Contii;c:it. :IS i: w-est.crlj- wiiicl a t a liipliw :i.lt.itudc. Thp I>ocnc!ary sm.f:!ce bctwccn t:hc \v;iriii and moist 11ppc1' current fi'iini tlic sea and the iw-cold lower current I If
opp Isitv clirection fli1ctu;it.c.s considernbly up ti.licl clo\+~.n.
t,hr :wst. wind somc.tin:cfis reaching as fnr dow; as t,o t h i
1110211 it niii slopes. where t,hc e:lst wind is tel1iPt lixrily
dni;~niecl up. However, when the e::.ut wind ngaiii hmilis 100s~ it. ik:5WJ?dS with n reil~~ubl~il violmce. The j 7 ;i i i i I
velocity 2 t t,he coast. of N o r ~~i ~y thereforc SCJnll?till?t!S
second (IUS kin. per Iiciur) in t.hc course of ii few xiiiiitit,ps,
which i-:i:iy of iv~irsc iiivr!lw coiiuic1cr;rl)lo liis:.; of life iin(.l
],ru;.~rL,.y t,o tltc iishiiip ])opul:!.t.ioii. 8t.i.iriii wmiiiip
bttscd i rii thew diwo;.t~riw Lg Yiiiitlstriiiii wiil sliort.:j- 1)'.
issued ljy thc Noiw*egiii.ii ;tuthmit,ies. Tho at.niosplieric circiilntioii clescribcrl here vu11 bo beau- tifully dcinonst,rated in a trough with walls of plate! ghss such as I hnre used for liydrodynamical esperinic?nts (1er:gtli 100 mi.; width 30 cni.; depth 22 cni.). Th trough is divided traiisveisely by a triaqplar block ur"
rubber reachii?g hnlfmiy up its sides. Olio prtition is filled with ice, and oil t,he bottom of the ot,lier is plnced a
wliorc? a wnrni current collides with a co f d one, or wliere it
rails. The effect on t. t ie a.tiiiospheric circulntion is dso
ii:ipct,us froiii thc high c P ensity o i the a.ir (which is in-
iiici-cwcc: frt:l11 :i gei1t.k b ~e z t . t.o i l gill(: of :iO 11iiAtt*rs p ~r
small metal bos with hot water representing the Norwe- gian Sw. By eslpaliii some tobacco smoke into the trough, the circulation % monies plainly visible, and one
sees a cat.amct of ice-cold air doscending the slope of the rulhcr block aiid rising again abruptly over the hot metal, whereas an up er current inimecliatdy under the [glass?]
ditions which are probably it rather less intense combi- :i;ition of both types prevail over the Japanese islands, which are also recognized as one of the stormiest regions of the world. Predictions of the storni frequency based
on ocea.n ographicnl observations appear, thereforo, to be feasible for Japan t h o .
111 his pnper Pettersson also calls attention to the fact recognized already in 1579 by the Danish meteorologist, Hoffmeyer (Ztschr. (1. Oesterr. Ges. f. Meteorol., 1878, 13, aiid 1879, 14), namely, thtit the distribution of the air
ressurc sliows distinct eff acts of oc.eanic influences. Esi>l?oially in the cold seasoii is there a tendency for sta- tioi:iiry barometxic niininin to develo and to remain
geiicwl cousidernbly warmer than the surrounding laad s u r h m . A chart of the average isobars for the wintmer
mol:tils plsinly shorn how- c.losely these follow the outlines of t h colitillellts. These regions of relatively low pres-
sure are of considerable importance in our latitudes on account of their evident close relation to the movements of t h tmveling barometric depressions , i. e., the cycloi~es.
Mtt 1s of t,lm priiwipal storm tmcks, such as those bv vaii
BoJlwr (see iig. 21, also show t h t regions like the North
Sen wit.11 t,hc Sl~igorrack, the southern Baltic-in u less d e p w even large litkes like VBnnern (Sweden) and Lrtdngs (Russia) as long as they remain unfrozen-are touchoil hy t,ho most freclueiited storm tracks. A close
and iwiit.iiiiii)us study of the surfilce winter tern eratures
drav:iiig up dso of short-range weather forecasts.
cover is strong 'I y drawn inward hack over the ice. Con-
stat~i,mary over wltter surfaces, as the \ atter are then hi
of t,hoso regions therefore ought; to bo usefu f for the
FI.:. ?.--;t.+m Inirks fw nurll~~~estrrn Yurnpe, after van Urbbcr
The ii;imrnse iniporttmca for our winter climate, of the lnrge low-pressure region which prerails in winter over the wnrni ocean surftice tu the south of Iceland, was also pointred out by Hofliiieyc~r and has been the subject of
JUNE, 1916. MONTHLY WEATHER REVIEW. 341
more recent remarkable studies by Teisserenc de Bort, and his collaborator, Hildebrands~on.~ In fact the estent to which atmospheric cirr.iiliLtion depends on oceanic conditions, wlirn once fully recog-
nized, will inake a close cooperation between nieteorolo- ists and oceanographers a condition sine qua non for the !ormulation of successful weather forecasts.
“HE INFLUENCE OF THE OCEAN ON RAINIO’ALL.
From a theoretical point of view t,he prevsiling condi- tions of the surface sheet of the ocean ought to haw a de- cided influence on the amount of rRin precipit,atecl over
the continents. However, so far no very definit,e proofs have been produced for the actual existence of the sup- The weatvest) part of the rain which
!ab over Euro e is no dou%t fornied by e-riiportttion frmi
oration, which is a func.tion of several varinbles, J-iirirs rapidly with temperature, a rise of 5 clegrces from + 1O0C.
to t 1 5 O C . c.orresponding to an increase in thc vspor pressure of some 40 per cent. It would therefore nppear
very likely that an unusually low surface trnipcrature over large parts of the North AAtlantic niny be followed by a reduced rainfall in a coming season. 0. Pettersson has proved that a t times when an outburst of icebergs tnkes lace in the South Indian Ocean the monsoon rains over scanty, which inevitnbly results in a niore or less fn.ilure of tlie crops and subsequent faniine of 1iuni:~n beings. The greatest outburst of icebergs in t,he South Indian 0ceii.n
occurred in 1595-1897, and 1S96 was ii ye:w of esacp-
tional drought in India; the crops there failed over sev- eral hundred thousand square niiles and inillions of the
PO ulation were reduced to a state of famine. 8nce we have full knowledge of the surface tempern- ture (and the salinity) of the most re reseiitative parts of the North Atlantic Ocenn, and can P ollow their vnrin- tions from month to month, perha s a kind of long-range
I have tried to point out that from the stiintlpoint of
the meteorolowist a systeniatic and regular ohsrrvittion of the North h n t i c is a task which ought t o bc unclcr- taken without delay and that it would have every pros-
pect of gaining results of iminense prac tictil viiluc to
mankind. ivc brief descrip-
make it possible to take isolated or continuous observa-
tions of -rarious ocean0 aphical elcnients wit’li zi niini-
osed relationship.
the surface of t g e North Atlantic. Now the rnt.e of eviip-
%dia will, in general, be
forecast of the rainfall also may B econie feasible.
In a succeeding paper I intend to tions of sonio novel instruments ant ? niet,hods which
mum o t labor and a hig #!- degree of accuracy.
PBECIPITATION OVEB TEE SOUTHEAST BOCKY MOUNTAIN SL0PE.l
By CLEVE HALLENBECK, Assistant Observer.
[Dated: Westher Boresu. Roswell, N. &lex.. June 14, 1916.1
Nearly all of the generd rains over the region eiiibriic- ing eastern New Mexico and the extreme western portion of Texas come with easterly winds. The greatest frc-
quencv of precipitation at the eastern border of this artw
is with southeast winds, gradudlv sliiftin to northeast-
erly by tho time the Pecos Vahy in 8ew Mesico is
reached. It can be said that in this region rains coniiiig
b Srr tlicir work “C!ntres daction de 1’8tma~phtc.”
e Sveuska H drogra8sk-Biologisk8 Komurissioncns Sluiflrr V.
1 Accompmh by Charts SLW-72 to XLIV-iG. Figures 1 to b.
from any other directions than southeasterly and north- easterly are so infrequent, as to he negligible. It, follows that m y distribution of pressure gi-ring risc
to stcitcly eastcrly winds over this region pvill be produc- tive of precipitation. The most depenclahle condition is a low pressure area to the south; m general, south of the 35th prude1 of latitude. precipitation usually brgins when the low has moved far onougli edwnrd to brhg the incurving winds from the Gulf ovor west! Tesns
as easterly and o-rer emtern New Mexico as northeast.- erlv winds. While these lows moving dong the southern hordcr ne:i.rly dw-:iya produce recipitntion, thoso t ,l d 1 ~s nort,li of the 35th parcd P el seldom cause precipitation
mer t,his n.ren. This is dut? t.o tho t,opowapliy of t.lw
Sontliwest. The C!ont,incmtaI Divide, es tcncbng southart1
from C!olorndo, divides in northern Ncw hhsico in to two .hrnnclics : t,lw west, brnnch estrnds s o u t ,l i ~~-c ~t ~~n r ~l nearly to the corner of the State, while the east. hrnncli extends southward, scparn.tii:g the Chnndc nnd tho Pecos Valleys. The Divide lias an elevation of 10,000
t,o 13,000 feet- in the nort.liern p r t of the State; t.hrough thc ccntrn.1 and southern portions of thc State t.lw two l?rn.nclies have iibout thc sanie ~lcvnt~inn of F,000 fect,. The mst.ern branch, however, extcnds clown into Texas to the meeting of the Grnnde and Pecos, and thus be-
conies a controlling factor in the distrihution of procipi- tation over this area. That part of Texas lying inime- dint,tdy east of this range is the clryost. portion nf tho St*atr. Westerly winds over -4riz011:1, and New Mexico are
‘ upslope ’ wiiicls until they reach the wcstrrn branch of the chide, t,lien their direction averages about. horizonti11
unt,il they piss tho crcst of tlie eastern branch, after which t,liey are “clo~viislope” RS long as their direction has an cilst,warcl component. Tnkiiig now the case of B depression centered near the Iiort;hcrn hortler of the St.at,e: over its western quadrant, t.he wintls are generally “upslopc,’ ’ and therefore favora- ble t,o prccipit,at,ion. Also over its soutlienst~ern cluaclrant tlie ivintl is nioving “upslope” froiii t,he Gulf. But over
t.liat port,ion of it.s southern c uaclrant lying east of the
the Gulf is the ttree enibracing eastmn New Mesico and est,rcnie w-est.erii TCS~S, where the wind is blowing ‘; doyn- slope.“ itnd t,hc?rrfore is unfnrornblo to 1)rec:ipitttt.ion. For this reason n at.orni ttrea piissing eastwiircl approsi-
nmtcly nlong tho northern border of thc State causes
gc.nc+rd prc-.c.ipit.at.ion oyer all the area under its influence, with t,he except,ioii of this “dry belt.” The accompanying chart (fig. 1, ~~1 v -7 2 ) shows an idetil wind circulution around one of these depressions. The precipitation areas of many LOWS show remark- able conformity to t,his theoretical one. A large number of the southern LOWS are southern cciit,crs of low trou hs. But even in depressions that are
t.h(: Gulf winds to hlow in from tlio southeast, often hlow- iiig in a st,raipht line, or even recurving slightly to the iinrt,li, when tlie low cent,er is 1oc:atecl as shown on the t,lit+oret,ical circulntion. On i,lie cliart present,ed this t.cnc1ciic.y is cniphasizetl to show the wind-shift line,
which 11n~n1sl.Ly marks tho west.twi liniit of prccipitntion. Thc distribution of prtvipita.tion around ti rleprwioa is illust.r:i.t.ctl in thc four composite maps prcacnt,cd in ijgi1i.e~ 3, :3 , 4, 5 . Thcuc W C T ~ const.ructrd from the
nrc!r:i.glx h t ,a for a number of LOWS in :i.I)Pr~Jxiiii:i,t,cl~ t.he
samp posit,icms. t i .1 ~1 show the nvern.ge pressure, wiiid (lirpct,ion, and prcc.ipit.ation frequency for LOWS cen-
divide and west of the belt o / southeasterly winds from
rtvgularly foriiied, 6 f icre seenis to be a st.rong tendency of