MONTHLY WEATHER REVIEW
CLOeED NOVEMBER 3, 1922
-- SEPTEMBER, - 1922. IeSUED nECEMBE8 4, 1932
VOL. 50, No. 9.
W. B. No. 786. -
~
. _-_ -- -.___-- ___
THE PREPARATION AND SIQNIFICANCE OF FREE-AIR PRESSURE MAPS FOR THE CENTRAL AND EASTERN UNITED
STATES.’
By C. LEROY MEISINQER, Meteorologist.
[Author’s abstract.]
[Weather Bureau, Waahington, D. C., Oct. 15,1~2.]
NOTE: There has been issued recently under the above title
MONTHLY II’EATHER REVIEW SUPPLEMENT NO. 21. It haa been the practice to resent in the hIoNTRLT WEATHER REVIEW rather full abtracts of &e material contained in the SUPPLEMENTS. This step is made in the interest of printing economy. for it is the hope that the eneral reader may be satisfied in this way without neceasitating a
L. edition and without entailing waato through indkimhate distribution of the SUPPLEMENTS. Therefore, those who find this subject one of special interest, an? who desire to procure the corn lete discussion, may do so hy applymg t,o the Superintendent of gocu- menta, Government Printing Ofice, Washington, U. C., who carries a limited supply in stock. The publication contains 77 pages, 31 ligures. and 22 lithograph charts. The price ie 25 C~U~S.-EDITOR.
It is the purpose of this abstract to present concisely that material which has not been used m previous pub- lications dealing with this subject. The material is
large1 excerpted from the extended ublication men-
have not been reproduced. I t is hoped, nevertheless, to illumine the salient features of the discussion, allowing the reader who so desires to seek the details in SUPPLE-
Those who have followed previous papers concerned with this research,2 are aware of the principal difficulty confronted in the attem t t.0 reduce barometric preqsure from the elevation of tr ie barometer to some arbitrary level. It is the difficulty of knowin accuat.ely the mean
reduction is to be upward, the air column is real: if it is to be downward, as a t present in reducing to sea level, there is 110 actual air column, and, in t h s 1at.t.er case, the roblem is to find a temperature value which will is concerned with
air temperatura. There is scarcely a meteorological element that is more irregularly variable with change of altitude than tem- perature. The value of e that would bo ideal may he defined as a mean determined by integrati the current
air column under consideration. But this is manifestly
impracticable. The equipment necessary for the obser- vation of free-tlir temperatures can not be provided for
the individual stations: and, if it could, the time required for the reduction of observations would render the plan
tiondabove. Maiig t.alles, numerous H gures aid charts
MENT 21.
temperature of the intervening co K umn of air. If the
yiel a smooth isobars.
vertical temperature curve t,hroughout the i ength of the
reduction upward, hence, of actual
iiieffectual for the forecasting of weather. The resen
mder, observable Cct the sw$ace, that ud! lead to a close a roximatim tf t 7 ~ mean temperature of the air column. #[ell one considers the almost. infinite variety of surface and free-air conditions that bear directly upon the temperature of the air column, the search seems, at the outset, rat,her unpromising; but the way is opened through the use of the surface-wind direction as a basis of classification. The attempt to draw free-air pressure charts is not new. It has been attempted in one form or another and for various purposes by Teisserenc de Bort; Koppen,’ Bigelow,5 Sandstrom,B Dietsch,’ and Fu’iwhara.8 As long ago as 1882, Professor AbbeD said: “I’!i fact, for all
cases, the only natural method of reduction would seem to consist in an attempt to reduce upward through the actually existing atmos here (whose temperature and moisture can be observe CY to a uniform altitude.”
problem, therefore, becomes that of jndiiq some c f ue, or
WIND DIRECTION A 8 AN INDEX TO TEMPERATURES IN
THE FREE AIR.
The need fx an. index.-In reducing to u per levels
distance between the station and the reduction level,
(2) the surface pressure, and (3) the surface tern erature.
tqniperature of the air column, and (2) the mean vapor pressure of the air column. The mean temperature affords the greater roblem, for the water vapor effect
is less influential. !f the mean temperature can not
three elements are accurately known-(1) t f e vertical
There are two elements to be determined, (1) t g e mean
be observed, it is in1 that some index, o b s e d l e What shdl this index be? are two qudifi- conform: (1) It must have a direct physical relation to the temperature in the free air, and (2) it must be one of the weather
htude sur la drculaticm g &n M de I’atmosph8re. Anndu du Bureau m a l
.Vit+rol fqur de France lM, Part IV, MEthrologie Csral. seeand partie. pp. a5-41.
4 Upr% Oestalt der hobarm in !hrer lbh8ngipkek von S d 6 h e und Tcmperatur- Verteiluy bfel8orologthc ZrUdchrtfl December 1tW pp. 47@481.
5 Re or on the barometry ofthe Uni‘ted Statw b d a , and the West Indles. Brporl Nthe EhWZthe Weather Bureau. IaOeteOl I d 11.
e On the wnstfuetion of Isobaric charts fdr high levels In the earth’s atmosphere and T~n~adl0n8 qf the Anmicair Philaaophbal Sacid#, N. S.,
?:.r:fx“E?frt?/$pp. 31436. rUntersuchungm fiber dle hdenin des Widea mlt der H&e In ZyHonm. Vm5flcntlicAunqrr de8 c7ropAplkaliachm~itut~ der Unfr:er&dlt Lei&, Band II. Heft 5, 1918, pp. 197-234. Abstrwt in YO. WEATEEX REV July 19% P. 402.
8 Pressure maps at 3 L~ometas In Japan. Mo. W~TIIER’REV., OctobEs, 18aL pp. 571-572. SAppendLx 61, Repod o/ IAr Chit/ Signal Offrepr, 1882, Part 1, p. 626.
lflcsnce.
463
154 . MONTHLY WEATHER REVIEW,
elements regularly observed at stations. The first of t,hese qualifications is scientijically pertinent; the second is practical. It is often inadvisable to infuse too much of the purely practical into the preliminary research rela- tive to such a problem as this; but in this case it seems evident that the factors of simplicity and practicability me inse arable from the scientific. solution of the prob- lem. dr that reason it is desirable to impose this qualification upon the element, selected. Comparison. of wind &recti.on. a.nd a.tmospheric presswe as temnperaturs coi~trols .in fhe lowest levels.-A careful con- sideration of the various elements observed regularly a t meteorological stations will invariably eliminate all but one (excluding temperature and pressure, which are essen- tial to the formula as stated above), and that one is zcrind direction. Surface pressure and temperature do, how- ever, call for a word in this connection. Leaving the quesbion of the cause of irre,plar barometric fluctuations, i. e., the cydones and anticyclones of extratropical lati- tudes out of consideration, it is sufficient to say that tem- erature changes in the lowest levels of the atmosphere
i ) only the lowest 2 kilometers will be considered here) are the result of the im ortation of warmer or cooler air.
and the blowing of the wind is the result of a pressure gradient, however established. Many important discussions have centered about the cause of temperature chan es in the free air, some met.eor-
t i 4 dynamic m character,'O and others that the changes res t t h o u h the import,ation of air of different tem-
with conditions at hi her levels than are of interest here. It is generally agreef it seems, that in the United States the temperature, owing to the continental charnct.er of the weather c.ontrols, at least below 3 kilometers, is more strongly related to the cource of the air (hence to wind direction) than to such effects as dynamic and radiational heating and cooling. In 1919, correlation coefficients were worked up by W. S. Cloud, at t.hat time assistant in the Aerological Division of the Weat,her Bureau, showing the relation between temperature, pressure, and the south component of the wind at the surface and at 3 kilometers, based u on
o posite in sign t.o those determined in Europe, and led
&. Greg to conclude that the factor of coiitinentality with the attendant marked effects of wind direction was the cause of the obliterntion of such dynamic effect.s as were pointed out by W. H. Dines, in En land. Mr.
relation coeflicients] confirm the conclusions already given, viz, that in the United States, particularly in the mterior portions, wind direction exerts a greater influence on the av temperature than does the sea-level pressure." One may attnbute wind direction to pressure distribu- tion, but, in such event, tho relation is, a t best, only
indirect between pressure and temperature, whereas be- tween wind direction and temperature it is direct. This disposes of the measured element of surface barometric pressure aa an index to upper temperatures. Wind direction and tern eratzcre at the su,;lce in relalion to freeair temperatures.-&milarly, there is a good wrre-
Such importation is t R e result of the blowing of the wind,
010 .sts adhering to the be H ief that such changes are essen-
perature." % u t these discussions concern themselves
approxim-ately 200 observations. These gave va P ues
Gregg states, in conclusion: "These figures [ 8 loud's cor-
- ..
1oDiaes. W. H: The rhararteriqtirs of the free atniosphme. GcopAssiraf .WernoireR No fS Brlt Meteorol al OR., 1919. Abstract Yo. WEATHEB REV., September, 1919:
p ~~~&~~ G%Zg&.m mature distribution in the lowest 5 kilometers of cy-
ClonPS Bud ButicyCh~ YO. &?PATHFR REV., 8 t a b =, 1919, p. 647449.
Journal dthc Royal dfelrwolopfeal Sodcty, Jmuary, 1921, pp. 23-46. ~mgias c. K. M.: Temperature vsriatims in %e lowest 4 d k e t e r s . ~w r t n i y
SFPTEMBER, 1922
lation in the United States, between surface temperature and wind direction. This holds at least as high as 3 kilometers. While, in considering seasonal or monthly normals, there a pears to be a certain relation between tem erature at t R e surface and at some freeair level, it
hold uniformly under day-to-day conditions. The ver- tical distribution of temperature is not constant, nor even regularly variable, but one which varies with type of weather, time of day, top0 aphy, etc. It is apparent, then, that if the relation i? etween surface temperature and free-air temperature were used, it would have to be classified according to some other condition more repre- sentative of the t pe of weather. Wind direction would
it seems that wind direction stands as the lo ical element by which temperatures in the free air in the 6 nited States may be judged most readily; both surface pressure and temperature ar? but indirectly related to free-air temper- ature, while mnd direction a t the surface-the conse- quence of both these elements-is directly related. General pressure distl-ibwtion as an inder.-It has been sug ested that the position of the station with reference
the estimation of temperatures aloft. It is believed, however, that this criterion fails with respect to the qualification of practicality. An observer must be able, without knowledge of mihespread conditions, to apply whatever criterion is adopted Obviously, it is not until all the observations are collected upon the synoptic chart that one can determine the station s location with respect to the quadrant of the cyclone or anticyclone. Hence, this sug estion is, for current reductions, entirely im- practicaf If it were ossible, 3s it is in statistical sum- maries, there is the a c r ditional drawback of being unable to assign the station definitely to a pro er barometric sit~ation.'~ Wind direction, however, feing a direct result of the barometric situation, affords a satisfactory and sim le ex edient, and such'classification is, in the
The time dement in Tl-ind dwection.-One point has been raised in discussion that is worthy of careful consideration. Suppose that at a certain observation the temperature is -1.5' C. and the wind south, of moderate velocity, but that i t has changed to south only within the last hour. The observer using his observations as the funda- mental data computes the robable mean temperature of the air column upon the E asis of a south wind at the surface. Suppose, moreover, that the wind blows steadily from the south for 24 hours and in that time the surface temperature has risen to 0' C. under its influence. The mean temperature of the air column will be quite different in the two cases, as is the surface temperature; but, since the south wind was blowing a t both observations, the observer will consider that the difference between the mean temperature of the air column and the surface temperature is the same a t these times. Is such a device justified? This is a question of enuine importance for an answer to which one must fook to the testimony of observations. The only answer that the writer can give at this time is based upon some statistical data gathered from the aerological stations at Mount Weather, Va., Drexel, Nebr., and Elllendale, N. Dak., and published in connection with preliminary considerations of this question of reduction upward.13
is o E vious that such a relation can not be expected to
offer such a classi B cation for reasons given above. Thus,
to t fl e distribution of pressure should be the criterion for
last ann P - ysis, c. Y assification by pressure distribution.
I* Exccpt in the rsso 01 I erlol Inx-stl lims niicb as thc 3ud of thedJrnamIrs of qclonea and antl~yclmer, ths c l a s ~l ~~a t .~n has'been nhanddcfl gy the Unlted States 1 rather B:;teaii in favor of fhsaifiration by surlare wind direction. u Mdsinger, C. LeRoy: Prellmlner steps in the making of I&r pressure and wind ehsrts. Yo. WEATIIES REV., d y , lm, pp. 2sz67.j
SEPTEMBER, 1992. MONTHLY WEATHER REVIEW. 455
If there is a marked difference between the air column tam erature when the wind has just set in and after it hasteen blowing for a considerable time, the probable variation or standard deviation of the individual cases In other words, if the from the mean should be 1 probable variation is small, t e individual observations must agree closely with t.he mean, and the time interval between onset and observation must be of slight im-
portance. It was found trhats, taking the observations as a whole, at the three stations mentioned, the probable variation of the mean temperature of the air column from the mean in reducing t.o 1 kilometer above sea level (roughlv, an air column of 700 meters) is only
1.3' C.; to 2 kilometers it is only 1.8' C., being slight.ly greater a t inland than at coastal st,ations. In terms of pressure a t ordin conditions these variations are of the order of 0.5 "% m . and 1.2 mb. at the two lerels, respectively. It is believed that errors of this order of magnitude would not be serious, especially if t h 3 are distributed over considerable areas rather t im loc ized. This is not to say that the consideration of the time factor would not reduce t.he probable variation, but it seems that the labor and tedium of this further classiti- cation would hardly be just.ified by the degree of increased accuracy. The turnirrg ?f cc.i.titl wit?& tadt.ittidr.-Thc iwgunim t. may he dvanced t.h:Lt LL certain wind direction at. the surface does not necessiirily intlimte the turning of t.he uppw wind and that.. using the orgunient t,hat iniportntion is the chief factor in dete~~iii~iing temper:itures below 3
kilomet.ers, an upper temperature may be quit.e different at two observations when the surface temperat.ure and wind direction are the same: but. the same argument advanced abore will hold here. If such cases do occur (and they undoubtedly do), they are not, frequent enough to esercise npprcciable effects upon t.he reduced pressure. But there is another way to approach the problem, and that is to study the results of aerological ohserra- tions which have lately been sumniarized fnr the eastern and central Unit.ed St.at,es by t.he Weather Bureau Berolo 'cal Division." we %ow that., as greater ;mi grent,er elevations t i w
attained by kit,es, pilot btilloons, and other means, t.harc is frequently aiitl usunlly a turning of the wind, sonic- t h e s t.o t,lw right aiid sometimes to t.he left. It. is not
of much iniporttlnce whether this turning is much or
litt,lc, RO long a.!! ,it ia tA.6 m t i i r uridvr si.mi1a.r xiirfacr (YJIl-
d.itio IN.
It follows, t.liwefore, t.liat. if t,ht! turning is not the siiiiie
under d l conrli tions t.he tleriution from surface direction should be small, if surface direction is to he a relinhle index. In other words, surface wind direction may be regarded as a satisfactory guide to upper temperatures
(1) if turning with alt,itucle is const.ant., or nearly so. or
(2) if. deviation from surface direction is small. Mr. Cfregg has arranged t,ables extremely conrenien t for t.liis test,.1s In Table 1Oc is given t*lie average deviation in de ees of the free-a,ir winds from surface direction at,
In Table 15c is given the tiverage percentage frequency of
clockwise and counterclockwise furning for the sanie ele- vation and the saiiie stations. The curves in the accoin- panying figure 1 a.re ca.n-ied to the 3-kilometer level in order t a t the proper t,rend at. the 2-kilometer level may be shown.
7-
d -F I erent elevations above the several kite stations.
-
Aerologlcnl Survey ofthe United States. Mo. IV~C.~IER REV. SUPPLEMEKT No. M.
15 Ibid., pp. 55 and 6R.
It will be seen that the avera e elevation of the six
above sea level. For each wind direction a t the surface
(8 points) there are three curves-summer, winter, and annual deviation. The small numbers just below the surface level indicate the wind direction in degrees, measuring clockwise from north, while the wind direc- tions indwated near the 3-kilometer level are for 16 coni- Above each of the e' ht points of the sur-
percentages of the total observations. T%e darker shad- ing indicates clockwise and the lighter shading counter- clockwise turning, while the unshaded portion indicates no turning at all. The deviation curves for the southeast wind, owmg to their marked and rapid turning toward the west, are to be found in the lower tier at the left. It will now be instructive to examine these curves with
s ecial reference to the consideration set forth above. In
this we may study the eight polygons. Taking a broad survey of the eight graphs, we find that there is a marked seasonal difference, as well as a difference between the several directions. The outstanding features of the polygons may be e.numerated as follows:
(1) There is more frequent turnin of winds from sur-
at the 1-kilometer level. This is a well-known fact, but a
short table of the percentage of times tlwe is n o change is of interest, the annual average only being considered.
(anIclLn2 crrCmp9.
stations is 250 meters and that t 7 ie elevations are given
ass points. face wind are shown fre uency PO P ygons of the average clockwise and counterc P ockwise turninm expressed as
t E e first place, what of the constancy of turning? For
face direction in all seasons at the 2- a -ilometer level than
TAHLE 1 .-Pcrceritiigc Jiiqilpncy of 'iw turilJ,iig qf wind with altitrck
. .. .- __ ~-
Elrration.
2 km .._. ... . .._.__ .... ._.. . 44
It is clear from this table that northerly winds are the dee est and most likel to persist up to the %kilometer
likel to turn below this level. (2y At the 1-kilometer level clockwise turning occurs most frequently with southeast winds and least frequently with northwest winds, the t.ransition from one to another
be;Y 1 t the 1-kilometer level counterclockwise turning occurs most frequently with northwest and north winds, and least frequently with southeast and south winds, the frequency of clockwise turning beina rather small.
(4) At the %kilometer level clocxwise turning occurs
most frequently with southeast winds and least frequently with northwest or north winds.
(5) At the 2-kilometer level counterclockwise turning occurs most frequently with north and northeast winds and least frequently with southeast winds, t-he frequency relative to clockwise turning bein considerably greater, es ecially with northerly winds, t 7 ian is the case a t the 1-klometer level.
(6) The h t precept-that conceiming the greatest constancy of turning-is fulfilled by southerly winds, es ecially southeast. %et us now examine the portion of the diagram dealing with the average deviations from surface direction, bear- ing especially in mind the importance of the amount of the deviation and its relation to the constancy of turning
leve, P while east, sout l east, and south winds are most
uite gradual.
456 MONTHLY .WEATHER REVIEW. SEPmMBEB, 1922
SOUTHEAST
S - SUMMER
W- WINTER
A - ANNUAL
Flo. l.-Summer, winter, and annual average turning ol winda with altitude from theeight surfwe direotions, and the pemt8g0 freswmop OIclockWi~ and oountemlockwlse tuning.
SEPTEMBER, 1922. MONTHLY WEATHER REVIEW. 467
discussed above. The following features appear worthy of consideration :
(1) At the 1-kilometer level all winds exce t northwest
viation is clockwise also by a very small mount, but the summer and annual curves tend counterclockwise. (2) At the 1-kilometer level the maximum deviation in the clockwise direction occurs with east and southeast winds. In winter this deviation amounts to more than 45’.
(3) At the 1-kilometer level the clockwise deviation is much less in summer than in winter; with northwest winds, mentioned in (l), the summer deviation is counter- dockwise.
(4) At the 2-kilometer level all winds, exceptin
exce tions the deviations are decidedly counterwise. (5rAt the 2-kilometer level the maximum clockwise deviation occurs with southeast winds, where, in winter, it amounts to as much as 75”.
(6) At the 2-kilometer level the m&uimum counter- clockwise deviation occurs with northeast winds, where, in winter, it amounts to about 30’.
(7) At the 2-kilometer level the clockwise deviation is
more pronounced in winter than in summer. This is also true in the case of the three northerly winds mentioned in (4) with respect to counterclockwise deviation.
(8) The second r e c e p t t h a t concerned with the occur-
the northwest wind. It thus appears that the earlier contention that wind direction is a reliable index to u per temperatures is veri-
quency polygons that the turning is most frequently the same aloft when the winds are southerly, but that the average deviation from surface direction is least when the winds are northerly. The greatest average deviation occurs wilh greatest r e l i d d ~t ~ of turning; the hast deviation occurs with the leapt reliabilzty of turninq. The conclusion is, therefore, that the available statis- tical evidence is decidedly favorable to the use of surface wind direction in the capacity of an index to the thermal conditions aloft.
deviate clockwise. With northwest winds t R e winter de-
northeast, and northwest, deviate clockwise; in t i? e three
rence of the smal P est average deviation--is fulfilled with
fied by these considerations. f t was seen from the fre-
TBE EVALUATION AXD GEOGRAPHICAL DISTRIBUTION
OF A.
The dafa.-The collection of data consisted in going over the individual kite flights and determining the mean temperature of the air column. finding its difference from the surface temperature, and classif ing this difference by wind direction (eiaht point,s) a n i by months. The record sheets of the Rights prcpared in the Aerological Division of the Weather Bureau give the conditions of temperature, pressure, vapor pressure, etc., at frequent altitude stages, and the simultaneous surface conditions. In determining the mean temperature of the air column between the surface and a chosen level, say 1 kilometer above sea level, it was necessary to weight the mean temperatures of several layers intervening according to the de th of each layer. In this way, the moan temper*
several layers whose mean t.e.nipcmt.ure could he very
accurately determined. Inspection of the vertical curves of temperature indicat.es t,hat, when carefully performed, this method of inte ation is sufficiently accurate. .
Dqinition of A.-Esperience has indicated that con- fusion may arise in the use of the rather prolix expression
ture o P the required air colunin was t,he weighted mean of
“difference between the mean temperature of the air column and the surface temperature,” a phrase that has been used a great deal. A s-pbol for this quantity is desirable since it will be necessary to refer to it again and again. Consequently from the following relation,
t +A =8 ,
in which t represents the surface temperature, e the mean temperature of the air colunin, and A the difference between the two, it appears that, A will fulfill this need satisfactorily.’e It has been used throughout the paper. The aerological stations.-The data were drawn from the seven kite strations of the Weather Bureaiz, two of which have since been discontinued. They comprise four sta- tions in an approximately north-south line in the Middle West-Ellendale, N. Dak. ; Drexel, Nebr.; Broken Arrow, Okla.; and Groesbeck, Tex.;-a station at Royal Center, Ind.; one at Leesburg, Ga.; and one a t Mount Weather, Va. The last two stations are no longer in o eration, the former having been replaced about the enfof 1920 by
one more satisfactorily located at Due West, S. C., and the latter discontinued in 1914, after seven years.of active work. The preliminary studies were concerned only with Mount Weather, Ellendale, and Drexel, but the present paper has been based upon all flights occurring about 8 a. m., 75th meridian time, a t all stations up to the beginning of 1921. The data at Due West have not been considered because this station has been established so recently that the means of the difference between the surface tem erature and the mean temperature of the air
affords an interesting opportunity to test the accuracy of the computed pressure maps. The total number of kite observations examined and tabulated is over 5,000 when all stations are considered. It is seen that even when this number is subdivided and
column wou s d not be very reliable. Moreover, Due West
of observations is a
meters above sea level, respectively, were chosen. There are several reasons why these particular levels seemed most desirable, namely, (1) the largest amount of aero- logical data was available for these lower levels, since the number of kite flights that reached altitudes greater than
2 kilometers above sea level falls off rapidly; (2) it is at about these levels that the greatest amount of flying takes place, hence maps of pressure there would be of the great- est benefit to aviators; and (3) above 2 kilometers the
16 A clear dutlnrtion should be made by the reader between the tsnn Af commonly ucad in aerology and the symbol A used In thls Thc former Is a svmbol uaed to denote the ralc of r l n p r of Ytmpmturr rirh a!titt$ Ii la calied the verticil temperature gradient or lapse rate and uslurllv is the amount of temperazure change pnr altitude chauge of 1On meters. Its dm is pasitfw when tpmpraturefallr with Increase olaltltude and nrqdirr in the case of inrrnion. The followmg quotation from the recommendation of the Elubcommlssiou on Interuational Publications at Monseo, in lW, gives the hasla for thls ursge: *‘La SouoCommii4on recommande d’adopter pour la d6flnition du sl e du madient verticnl de L tem~ratuw le system ou ie gradient sera paitif lorsque% tpmperature dimlnue & m?sure que la hauteur ougmente. et que le pdient sera ndgatif dans la can contraire.” C.9i:ikrnf Rruninn de la Cnmmiubion lntrrotronilr pour I’arrwfuhm seirn- lifirar h Monnro du S I m m au fi avril, r9r)r). BtrasbourR, 1910.)
On the otlier hand the s,vmhd 3 does not represent a lapse raft: it represents an accual di,Tcrencr frmntrafhre between two pants I n a vertical in the atmosphere, one at the surface at$ the other In the free air at a level representative of the air column In ques- tion. Helice, when the temperature /alia wlth altlrude, the value of A Is nrgafiiv, and when temperature rim tho s e n is posifiac. Owing to this bUcrence of sign, i t is mper- 8 t h that A1 and A be not confused.
per
458
JAN. I
MONTHLY WEATHER RlWIEJV.
2 KILOMETER
h?R. I .‘ULY 1
F I
I .
I
SEPTEMBER, 1928
:oc1 I
FIQ. %-Specimen ohart6 ol the distribution ol A at 2 kilcrmtlm
SEPTEMBFA, 1983. MONTHLY WEATHER REVIEW. 459
I
!
Altitude above m.s.1.
tendency for isobars to lie prevailingly from west to east becomes a parent. The weaker pressure formations may
found influence upon the surface went er. Below 1 kilometer, the effect of surfaceinduced tur- bulence may, and does, have considerable influence in deflecting winds from the gradient direction and in pre- venting them from attaining gradient speed. Thus, the
1 and 2 kilometfx levels seem acceptable as trial levels for
pressure reductions. The time of ohsrrvation.-In selecting the flights from which the data were obtained, an attempt was made to take only those represent,ative of conditions about
8 a. m., 75th meridian time. The aver e time of kite flights a t various aerological stations is ”6 a out 10 a. m., but flights occurring much later or much earlier than
8 a. m. were not included in this study. The purpose in using only t,hose observations was to get temperature relations t.hat would be applicable to the regular morning observations at Weather Bureau stations. Therefore, while it is undoubte,dl true that the mean time. of the observations containe B herein is not precisely 8 a. m., it must lie within a few minutes of that time, and, for all practical urposes, the change in temperahre owing to
Owing to the small number o f # hts occurring during the evening hours, no attempt has ieen made to prepare similar dnt,a for the 8 p. m. observation. Work must, however, be done along this line, possibly as the next ste in this research. !&dmt.nt of origina.2 data..-The data, after being compiled as indicated above, were modified so as to include the thermal effect of mean monthly vapor pres- sure and smoothed graphically by means of lines showing equal values of A on coordinates of surface wind direc- tion and mont.hs. This was done for each level and for each of the kite stations. The smoothed values of A were then lotted on maps of the eastern United States and lines o P equal value of A were drawn, thus showing the geographical distribut.ion. Figure 2 shows some o€ these maps. From such charts it was possible to inter- olate for intermediate reporting st,ations of the Weather Bureau. The selcction. of station.s.--An attempt was made to select about 30 regular Weather Bureau stut,ions having good anemosco e es osures. It must be confessed that this wa.s dificupt. #he location of wind vanes on high buildin s in large cities where they are affected by eddies caused % y surrounding buildings or architectural features not infrequent1 renders the recorded mind direction at variance with t K e direct-ion at surrounding stations and the general flow indicated by the isobam. Again, certain anemoscope exposures tire not comparable with those at surrounding stations because of topographic irregularities, valleys, slopes, etc., which deflect the vane from the direction the ressure radien t requires. Perhaps there have been inc f uded in t 5 is list some that are not the best from t,his viewpoint. Another qualification, however, is that they shall be rather evenly spaced about the countr within the limits deemed safe for interpolation. The fo 9 lowing table gives
a list of the 33 stntions selected, their altitudes above sea level, and the length of the air column to the two reduc- tion levels:
E
not exten B above that height, yet. the may hare a pro-
this time x ifference would be ne li ible.
Length 01 air cd- umn to-
lkm. 2km. above abow m. s. 1. m. s.1.
TABLE 2.-The 8tnthn8, th& altimde8 und distanced from the two rcduc- tion h e l 8 (m.eter.9).
I
Statlon.
Figure 3 shows the distribution of the interpolated reporting stations which were used aa reduction pointa in the maps that follow.
TESTING THE METHOD.
The nature of the ksk.-Before proceeding with the values of A deduced in the foregoing ages to the actual
scheme to as many tests as possible. It is practically impossible a t the present time to know the absolute synchronous pressure distribution at free-air levels; hence comparisons of pressures com uted b this method with
of approachin such tests suggest themselves, the h t
dependent upon well-known and theoretically sound physical relations. These methods are:
(1) The comparison of computed pressures with those actually measured with kites. The Due West, S. C., station was inaugurated so recently that it was impos- sible to include m the data of this paper observations from that station. Such values of A as may be ap lied
previous chapter and are in no way influenced by actual observations at that point. If, then, such interpolated values of A yield temperature arguments giving com- puted pressure in good agreement with those observed, the method may be regarded as satisfactory.
(2) Having made maps based u on observed free-air
ossible to make comparison maps computed from sur-
face data. If the two maps agree, confidence in both will be increased.
making of upper maps, it is desira Fl le to subject the
those actually existing are di d cult .5 ut three methods
being direct w a ile the second and third are indirect but
to Due West must be interpolated from the maps o !? the
wind velocities and gradient win a relations, it is now
460 MONTHLY WEATHER REVIEW. SEPTEMBER, 1922
(3) On certain da s when widespread pilot-balloon ob-
to prepare computed maps for the 1 and 2 k ometer levels and, u on the basis of the gradient wind relations,
with the trend and s acing of the isobars.
Corn utations for L&e Wat, S. C.--Considering the first
served pressure values as obtained y the meteorograph at Due West was aa follows:
(1) Tbe aerological records of 42 kite flight8 made at about 8 a. m. and reaching the 2-kilometer levcl were selected at random exce t that a general seasonal dis-
tween March 6, 1921, and January l l , 1922.
(2) The surface data at the beginning of the fli hts were the bases for the cwnputations and consistefi of
P b l e servations were ma B e up to 2 kilometers it was
to compare t ?I e observed direction and speed of the wind
plan, t 1 e procedure for comparin computed with ob-
hibut-ion was sought. A ese 42 flzhts were made be-
%
These e.ffects were estimated at 0.2 mb. and 0.3 mb., respectively, for the 1 and 2 kilometer levels; and, being of the roper sign, when applied to the average dXer- ences 8 t ove, decreased them to - 0.3 mb. rimd - 1.1 mb. for the two levels respectively. ressure a t ti per levels during the
change of tcm erature in the low f&els of the titmos-
outstanding difference to -0.2 mb. and -0.9 mb., rcspectivel y. (3) The rwidual negative tendency is small enough to exert but little effect upon the whole map when it is con- sidered t.lint the isobars will be drawn for intervals of
2.5 mb., horizontal difference of pressure. Moreover, such tendencies will probably not. be localized at certain
(2) Change of int.erval nientione s in (1) resu P tina from the diurnal
. This e 8 ect w.w estimated at 0.1 inb. and 0.2 mb.
rh." 01 the txo levels, respectively, and further decreased the
wind direction, rasure, and temperature. These were
1 and 2 kilometer levels. (3) Values of A interpolated from the maps were B -
up r level com uted by the h sometric formula.
-0.5 mb. and for the 2-kilometer level - 1.4 mb. This average diflerence, it will be noted, is negative. indi- cating that the computed pressure is, on the average, too low. "he followvlg effecte may account for such R
tendenc :
(1) dm e of pressure at upper levels between the time of kite IaunLng, or, more precisely, tshe epoch of the coni-
putation, 8 a. m., and the time of the arriva.1 of the kite
at upper levels. "his may be the result of irregular variations of pressure due to the approach or recession of cyclonic areas, or t-lie diurnal variation of pressure, or both,
tabulated, toget E er with the observed pressures, for the
plied to the surface temperature and the pressure a t t i e P
K e average $ ifferencc for t F e 1-kilometer level was
st.utions but will, in general, be operative over consid- erable areas, with the result that the horizontal gradi- ents of pressure will be but little aflected. It EI the gradient rather than the absolute pressure that is of chief concern. Presxure mms fn,m obetwed w i i d velocif ;ee.-From tile
gradien t-wind equations it is possible to compute tlw wlocity of the gradient: latitude. densitj-, and radius of
curvature of wind path are known. Conversely, if t.lw gradient dp,'d/i is unknown, but the speed of the wind is observed by kite or pilot! bqlloons. it is ossible to sol1-e theequationsfor thegraditmi. 1f this is c P onefor a numbtv of stations. the distance lwtw-een isobars cnn be coni-
putetl and tQr distribution of pressure at the u ,per l e d determined. the trend of the isohis hcing inc /. ica tecl by the ohscrretl wind directions. -1 few kite flights reach- ing the required levels enahle one to assign nctunl nducs
to the isohairs. thus vompleting the i m p .
REPTE-, 1922. MONTHLY WEATHER REVIEW. 461
In 1920, as n matter of interest, this was
ures 4, 5, and 6 show: respectirely, the tion a t 8 a. m.. March 37, 1920. at seti
FIG. 4.-Sea-level weather map, March 27, 1920, 8 s. m., 75th meridian time. (Reprinted from MO. WEATHER REV., lSz0, p. 100.)
and 2 kilometers above sea level. These inaps are re-
printed from that article. The wind arrows show the directions from which the tsrend of the isobars was deter-
FIO. 5.--Pm~re at 1 kilometer above sea lwd, in minibus, Marob 27,1920,8 a. m.,
76th mwidian tima (Reprinted from Mo. W u m m ~ h., 1920, p. 700.)
mined. At that time it was impracticable to compute the pressure a t theupper levelsin any other way; with the com-
thus intlt~pmtlently tu.ri\.cd at indicates not only that both
arc substuntially c0rrec.t but also that the A-method of computing pressures is reliable.
FIa. B.-Pressure at 2 kilometen, above sea level, in m i l l i b , March 27, lolo, 8 a. m.,
75th meridian time. (Reprinted from Mo. W ~T E E S REV., 1920, p. 700.)
r5
The reader is therefore invited to compare Fi and 6 with figures 7 and S. It is needless to say t at the agreement is striking.. C'omputed maps aid obscmred uGnds.-In line with the above idea is that of drawing free-air pressure maps by
462 MONTHLY WEATHER REVIEW. SEPTEMBER, 1922
computation done, sild then comparing them with free- air wind observations at as many stations as possible. This is substaintidly the Fame iden as that employed
Fmn. i-S.-Prssum distribution at 1 and 2 kilomctm above 88. lev& March 27, lW, R a. m., 75th meridian tlme, computed from aur&m &ts.
above, except, that no iuap is drawn from the observed wind relociiiies. The Aerologicul Division. from an ins ection of its records. supplied :I list of dates when wi I f espread pilot-
balloon observations were made a t the several stations
of the Weather Bureau, the Signal Corps, and the Navy. Pressure iiiaps, hnsecl entirely upon surface dat.a, were computed. The observed winds were then compared with the nizxps. It must be remembered that. the wind arrows were entered upon the maps @er t.hey were dra.wn, and were in no wag influent>ia.I in determining the dis- trjbut.iion of t.he isobaw. Figures 9 to 10 sliow the re- sult. C!ompn.risoa. maps at 1 and 1 kilometr?rs.-In general, the inttys (Figs. 9 and 10) spcak for themselves, and, when studied with res ect to the various considera- bions presented earlier in t. !I 's section and in connection with the t,nhle of observed wind speeds at the two levels (.Table 3), the agreement, in nearly all cases is strikingly sksfar tory. In fact, only one inap (several additional dates were sbudied which are not presented .pa hically
t>hhat were not in good agreement with the isobars. Table
3 . howercr. shows t.hat, with very few escept,ions, the wind speeds over the entire country were low; in fact, t,he air was generally stagnant. There were only two observdons where the vnnd exceeded 10 meters per second, namely, at the l-kilometer level ut, Broken Arrow, Okla., and rtt t.he %kilometer level at Lee Hall, Va. In t,he vicini@- of t.hese stations, however, t.he winds were li ht, and, since the pilot-balloon speed is redly the result
o F only a momentary observation, it is altoget,her possible that the balloon was under the influence of local con- vection, w1iic.h iniglit giw rise to a momentary lurch or
in t.his abstract), t.hat of August 33, showed winc P a.rrows
--
turbulent gust. Kite flights were made at the same time at Ellendale, Drexel. Broken Arrow, and Roval Center. At all places, the direction given pilot ballo&s and kites is in substan: tial agreement, and the same can be said of the speed, except at. R o p l Center, at the 1-kilometer level, where the pilot-ba.lloon s ead was 4 m. p. s. as opposed to 10
It shouIcl be noted that the horizontal tcm erature
12' C., and t.he 8-gradient from which the free-m pres- sures were comput,ed were only 6.7' C. at t.he l-kilometer level and 8.6' C.. at the %kilometer level, between Duluth a.nd Jacksonville. This is in accord wit.h the point which has often been made, that, under conditions of a week latitudinal teniperature gradient at the surface, the up er winds may be expected to be light and variable; wit{ a steep tem rature gradient, the sea-level ressure for-
tead to lie from west to east at relatively low eleva- t,ions. It is ensily seen, therefore, from these ma. s, that when
agreement of observed winds with computed isobars is
best; when the map is "flat" and the pressure formations vrtpe and irregular, the agreement is not, so dependable. This, however, does not militate against the accuracy of t.he corn uted isobars, for it is a well-known fact that light an R variable winds always accompany such pressure formations when the horizontal temperature radient is
weak. These ma. s have their particular a p7icstion to
]em of aerial navigation is correspon mgly simplified. On such stagnant summer days, the aviator is more concerned with the vertical component of the air than the horizontal. .On August 28, 1921, there was robabl not a single sec- ivers where the
in t.he navigation of either lighter-than-air or heavier-
ni. p. s. registered E y t.he kite.
gradient between Duluth and Key West was on P 9 about
mations c K" ange rapidly with altitud'e an B the isobars
t,he pressure formations are relatively we1 P defined, the
aviation, but, un er conditions of li ht win 3 s, the proh- R
tion east of the Mississi pi an g p t Ohio wind speeds were of su 8 cient magnitude to be a factor
c f
SEPTEMBER, 1WP. MONTHLY WEATHER REVIEW.
JAN. 15. 1921
2 KILOMETERS
FEB. 21. 1921
2 KILOMETERS
463
464 MONTHLY WEATHER REVIEW. SEPTEMBER, 1922
APR. 5. 1921
V ’.-. ..J
1 KILOMETER
FIG. ~o.-Precunue distrlbutim at ses level and at 1 and 2 Luometers abovn aea level on Aprll 5,1921, and May 21,1921, ampared with winds observed at amdoglcal rtatlons.
2 KILOMFTEHS V
SEPTBIYBEB, 1929. MONTHLY WEATHER REVIEW. 466
than-air craft.’8 West of these regions, where the observed winds wem stronger, the pressure formation was more definite and the eement. m direction better.
ressure protruding from the Southwest into t e Missouri
t d e y was more pronounced at the u per levels than at r
Attention is called to the T act that the ton e of low
sea level, and its realit is confirme : strongly by t.he Ellendale, Drexel, and B roken Arrow observations.
TABLE 3.-VeW& of obasrvsd winds at pilot balloon a t n t h (m. p . 8 .).
StrtlOll.
Burllugton, Vt .....
Ithsc8,N.Y .......
l5tohall Field......
Lskehmt, N. J. ...
Wcrshington, D. C..
Lee Ha, Va .......
Camp Bragg, N. C.
Due We&, 8. C.. ..
puricl Islend, 8. c..
Key West, Fls.....
Peauncols, Flails.....
Camp Benning, Os.
Camp =ox, Ky-..
MaCook Field, Ohlc
IIOJral center, Ind..
LansLug, mch ......
l&umal# Wls......
Ellandale, N. DaL
Fat Omaba, Nehr.
Broken Arrow, OW
Growbeck, Tax.....
Fort Si& Oh.. ...
Ellington Field, Ta
Kelly Field, !l’ex.. .
...... ...... ._._.. ...... 11
16
9 12 2
...... ......
...... ...... ...... 3 2
5 4 1
3 4
5
5 4
8 2
...... ......
...... ......
...... ...... ...... ...... 14 14
14
...... ...... ...... I
(1
6
3 3 2
...... ......
--
1 Kite observntlaa a Kite observation at Draxel, Nebr., about 20 miles west of Fort Omeha.
These tests could be continued indefinitely, but it is believed that sufficient evidence has been brought for- ward to justify the conviction that these free-air maps are accurate. Are we not, therefore, justified in carrying on? If the answer is in the afbmative, we are obliged to give the maps the most thorough study ossible in order
Their value in relation to aviation is unquestioned, and, If the exam le of the Japanese meteorologists is .to carry Et with us, we can not escape the belief that any these dai y charts of free-air conditions will be useful in general forecasting.
to ascertain their value for day-to- c f ay forecasting.
TEE MAPS.
The selctbn of dates fbr mup drawi .-It is mani-
mynad mteresting casea in which capricious nature checkmated the forecaster. Instead of selecting dates at random, however, it was believed that greater interest
featly impossible to cover XI a limted num Y er of maps, the
would attach to those in which the unexpected occurred. C. L. Mitchell, forecaster a t the central office of the
comment
first is a co y, so far as isobars and isotherms are con-
been converted. The blac- lines are sea-level ressure, the red, surface temperature, and the black sha ed areas show re ions in whch precipitation occurred in the 24
not the same as that shaded area on the dail weather
the following day. possible prognostic value of the free-air charts for precipi- tation. The two smaller maps show the pressure distri- bution, in black, at the respective levels. The red lines on these smaller maps are not isotherms in the usual sense, but are lines of e ual value of e or lines of equd temperature argument. f n general, this conforms to the distribution of surface temperature and is somewhat representative of the distribution of free-air isotherms, but care should be taken not to inte ret them as iso-
intermediate level.
The are three maps; the
CY K
cerned, of t B e weather ma s except that the units have
houmfo E owing the time ofthe map. This last feature is
map, but corresponds to the shaded area on t l e map of This is for use in connection with the ,
therms corres onding to the level of t ‘% e isobars. They may be consi R ered as approximate temperatures at some
OHABT 1.
December 17, 1919.-The especial features which ren- dered this date remarkable have been discussed by hh. W. R. Gre g.lB The highest velocit ever observed
per second, was observed at an altitude of 7,200 meters on this date at Lansing, Mich. This observation was corroborated by numerous observations of extremely high velocities at the levels reached and a ve increase of speed with altitude at other stat,ions, a thou h no other observations reached so great an altitude. tt was of interest, therefore, to draw the upper isobars for this dat,e in order to see to what extent the computed isobars bore out the testimony of the observations. The result is strikin .
is the com lete obliteration of the s e e eve1 pressure con- figuration % elow 1 kilometer. At 2 kilometers, the iso- bars are parallel over the eastern United States and the gradient steeper than at 1 kilometer. In other words, what appeared to be a “rid e” of high pressure at sea
bend of the lsobars over Arkansas and Missouri a t 1
kilometer. Even the southern development of the cy- clone on the Atlantic coast is overrun at 1 kilometer by northwest winds.
in the free air be 9 ow 10 km. in the United B tates, 83 meters
’9 rapid
Perhaps the h t B eature that will ap ear to the student
level has disappeared except s or a suggestion of it in the
P
Nota on nigh frm-air alnd relcdtiea obmrved Dsc. 16-17.1919. UO. WUmrSB REV., December, 1910, pp. 868-864.
406 MONTISLY WEATIIER REVIEW. SEPTEMBER, 1922
The reason for this sudden chan e within (t small limit
gradient of surface temperature which is amen tuated and somewhat smoothed in the 1-kilometer distribution of 8. It is clear that the temperature distribution acd not the surface pressure was the dominating influence on this day. The wind, in attempting to flow from above the warm region to the cold re ion was deflected by t,he
parallel to the isotherms.
The forecaster doubtless would have forecast westerly winds at a quite low elevation, but it is a question whether he would have anticipated that o posin
would have a force a proxima.t.el e ual to half the air speed of a commerciafairplane. 6r &at a high- owered
had its speed reduced to less than that of sn express train. Or that the same craft flying from Chica o to
nation several hours sooner than usual. These are facts of the greatest importance in the dispatching of mail and will be of greater importance as the activities of com- mercial aviation multiply. Bee-air maps disclose the facts without surmise or guess.
of altitude is not hard to find. 1 t lies in the marked
earth’s rotation wit.h the rem ? t that it flowed nearly
at ordinary flying levels between New Yor E and Finds hicago
commercial dirigible flying the same route wou s d have
New York would have been able to arrive at the 5 esti-
CHARTS II-ITI. ,
Murch I4-lefi, 1919.-Preceding the first day of this period, a low pressure area had traveled d o a curved
level map (d m t 11). A well-marked high-pressure area is found over Ontario, and between the high‘and low centers lie evenly spaced isobars re resenting a difference of pressure of about 54 mb. The Zirection of movement of the low center is of chief interest in this series. Judg- in from the forecasts of colder in the west portions of
apparently anticipated that the c clone would more more to the east than it actuall &d. In fact,, as t.he
exceeded that of the east com oiient and the center
&e map for the following day does not extend far enough north for one to ascertain the lockition of t,he lowest pressure, but it is. not unlikely that it lies in a wenera1 line with the direct,ion of the previous dit
!h i s motion was from II little south of southwest.. T& see-level isobars the h t day had a trend from south- east to northwest over the eastern United States, the second day more from the sout.h, the third day from the southwest. On the h t day, instead of hgh pressure so far to the north as pictured at sea level, it a e m more as a general &h pressure region east of the R i t e d States producing wmds at 1 kilometer below latitude 40’ from the south- southwest and north of that 1at.itude from the southeast. At 2 kilometers, the isobars are from the seuthwest. With general high pressure to the east, and tt well-defined isobaric trend from south-southwest to north-northeast at 2 kilometers, it is quite apparent that the movement of the c clonic center is definite1 related. The 15th showrs t&s southwest current. st& better established and, ‘udging from the closer isobars, of higher velocit .
period is parallel to these isobars. The developin of high pressure in the free air extends paralle to the
path from the middle of the Ore on coast to T t le point in
the nort,hern art of Colorado, w % ere it is seen on the sea-
Ne 5 rash and South Dakota for the following day, it, was
map for the 15th shows, the nort K component of motion
a peared in north central Nort f l Drtli0t.a (Cha.rt 111).
What do we find aloft ?
The d mction of movement of this cent,er throughout’ t. I7 e
Ywau’l
Atla.nt.ic coast-& fact not discernible on the sea-level nort,hward bulging of the map. The 8-isothmms s ow8 the effect of the importation of warm, southerly air. It is apparent here, a~ it will be in later maps, that sea- level anticyclones, a.t least in winter, are not reproduced in the free air with the positiveness charmcteristic of cyclones. The Colorado cyclone, for instance, is appsr- ent. at each level, but the ea.st.ern high pressure does not conform to that a t sea level, and the small high-pressure region in Nebraska and South Dakotn on t,he 16th does not oven appear on the 1-kilometer map, being overrun by westerly or northwesterly winds.
r*,,;V,
CHARTS IV-1‘.
October 16-17, 181/t.-Iii the original publicntion, this series of ma s extends froin October 11 to 18, 1914, but
here. Preceding the first of these two maps, on the 13th, there ap eared on the sea-level map what might be
country from the Gulf of Mexico. On t.hat ate this does not appear in the free-air ma s. The oidy response to
when the low center at 3 kilometers is found over southern Alabama, while another center of equal intensity is over Lake Superior. The following day there has occurred wbat R pears as (t merger of the two with a very much
This disturbance is nearly circular, its importational power is most perfectly developed, and low temperature is found intruding from the west uiitil the temperature of the south half of the formation is lower than that of the north half. This iy significant with respect to rainfall. Here is cold dr-y air imported from the ukst and north- west; the warmer, nioisture-laden air from the Gulf and South Atlantic coasts is drawn wit.h inwheel symmetry into the north half of the storm. Tks effect, apparent at both lerels in the free air, but complicated at sea level by the tri-centered low area in the east, seems to ex lain
storm and but little sout,h of the free-air center. The shifting of the center to the west in the free air is also the result of the marked development of the tempera- ture contrast between front and rear of the cyclone. Having actually curled about itself streams of Sir of considerable teinperature difference, it is found that on the 17th there is an isolated niass of cold air cast adrift in the southeast quadrant (1 kilometer) and a symmetri- cally located mass of warm air detached north of the pre- vious storm center (2 kilometers). This latter warm air was certainly influential in splitting the low pressure in two a t the 2-kilometer level. In general, this might l e d to the suggestion that the anoinrl.lous temperature distri- bution is the cause of the rapid disintegration of what seemed on the 16th to be a well-developed circular cyclone in t-he free air. In other words, it is conceivable that the storm literally destroyed itself, at least in the lower layers, through ita own vigor in mixing air currenb of markedly differing temperature and humidity.N On the succeeding dates, the baroiuetric si t.uation changed rapidly, and a new rt5giie was begun which would intro- duce a whole new series of charts.
only two o 4 these have been selected for reproduction
describe K as a tongue of low pressure overs reading the
low-pressure activity in the Gu E region comes on the 15th,
intensi R ed center in west-central Illinois.
B
t.he occurrence of precipitation in the north half o F the
the source and the dl&duUod al the cyclone foll&e.
L
I
..
2-Kilometer level
P
2-Kilometer level
P
2-Kilometer level
P
2-Kilometer level
P
I
I
September, 3.922. M.W.B G-131. a. I.. x. Chart VIII. December 9, 1919.
A
SEPTEMBER, 1923. MONTHLY WEATHER REVIEW. 467
Directton..
Direction.. Velocity.. .
velocity.. . Direction..,
.................. ... Ithaca, N.Y.. Velocity
................. IAIISiIIp, Mi&.
................. Direction.. West Point, Ky
Attention is again invited to the importance of these maps in aviation. Here we have had a wide expanse of country largely cloud covered. Had aerological stations been functionin in 1914, they could have afforded little
circulation aloft was well defined, but it was not so
apparent at the surface; the gradients aloft were steep and it is likely that high winds prevailed. T8:% these maps have aided the aviator?
assistance in t E e matter of free-air movements. The
CHART8 VI-VIII.
December 6,8, and 8,1819.4n the 5th and thc follow- ing days the low-pressure center in the free air was north of the sea-level center; in fact, when the sea-level cyclone was centered in eastern Oklahoma the up er
the result of the low temperatures prevailing in the north. With the establishment of a strong s0ut.h- westerly drift, the temperature contrast was accentu- ated so that the isobars followed the trend of the isotherms
center was in Minnesota. Such shifting was undoubt-e : lg
W. .............................. 2 .............................. NNE. .............................. 1 SSE. .............................. 13
N. SW. NNW. ..........
i I ..............................
..............................
ver closely. !&e low center in the Lake re ion, being circular and
in the Mississippi Valley, while the front was being fed with a steady stream from the southwest. The meetin place of these two drifts of differing temperature occurre dong a band from northeast Texas to Indiana, where the surface tem erature radient gave a simultaneous tem-
south ends of Illinois on the 9th. The map for the 9th gives a splendid example of the shift of the low-pressure center toward the west and north- west when there is strong temperature contrast between front and rear. I n this case the sea-level center is in
slow moving, develo ed considera 5 le intensity and intro- duced extremely col B air, farther and farther to the south
%
perature di B eronce o 9 20' C., between the north and
tions to substantiate the rather startling shiftiw of the low-pressure center revealed in this series, Tafie 4 is offered. Pilot-balloon observations were limited in alti- tude practically univenally to 1 kilometer or less, but a t several of the kite stations the altitudes attained were somewhat greater. Summy.-This cursory study has indicated that the fundamental considerations regarding the interaction of surface tem erature and sea-lepel (surface) pressure in
henomena, influenced, as they are, by a multiplicity of factors, are often difEcult to study, and these maps are not exce tional. Nevertheless, it is believed that the
clearly discernible rn the free air than at the surface, because the am not subjected to the modifying influ-
The evidence of the dissipation of an area of low pres- sure in the lowest lev& as a result of its own activity in
influencing P ree-air pressure are justified. Meteorological
results o P the interplay of these factors are much more
ences of su r9 ace friction and turbulence.
................... Madison, wis
im orting air of stro ly contrasting tem erature, seems
through strong temperature contrasts, the conduct of free-air secondaries and their influence on surface condi- tions, the relation of these effects to the distribution of precipitat.ion, are all subjects rich in importance to general forecasting. The importance to aviation interests has already been emphasized. The perusal of these charbs shows clear1 that a correct aumse of what the winds are doing alo 9 t is often impossible from the sea-level chart. This knowledge is, however, absolute1 essential to the safe and efficient conduct of aerial tr&c.
hig Il y suggestive. %e displacing of P ree-air centers
Velocity ...I 3 12 10 I ..........
TABLE 4.-fiee-nir uinds nl aerobgicat stations December 8-9.1908.
[Speed in meters pm second.]
I. OBSERVATIONS WITH KITES.
I
Dec. 8. I Dec.9.
Station.
I-. a h . ilan. a h I - - - -- -
.................... ....................
Ellendale, N. Dak 7:;
BE. 12.7
Royal Cater, Ind
Drexd, Nebr..
Broken Arrow, Okla..
Grmbeelr, Tex.
lFi wNF9 wsw' 7.4 ..............
..................
SSk! I NNW. 13.4 IWNW. m a
... 7.2 .......... i 17.3 ..........
... 12.5 16.7 16.2 17.6 ................ .......... I (NNW. ..........
11. OBSERVATIONS WITH PILOT BALLOONS.
1 1 p. m. obwrvtion. 2 1.815 meters. J 1,Ma meters. 4 66B meters.
CONCLUBION.
This study aims to carry the process of free-air reduc- tion to the point where it can be performed at meteoro- logical stations with a facility equal to that with which sea-level reductions are performed at present. The process is fully explained, the accuracy of the result- ing maps is supported by much evidence, and a few
s ecimen maps are drawn. The red test of the value of
stration must rest with the orecaster, experienced, as
he is, with daily weather controls, and sym athetic, as
him pictures hitherto ap arent only as fragmentary im-
more ade uate conception of the processes at work in
ciation of three-dimensional weather.
The method is both practicable and practical. 'JXere is no unsurmountable obstacle to the accomplishment of its objects and it can be turned at once into useful channels. It will assist in the most economical manner with the visualization of the third dimension of the weather. The labor of reduction can be performed with no more experience than is required for reduction to sea level, the basic material for the reduction embraces no observation not already required for the daily re- ports, and no more of the station observer's time will be required than is devoted at present to sea-level reduction.
9
t R ese maps can not be made b the aut.hor; that demon-
he must be, with the new charts, which wil .p reveal to
pressions. It is hoped t Y l at such a trial will lead to a
the lower 7 evels of the atmosphere and a deeper appre-
468 MONTHLY WEATHER REVIEW. SEPTEMBER, 1928
The map would ‘ve additional knowledge of the free-
obvious that many cases occur when the free-air condi- tions can not be accurately judged from sea-level data. These often occur a t times of stormy weather when pilot- balloon observations are not available. Such times are likewise trying to the aviator, and it is then that he wants the most reliable advice. It is believed that t,hese maps will eventually have ficance for general forecasting because of the close r sY ation between free-air conditions and certain hases of surface weather, precipitation, cloudiness, a n 8 tem-
air winds. From t f? e examples given in this paper it is erature. If these maps axe found useful in the eastern fJnited States and those tentative plans which have been suggested above for attacking the pleateau are found fruitful, there is the encour ng possibility that,we may
to coast with a weather map of three dimensions. This may not be a universal panacea for all forecasting ills but it will a t least afford a glimpse of the ph sicai processes at work, and lift u8 from the annoying Asap- pointments of em iricism a little nearer to that ultimate goal toward whic % all st.udents of wea.tlier forecasting are striving.
realize an ambition to blan T et our country from coast
J. BJERKNES AND H. SOLBERQ ON THE LIFE CYCLE OF CYCLONES AND THE POLAR FRONT THEORY OF ATMOS- PHERIC CIRCULATION.‘
By ALFRED J. HENRY.
[Weather Buimu, Washington, n. C., Nowmber 3, 1BP.l
These two young Norwegian meteoro1ogist.s have our best thanks for the clear presentation of t,heir views on that perpetual1 interesting question of the origin and
the REVIEW have had some intimation ol the research work in forecasting that is bein conducted at the
the August issue.2 That article will serve tls a prelude to the more formal presentation of t,he subject in the pa er under review. %e elder B’erknes, from a stud of meather charts on
siderations, was led to a theory of the formation of cy-
clones and anticyclones, the erms of which, according to
and Helmholz. The latter in a paper on Atmospheric Motions3 has shown that thore is always a tendency to- ward the formation of B surface of discontinuity between air strata of different density which lie contiguous one above the other, and that at the bounding surfaces of such strata the conditions me ripe for the format.ion of atmospheric waves as soon tis a lighter stratum lies above a denser one. Professor Bjerknes has develo ecl these ideas und applied them to the ex lanation o P the origin and main- tenance of cyclones an$ anticyclones. 11ie air strat-a of different density most fre uently met in nature are the two great currents, one lowing toward the Pole, the other toward the Equator. In n sense thesc are the coun- ter currents of Bigelow and the o posing currents of Dove. How these currents act unft react to forni 2y-
clones and antic clones is erhaps best visualized from
a drawin “Idealize % cyclone.” This diaoram appeared in the August REVIEW, on page 404. f i e y describe the princi- al features of the cyclone as consistmg of two essen t i d y h e r e n t air masses, the one of cold, the other of warm origin. The two air masse3 are separated by a fairlj distinct boundar surface which runs through the cy-
clone and whici the authors believe may continue more or less distinctly through the greater part of the troposphere, being everywhere inclined toward the cold side at a small angle with the horizontal, say 1’ or even 0.l0. .
maintenance o ? cyclones and anticyclones. Readers of
Bergen Geophysical Institute from 5 liss Beck’s paper in
which lines o .( flow were depicte B and from other con-
his own statement, are to be B ound in the writings of Dove
whid the autiois P present under the title
~ ~
I GcoJWake ?wdikdow, Vol. 111, No. 1: Iiristi3nin, 1922. ,
’
Pagea 88EIMo.
8 The mechanics of the earth’s atmosphere. A cdection of translations by C. -4bbe. Smitlimmn CWletMom No. 8.4%
In the Northern Hemisphere the waim air is conveyed l.y a southwesterly or a westerly current on the sout,hern side of the depression.‘ ,4t the front of this current the wwm air ascends the wedge of colder n.ir and gives rise t.0 precipitation (warm front rain).5 The warm current is simultaneously attacked on its
flank by cold J r masses from the rear of the cyclone. Thereby part of the warm air is lifted and precipitation is formed (cold front rain).’
THE LIFE CYCLE OF CTCLOZEB.
The authors say that the more recent. investigations have shown t,hat the type OI cyclone above described represents a certain stage of development. in t,he life of a cyclone. The successive clitmges in form and structure are schematically shown in Figure 1, in which type c or d corresponds to the “ ideal ” cyclone mentioned above. In the earlier stages the smiie cyclone has the structure shown in a. arid 7, and i t will successively pass throu h the forins e, f, 9, and h of Figure 1. As inay be seen % y that figure, the initial stage of forimtitm is pictured in (G
wherein two oppositely directed currents a cold easterly (from the east) adjacent. to and on the same lerel with a warm westerly (from the west) is separat,ed by R nearly straight boundary. At the place where the new cyclone is to be fornied this oricrinitlly straight. boundary bulges out toward the c(J1d si& as in b, aiir~ the center of the cyclone will be found at the top of the projectina tongue of warm air. The tongue of warin air is ident-icay with the warm sector ’cone, and the ascendin air from this warni c?o:nis the “warm front ’’ rain and the “cold front” rain shown in c und d, respectively. This newly formed cyclone follows t,he current of warm air eastward
ltlld is propagat,ed as a wave on the boundary surface between warm and cold air. During t.he eastward motion, the amplitude of the warm ware increases (in a horizontal N-S direction as in
4 For the eastern part of the United States I should sav that this spedAoation should be modifled to read the warm current is conveved by a sbuthwesterly to a southeasterly current on the mithern or Pastern side of the depression-EDITOR.
0 ThLs is of aurae, a very generrliEed statement to wMch there cue many exceptians
50 far as &I itation is concarned. In the casa of a shift of the wind born offshore to on! shore, &en t i e land surface Is ulte cold pred itation miin regardless of the positlon of the cyclone canter, partlcidv along the MPddle Atlantlc wast.-Eolros.
8 In the United State3 cold front pr~clpitatlon IS not strongl marked in wuter except as snow aurries in mountaln districts and on the lee shors~ o r a e Qmat ~s k w . in Hun- mer however the preeipitatron of a cvclone may be Collflned to cdd front &, whwh in d m y c a d is clearly assoclsrrd with the “wind shUt” line in the mX-EmTOR.