FEBRUARY1959 MOXTHLY WEATHER REVIEW 69

METEOROLOGICAL CONDITIONS OVER DURING , 1956 JOS~A. COL~N US.Weather Bureau, San Juan,P.R.’ (Manuscript received January 28, 1958; revised November 28, 19581

ABSTRACT The observations of wind, pressure, rainfall, and radar bands recorded orer Puerto Rico during the passage of HurricaneBetsy are presented and discussed. Thetrack of thehurricane across the Atlantic and eastern Caribbean Sea, which shows an apparent sinusoidal oscillation around the mean path, is also included. The radar obserrations are used to obtain the detailed path of the hurricane across Puerto Rico and to study the changes in the structure of the precipitation bands that resulted from the passage over the mountainous section. Photo- graphs are presented to illustrate the rariations in the radar structure. Some inferences are made in regard to the changes in the intensity of the wind and pressure fields. It is shown that snme weakening of the wind field and filling of the central pressure occurred with motion over the land area, and that rapid deepening followed as soon as the humicane mored orer mater on the north side of the island. The rainfall observations are compared with those recorded in the great “Sam Felipe” hurricane of September 13, 1925.

1. INTRODUCTION 2. SYNOPTIC HISTORY On August 12, 1956, hurricane Betsy, the second tropi- Hurricane Betsy mas detected on August 9, 1956, near cal storm of the season, moved across the island of Puerto 1at.itude 14” N, longitude 49” W. withthe aid of ship Rico. It mas the first hurricane to pass direct,ly over the reports [ 11. Reconnaissance aircraft penetrated the island since September 24, 1932-a period of24 years. storm for the first timeon the afternoon of August 10 and Hurricane Betsy was a. fast-moving storm of relatively reported a small but well developed system of hurricane small size and its effects, fortunately, did not compare intensity. The maximumwinds were 100 knots inthe with the larger and more severe hurricanes of the past, southwestern quadrant, 90 knots in the northern, and 75 like the “San Felipe” hurricane of September 13, 1928, knots in the eastern quadrants. The minimum pressure and the “SanCiriaco” of August 8,1809.’ was 979 mb. and the diameter 20 n. mi. The radiusof From a meteorological point of view the occurrence of the circulation \vas quite small wit,h 45-knot winds extend- hurricane Betsy was significant in that it provided an ing only 6011. mi. to the north and50 n. mi. to the south opportunity to obt,ain observational data of a nature and of the vortex. scope never before possible in the eastern Caribbean area. The lzurricane moved rnestvm-d to TT-est-north\.Tiestward The linstallation of storm-detection radar at San Juanwas and passed ol-er the island of , F.W.I., around completed afew days before thearrival of hurricane 1200 GJIT of Sugust 11 (fig. 1.). Considerable damage Betsy and provideda first handview of the changes taking was reported in the islandsof Guadeloupe, Marie Galaate, place in the radar configuration as the hurricane nmred Ilominica, and Les Saintes. The hurricane continued to across the mountain range of Puert,o R,ico. Observations move generally est-north~~estward ol’er the eastern Car- by airborne radar gave valuable information on t.he storm ibbenn Sea and arrivedon the southeastern coast of Puerto during the period previous to it,s arrival at Puerto Rico. Rico around 1200 GJIT (0800 Lsr) ,August 12. Considerable radarand aerological data were also ob- 3. OSCILLATORYMOTION IN THE TRACK tained after the hurricanepassed Ptlerto Rico andmoved along the net of stations in Islands. This During it,s motion over the Atlantic andeastern Carib- bean Seahurricane Betsy IT~S almost continuously report is concerned only with the meteorological aspects tracked by reconnaissance aircraft. Center fixesbased of hurricane Bet,sy over I’uerto Rico and the area to the on airborne radar obser\-at~ionswere. available at 30-minute east. Emphasis is placed 011 t.he changes in the structure intervals during a considerable part of the period from of the hurricane caused by the motion over the mountains of Puerto Rico. themorning of August 11 onward (fig. 1). If allthe fixes are taken at full value, a sinusoidal variation around I Presentaffiliation : National Hurricane Research Proiect.- Miami.~~, Bla. the mean pat,lz is quite evident. Such oscillations in hur- a Accordingto the traditional method of, nam’inghurricanes after the saint’s day onwhich they occurred, hurricane Betsy m’m baptized in ricane tracks have been the subject of discussion among Puerto Rico as “Santa Clara.”

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FIGURE1.-Track of hurricane Betsy, August 1&12, 1956. Aircraft positions indicated by triangles, positions from San Juan radar by circles. Positions for which no time is directly indicated are in 30-minute intervals.

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FIGURE2.-Track of hurricane Betsy across Puerto Rico as determined from San Jnan radar. Contours in 1O0O-ft. intervals.

Unauthenticated | Downloaded 10/07/21 11:12 AM UTC FEBRUARY1959 WEATHERMONTHLY REVIEW 71 tropical meteorologists during the last 10 years [3, 4, 71. where the coastal lowland is very narrow and the height Ordinarily people are inclined to disbelieve them because of the terrain rises rapidly. The highest peaks in the sec- the amplitude of the variations often isof about the same tion along the track are close to 3,000 feet. magnitude as the errors in determining the center fixes. All the centerpositions in figure 2 were evaluated from The two mostimportant sources of error indetermining the radar film obtained at San Juan, and indicate the center positions with airborne radar are the navigational approximate center of the radar eye. Up to 1300 OMT errors in the position of the aircraft and the errors in the eye was well defined (see figs. 3 to 7) and the accu- determining the exact center of the radar bands. In the racy of the fixes good. Between 1300 and 1400 GMT, while case of hurricane Betsy the reconnaissance aircraft were the center was moving over the mountains, the eye struc- flying at all times over or near the neighboring islands ture deteriorated and it was then more difficult to deter- and thereforenavigational errors were probably at a mine the exactcenter of thehurricane. Another cause minimum. The set of positions between 0700 and 1330 of difficulty in the tracking was the inability to operate GMT August 11 were obtained by an airplane operating in the radar continuously. After 1300 GMT, because of se- the vicinity of the islands of and Martinique at vere interferencewith local communications, the radar an altitude close to 10,000 feet. According to the reports equipment was operated only for 3-minute intervalsevery from the aerologistaboard, the storm consistently pre- 15 minutes. With such a gap in the observations it was sented excellent eye targets,with a precipitation eye lnore difficult to maintain continuity of the eye opening. mostly circular, 9 to 15 n. mi. in diameter. The naviga- St 1400 GMT and afterwards the center was located in tion was reported as accuratewithin 3 n. mi. Between the northern lowlands and reformed rapidly. The fixes 1900 GMT August 11 and 0300 GMT August 12 a different after that time are morereliable. In the evaluation of aircraft tracked thecenter mostlyfrom the right or north-t,he track proper consideration was given to theindepend- ward side of the hurricane aroundt,he islands of Antigua, ent fixes obtained by the radar technician at the time of St. Kith, and St. Croix. After 0330 GMT a third aircraft the hurricane. operated from the vicinity of Puerto R,ico and the Virgin The path in figure 2 has been drawn connecting most Islands. Judging from the reports of the reconnaissance of the fixes, butno significance is claimed forall the aircraft and from the appearance of the hurricane when minor variations. Some changes in the direction of mo- it came within range of the San Juan radar(fig. 3), the tion, however, appear to have been significant. For ex- radar eye was well-defined and generally closed so that ample, between 1130 and 1300 GMT the average direction errors in placing the center should also have been small. was around 290-295"; between 1300 and 1400 GMT the Throughout the period, errors in thecenter fixes were re- motion was toward 315-320"; and after 1400 GMT it was ported as from 3 to 5 n. mi. which is well below the esti- around 300-305". It isinteresting to see that these mated amplitude of the oscillation. There was only one changes in direction fit rather well into the patternof the serious discrepancy in the continuity of the center fixes oscillation evident previous to the arrival of the center and it occurred between 0300 and 0330 GMT, August 12, at over the island. the time of a change in the observing aircraft, when the first position obtained by the new observer did not make 5. CHANGES IN THE RADAR STRUCTURE good continuity with the last position of the departing one. However, all factors considered, it appears that the Figures 3 to 14 present a series of photographs of the San Juanradarscope showing the structure of the hurri- accuracy of the reports shown in figure 1 is as good as cane at different positions over the island. The approxi- can ever be expected. mateposition of the storm at the time of each photo- The wavelength of the oscillation of the track (fig. 1) was of the order of 160 n. mi. in the easternsection of the graph can be determined from figure 2. In this series of track and decreased to around 100 n. mi. farther west. pictures the echoes from the southeasternside, between di- rections 120" and 150' be,aring from thecenter of the scope, This indicates a period of about 7 to 12 hours. The am- are partially obscured due to physical interference from plitude was 10 to 15 n. mi., but decreased and was practi- the San Juan AirportControl Tower, located on the roof cally insignificant in the vicinity of Puerto Rico. These a shortdistance from the radar antenna. Because of values forthe period andamplitude are considerably this interference the hurricanewas not completely visible smaller than those reported previously in the literature on the radar screen until just a few minutes before the 13~4~71. time of figure 3. Figure 3, taken at 1207 GMT when the hurricane was 4. PATH OF THE STORM CENTER ACROSS THE ISLAND centered over the southerncoast, shows a circular andwell- Figure 2 shows the path of the center across the island defined inner band. The outer diameter is around 30 n. on a base map that contains height contours at 1,000-ft. mi. and the inner diameter 13 n. mi. so that the inner intervals. The mountain system consists essentiaIIy of band is 8-9 11. mi. wide. There is a sector of weak echoes, an east-west oriented range with highest peaks a little almost an opening, on the west side. The outline of the over 4,000 feet. The storm moved inlandin a section bandsnear the center of the scope is obscured by the

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FIGURE3.-Photograph of radarscope of San Juan radar at 1207 FIGURE5.-Radarscope at 1234 GMT. GMT, August 12,1956. Range markers 20 nautical miles apart.

FIOUKE(i."Radarscope at 1247 031~. FIGURE4.-Radarscope at 1230 GMT. mained practicallyunchanged during the following 15 ground return, but several bands can be identified on the to 20 minutes, as the storm moved gradually inland. By northeastern side of the hurricane spiraling toward the 1230 GMT (fig. 4), withthe eye center located about 4 eye. On the far or southern side of the storm one band 11. mi. north of the coast, the inner boundary of the eye is barely discernible following closely the 60 n. mi. range had taken a sort of rectangular shape. The edges of the marker. At the time of the photograph the storm was main band were also becoming ill-defined. A few minutes centered on the coast with the northern half over land later, at 1234 GMT (fig. 5), the eye showed an almost per- and the southern half over water, but this difference in fectrectangular form. This lasteda few minutes and the underlying surface does not seem to have made much then began to det,eriorate. At 1247 GMT (fig. 6) the cen- difference in the appearance of the radar echoes. tral band showed significant changes ; the inner diameter The outline and configuration of the radar bandre- was smaller, the echoes fromthe southern side were

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FIGURE 7,"Radarscope at 1302 GMT. FIGURE9.-Radarscope at 1331 GMT.

FIGURE10.-Radarscope at 1402 GMT.

At 1302 GMT (fig. 7) the center was still south of the rnain mountainridge. The inner brand showed a spiral structure open to the south-southwest. Figures 8 and 9, taken at 1312 and 1331 Gwr, respectively, show the storm when it was located on top of t,he mountainsat an altitude of over 2,000 feet. The eye opening was then very small, indistinct,, and irregularly shaped. The configuration of the outerb'ands on thenorthern semicircle had not c,llanged appreciably ; most of the changes occurred in the inner core of the system. In the determination of the radar ,track the center of

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FiGURE 11.-Radarscope at 1417 GMT. FIGURE 13.-Rada~scope at 1.532 GMT.

FIGURE12.-Radarscope at 1447 GMT. the opening was taken in each case as the center of the Some echoes from the terrain are visible in the southern eye, but it is possible that that point did notexactly coin- sectioll of the eye. Figure 12, taken at 1445 GMT shows cide with the center of the eye as identified in prel’71011s the storm centered close t,o the northern coastline, with an pictures. eye diameter of approximately 15 n. mi. The inner band The next photograph (fig. lo), which was taken at 1402 appears pract,ic:~Ilycomplete. In figure 13, at 1532 GMT, GMT when the hurricane had just moved down the n~oun- the hurriczne centerwas located over mater about4 n. mi. tains, shows avery small eye opening. After 1100 GNT north of the coast. The innerband shows a spiraling the eye began to widen and regain its original structure structure with an eye diameter of 18-20 11. mi. The last as the hurricanesystem moved away from the mountains. phot,ograph, figure 11, shows the hnrricnne at 1608 GMT At 1417’ GMT (fig. 11) the eye appeared already quitewide. when it was located about 12 n. mi. north of the coast.

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The central band was then almost completely over water. A rather strong shower, with a measurement of 0.30 inch Figure 14 shows a band structure quitedifferent from the of minfall, was observed at the San Juan Airport at that compact ring evident in figure 3. time, a.nd was the first heavy precipitation thereassociated There are a few significant points related to thechanges with the hurricane. After 0600 GMT there was a slight in the radar structure. First, the progress of the hurri- increase in thespeed and gustiness of the wind. The pre- cane system from a water to a land surface, and similarly cipitation became light, but fairlycontinuous for thenext, ,from a land to a water surface, by itself did not seem to -1- hours. have much of an effect on the structure or appearance of Anothersignificant change in the character of the the precipitation b'ands. Practically all the changesre- winds, and also of the pressure and rainfall, was noticed sulted fromthe motion over themountainous section. around 100 GNT, when the center was about 50 a mi. The rapidity with which the inner band reformeda,s soon awa?. There was a sharp increase in the wind speed and as it moved away from the mountainswas striking. in the intensity of the precipihtion. At the same time An interesting point concerning t.he speed of motion of the pressure, curve showed a more rapid downwlard trend. the center across the island was noticed and itmay be con- That ma,rkecl the beginning of the intense inner core of nected to thechanges in the radar structure,. The'average +.lie1lurric"ane. Thereafter the mind speed increased rap- speed of the hurricane center across the island wa,s 18-19 idly and steadilyto it,s maximum whichwas recorded from knots (a distance of 56 n. mi. from the point of entry to 1200 to 1245 GXT. The precipitationstarted increasing the point of depa,rture, disregarding the minor variations a little earlier than the wind speed land continued heavy in the path, in a period of 3 hours). The average speed unt.il short,ly a,fter t.he time of the peak gusts. The vis- in the 12- and 24-hour periods previous to the arrival of ibility cm're, which to a large extent, is a reflection of the the center on the south coast, and also after the departure intensity of the rainfall, made a sharp drop to around from the north coast, was in both cases only 15-16 knots. 1-2 statute miles about 0930 GMT and continued low until There was,apparently, a faster motion whilethe hurricane 1839 GXT. The maximum wind speed of laround 65 knots with wsts to 80 was recorded at 1235 GNT. Afterwards was moving over land. The fastest motion Seems to have h occurred between 1300 and 1400 GMT, bsut that was pre- the winds decreased rapidly ; at 1300 GMT 'the reading was cisely the period duringwhich the eye was more distorted 38 knotswith gwts to 52. By 1400 GMT the wind had and the accuracy in the center positions was less. Since reduced to 26 knots with guststo 43, the precipitation had the geometric center of the central opening, which TWS pmctically ceased, and thevisibility had increased to near changing shape and diameter rapidly, wlas taken in each its nolmal magnitude. At that time, however, the storm case as the eye center it is easy to see how discontinuities center was located only 28 n. mi. away. could occur. In fact, a s'tudy of the radar film suggests The maximum winds in the San Juan area were from that the eye formation t8hatdeveloped when the hurricane thenortheast; the southeasterly flow on the right rear moved intothe northern coastal areadid not follom quadrant was extremely light.The minimum pressure, in exact continuity from the previously existent eye that 1903 mb., was recorded at around 1800 GMT, a little later had deteriorated over the mountains. This could account than tbemaximum winds, but before the radarcenter had for the apparent acceleration. Unfortunately, due to the nrriwd ai the point of minimum distance from the sta- breaks in the radarpicture, the phenomenon could not be tion. The radar center was closest to Stan Juan, at a dis- verified unquestionably. tance of nbmout 22 n. mi., near 1315 GMT; that is, about 40 nlinute,s after the time of the peak gusts and 15 minutes 6. WINDOBSERVATIONS ,zfte.r the minimum pressure. There was evidently a grea.t Detailed wind observations were recorded at three sta- distortion in the hurricane circulation. The non-coinci- tions on the island : San Juan Airport (Weather Bureau dencc of the pressure eye with the circulation eye or the station) ; U.S. Naval Ba,se, San Juan, located about 6 n. radar eye has been reported previously [6] a.s a more or mi. west of theAirport; and Ramey AirForce Base, less normal occurrence. In the present situakion the mo- located on the northwestern corner of the island. atthe tion of the hurricaneover the mountainous area must have Naval Base, in addition to the hourly, airways observa- been the main factor in causing the observed irregular tions, a gust recorder giving a continuous graph of the dist,ribution in the met,eorological pamrnetejrs around the wind speed and direction was available. The winds in the San Juan'area (fig. 15) were from the center. 'northeast with speeds of around 15 knots during the eve- The observations a.t the Naval Rase, San Juan, agreed ningof August 11. Nosignificant change in direction r-ery well with those made at the Airport. The maximum or speed was noticed until around 0530 to 0600 GMT Au- winds, 46 knots with gusts to 67, and the minimum pres- gust 12, whena decrease to a near calmoccurred. At sure, 1001 mb., were both recorded at the same time as at that time the hurricane center was approximately 120 11. the Airport. The lower speeds rat the Naval Base were mi. 'away. The wind recorder at the Naval Base showed proba.bly due to the differences in exposure; the Base is that the period of calm lasted for about 45 minutes. The located on t,he wejstern side of the city and has greater speeds during the interval were never more than 5 knots. obstmc'tions to an easterly flow than t.he Airport Station.

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22 00 02 04 06 08 IO 12 14 16 18 20 22 00 02 04 GMT I8 20 22 00 02 04 06 08 IO 12 16 14 19 20 22 00 200 yz 200 I UJ I I 160gz - 160I 60.. ST. CROIX - 120I20

- 80 ula 40 - - 40 az +W 0- $.-

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GMTI8 20 22 00 02 04 06 08 IO I2 14 16 18 20 22 00 5- AUG. I I. 1956 AUG. 12 FIGURElti.-Meteorological observations at Hamilton Airport, St. Croix, V.I., duringthe passage of hurricane Betsy, August I, 1956 AUG.12 AUG 13 11-32, 1.956. Time scale inGMT. FIGURE15.-l\;leteorological observations at theairport Station, San Juan, P.R., during the passage of hurricane Betsy, ,211gust Ilnring the appro:~chof the hurricane the variationsin 12,1956. Time scale in GMT. wind speed, tdthough not the magnitudes, were about the Figures 3 to 6, which show the rada,r pictures clnring tlke same at the two stations; the wind profile in both cases period of strongest winds and precipitation in the Sa11 showed changes in the rate of increase at distances of Juan area, indicate that the activity there was associated about 50 and 30 11. mi. away from the center. The maxi- wi'th a radar bmdnext to the centralone. nlum wincls at St. Croix, 58 knots with gusts to 75, were The effect of the mountains in Puerto Kico on the iso- lower than at Sun Juan,but the difference was small and tach field around the center is revealed by the difference probably due to differences in exposure. For the purpose in the character of the wind speed variations at SanJu:tn of this discussion, however, this difference in magnitude and at St. Croix, V.I. (fig. 16). Bot,h stations m-ere lo- is notinlportnnt,. The major difference in the observa- cated on the same side of the hurricane and came within t.ions was in the cElaract,er of the wind speeds recorded in approximately the same distance of the center. The sts- t,lle rear or departingside of the hurricane. At St. Croix tion at St. Croix is located near the southern coast; the the windscontinued strong after the center passed the island is small and relatively flat so that distortions of point of minimumdistance. The highest speeds were t,he llurricane circulation due to t,he terrain should have from the southeast' and were evidently associated with the been minor. The pressureand rainfall observations at flow inthe right rear quadrant. At San Juan, on the the two stations showed very close agreement. St. Crois other hand, the observat,ions indicate a complete break- recorded a minimum sea level pressure of 1002 nib. and a down of the isotach field as soon as the hurricane entered total rainfall of 3.20 inches, as compared to 1003 mb. and the mountainousarea. As a result the wind circulation hac1 decreased considerably by the time the center arrived 3.21 inches at San Juan. Both stations recorded around at the point of minimum distance from the station, and 2.00 inches of rain in the 6-hour period of peak activity. the southeasterly flow in the right rear quadrant of the The minimum pressure at St. Croix was recorded at the hurricane was extremely light.Presumably, except for t,ime of minimum distance from the center; themaximum the effect of the mountains, the San Juan area, and simi- winds were recorded a little later. There was, therefore, 1:Lrly every other section on the island, would have been a better coincidence of the pressure and radar centers at underthe effect of strongerwinds for a considerably the time the hurricanepassed abeam St. Croix. longer period.

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I 06 08 IO 12 14 16 18 20 22 00 02 04 After 1500 GNT, when the hurricane center moved over water, there mas a sudden and rapid imrease in the wind speed ; the increase was even more pronounced after 1530 RAMEY A.F.B. GMT. The maximumintensity, 75 knotswith gusts to 100, was recorded short,lyafter 1600 GMT at about the time - of minimum distance from thecenter, and also at thetime - of t.he minimum pressure. The hurricane was then about - 40gE az 14 n. mi.away to the north of the station. There was, -0 L8 evidently, a better organization of the isotach and pres- sure fields around the eye, and a better coincidence of the pressure and circulation centers than when the hurricane passed close toSan Juan. The distribution of rainfall 29.90- (fig. 19) and damages show also heavier wind and rainfall v) 29.80g activity over the northwestern corner than in any other 0 z section of the north-coastal area. The pressure observa- 29.70 = W tions, discussed in greater detail in the following section, a: 29.60 2 similarly indicate rapid deepening after 1500 GMT. Fig- u7 ure 14 shows the radar picture at the time the hurricane 29.50 a was close to Ramey. Tlle peak wind and rainfall activity 29.40 d > there was evidently associated with the southern section W of the inner radarband. 29.30 2 W 29.20 u7 7. PRESSURE OBSERVATIONS

29.10 A series of pressure observations, including some baro- graph traces, were obtained at a few places on the island. Two sets of observations were takenpractically atJ the center of tlle hurricane: One at Guayama near the .^ southern coast, and the other at Arecibo, on the northern coast. There was in some cases uncertainty as to the alti- tude and calibration of tlle instruments. To remedy this difficul'ty all observations were corrected to sea level con- ditions by comparison with the San Juan data. In the cases in which a barogram was available a comparison of the pre,ssure readingsduring the daysprevious to or after t.he passage of the hurricaneprovided a reliable correction factor. In other cases correspondence with the observers srrved to clarify the accuracy oflthe data. All the ob- BUG. 12, 1956 AUG. 13 servations presented in the following discussion were in FIGURE17.-Meteorological observations at Ramey AFB, P.R., dur- that formcorrected to sea level. ing the passage of hurricane Betsy, August 12, 1956. Time scale The number of observations andable was not sufficient in GMT. lo establish the horizontal field of pressure and to study its chnges over land. Instead the central pressure was The observations at Ramey Air Force Base (fig. 17) conlputed at different positions along the track and the make possible certain deductions aboutthe intensific.il, t,'1011 variationswith time were studied. The resultsindicate of the hurricane after it moved from land to water on tlle filling of the ce.ntra1 pressure as a result of the motion north side of the island. The winds at Ramey mere of the over 'theland mass of Puerto Rico followed by rapid order of 20 knots with gusts of 3040 knots up to 1400 deepening as soon as thehurricane moved over water QMT when the hurricane, after crossing the mountains, again. was located about 40 n. mi. east of the station. After 1400 The procedurefollowed to estimate the central pressure GMT the center moved westward approachiug Ramey and \+-asto determine first a representative value of the pres- at 1500 GMT was located only 22 n. mi. away. Tlle winds, sure gradient in thecore of the hurricane and thenuse it however, remained relatively lorn ; t,he readings around to measure the minilnun1 pressure from observations in 1500 GMT were about 25 knok with gusts to 50-55; tlle tEle periphery. Various estimations omf the gradient were maximum reading by t,lla.t time was 30 knots with gusts made when thehurricane was centered nearthe south to 58. Theseobservations indicate a TT-eaker circulation coast of Puerto Rico and still not greatly affected by the than would normally be expected in view of tlle intensity terrain.The barograph tmce recorded near Ithe center of the hurricane beforeit entered the island. at Guayama (fig. 18) was 17ery useful in this respect. A

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TABLE1.-Tabulation of minimumpressures and estimated central pressures associated with hurricane Betsy, August 10-15, 1956

/ I I Date an( MinimumDistance Estimated Station time pressurefrom center central pres- (G"C (mb.) I (n. mi.) 1 sure (mb.) 29.70 I- I- Aug. 10 2 29.60 t USN Aircraft, 15O K., 57' UT._...... __ 1955 979 0 979 Ang. 11

Roseau,Dominica. ~ _.~_.____...... _... 1500 1006 29 981 Point-au-Pitre. Guadelouue. F.W.1.... 1800 991 12 979 Aug. 12 St. Croix, V.1 ___...____._._._..______0830 1002 22 982 23.4023.30 Guayama, P.R.. ~ - - __ .. ._.- ._. __ _. 1230 983 1-2 981 San Juan, P.R.______.....__._.______1300 1003 22 29.20 t 983 & I I/ Ponce, P. R-______.__...... _._...... 1330 IMX) 19 983 hreobo" ...._.._._._.._.__..-... '29.10 Toa Baja, P.R 1350 99a 13 985 989 0-1 989 987 14 09.00 ,VI 1 11 973 C.UTI6 18 20 22 00 02 04. 06 08 10 12 14 16 I8 20 22 00 02 04 06 AUG. 11. 1956 AUG. 12 AUG 13 USAF Aircraft, 22' S.,72' W ...... 972 0 972 FIGURE18.-Pressure profiles during hurricane Betsy,, August 12, 1956, as recorded in Guayama, Toa Baja, Arecibo, and Ramey tion 14 n. mi. to the south of the radar center. This ob- '' AFB, P.R. Pressuresreduced to sea level. See text. servation indicates a central pressure of about 973 mb. ; that is, a deepening of around 15 to 16 mb. to a value even value of 1.0 mb. n. mi.-l was accepted for stations located lower than hadbeen observed previously. About 22 hours within 15 n. mi. of the center. For the estimations from later on August 13 a reconnaissance aircraft measured the readings at St. Croix, San Juan, and Ponce, located 972 mb. at the center near 22" N., 72" W., which tends to 19-22 miles from the center, a gradient value of 0.9 mb. substant.iate the accuracy of the estimation from Ramey. n. mi.-l was used ; while for Roseau, Dominica, a value of Admit.tedly the method outlined above for estimating 0.8 mb. n. mi.-l was applied. A tabulation of t.he pressure the central pressure is rather crude. One serious draw- readings andestimated central pressures appearsin back is the fact that t.he track of the pressure center does table 1. not necessarily coincide with the circulation or the'radar A minimum sea level pressure of 983 mb. was recorded track. The Hydrometeorological Section, U.S. Weather at Guayama around 1215 GMT in a position 1to 2 miles to Bureau [6] fovnd t,hat the pressure center was most fre- the leflt of the radar center, and shortly after the hurri- quently displaced ahead and slightly to the right of the cane hadentered land. This observation was probably circulationcenter. The displacementwas mostly either close to, but not exactly at, the center of -ninimum pres- ahead or behind and not so much to the right or to the sure, and therefore the central pressure at the time was left of the circulation track. This is encouraging because somewhat lower, perhaps 981 mb. or even less. A mini- a displacement of the pressure minimum ahead or behind mum central pressure of 979 mb. was measured by a recon- thetrack, but still close to it, wauldnot seriously in- naissance aircraft when the hurricane was located over validate the above computations. Nevertheless, although the Atlantic on August 10. The minimum pressure ob- quantitatire estimates of the centralpressure and its vari- servations made during the intermediate periodat Roseau, ations have been presented the main conclusions can be Dominica; Point-au-Pitre, Guadeloupe ; and St. Croix, well sustained by qualitative reasonings. V.I. indicate centrad pressures that do not differ signifi- It is quite evident that if the pressure minimum was cantly from Ithe above. Evidently no appreciab'le filling displaced significantly to the right or to the left of the or deepening of the pressure field occurred from the time radar track theconclusion regarding the fillingof the cen- the hurricane was first reconnoitered on Angust 10 until tral pressure over land would still be valid on the basis of it arrived on the coast of Puerto R,ico. t,he observations at Guayama and Arecibo. Furthermore, A minimum pressure of 989 mb. was read at Arecibo if it is assumed that the pressureminimum was displaced to t,he right of the radar track, then the estimate of the in a position practically at the center, around 1500 GMT, at the time that the hurricane was leaving land. Again central pressure at Toa Baja should be higher than indi- the minimum centralpressure at the timemight have cated in table 1, thus corroborating the amount of filling been slightly lower. This observation indicates a filling over land. At the same time a displacement of the pres- of about 6 to 8 mb,. from the observation made on the sure minimum t,o the right of the track would not alter the magnitude of the deepening between the observations southern coast. Otherpressure observations were made at arecibo and Ramey. The only possibility that might at San Juan, Ponce, and Toa Baja (fig. 18) when the affect the results is that the pressure minimum was dis- hurricane was moving over the mountains; all three of placed to the leftof the track between 1500 and 1600 GMT. them are quite reliable. The estimations of central pres- In that case the conclusion concerning the amountof deep- surefrom these stations are subject to greater inac- ening between Arecibo and Ramey would be altered. curacies, but in general they corroborate the readings at Such a possibility, however, is not very likely; and it is Guayama and Arecibo. not supported by the observations in the rest of the is- About 1 hour after the observation at Arecibo, Ramey land. A detailed scrutiny of the available data suggests AFB recorded a minimum pressure of 987 mb. in a posi- that at thetime the hurricanewas moving over the moun-

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tains between 1200 and 1400 GMT the pressure minimum ably another bandof heavy rainfall over the oceanic area was probably displaced ahead and somewhat to the right on the rightside of the track. of the radar track. A comparative study was made of the rainfall totals There are, of course, other sources of error in thepres- associated with hurricane Betsy and those of the great sure evaluations, aside from the relationship b'etmeen the SanFelipe hurricane of September 13, 1928 [2]. The pressure andradar tracks. Allfactors considered, the path of the. SanFelipe hurricane aci-oss Puerto Rico pressure estimates listed in table 1 are quite reasonable was fairly close to that of Betsy ; it entered land in about and not unexpected. They exactly verify what one would the same location as Betsy, passed about 25 n. mi. to the normally anticipate insuch a situation. One thing that is southwest of San Juan, and left the island through the striking is the rapidity and magnitude of 'the deepening northwesterncorner. Onthat occasion SanJuan re- that took place when thehurricane moved again over corded a minimum pressure of 973 mb. and a maximum water on the north side of the island. This result attests wind of 160 m.p.h., which still rates as one of the highest to the tremendous influence of water surface on the or- hurricane winds ever measured. There were 11 consecu- ganization and maintenance of the hurricane circulation. tive hours of winds greater than 60 m.p.h. and about 6 hours of winds over 100 m.p.h. During hurricane Betsy 8. RAINFALL OBSERVATIONS San Juan recorded only 1 hour of winds greater than 60 Figure 19 shows the distributionof rainfall over Puerto m.p.h. On the basis of the observations at San Juan the Rico during hurricane Betsy. The amount,srange from radius of the intense inner core of theSan Felipe values of about half an inch in t,he southern coastal areas hurricane was estimated as about 180 n. mi., as compared to over 8 inches in the northeastern and cent,ral moun- to only 50 n. mi. for hurricane Betsy. Hurricane Betsy tainous sections. The rainfall was greater on the right was, therefore, relatively small. side of t.he track south of the mountain range and on the The rainy period at San Juan during hurricane Betsy left side of the track north of the range. Evidently this lasted 13 hours and 3.19 inches of rain were measured. distribution was mostly the result of orographic effects. During hurricane San Felipe 9.37 inches were measured The southern coastal areas to the left of the track were in n period of 33 hours.Dividing the total rainfall shielded by the rnounhins and thus the rainfall andwind by the lengthof the period gives a value representingaver- effectswere reduced. On the other hand the sections ex- age rainfall per hurricane-hour. This came out as 0.28 posed to the a,ction of the wind mere affected more. The inch perhour forhurricane San Felipe and 0.25 for zoneof heavier rainfallextending southeast-northwest hurricanc Betsy. Similar computations were tried for 36 over the northwestern section of the island was probably stations over the island that had rainfall records in both associated with the regeneration of the hurricane circuln- hurricanes. It was assumed that the length of the rainy tion after the center moved over water. There was prob- period at eachstation during San Felipe was greater

Unauthenticated | Downloaded 10/07/21 11:12 AM UTC 80 NOIYTHLYREVIEW WEATHER FEBRUARY1959 than during Betsy in the same relation, 33 to 13, that was In part,icular the contributions of Mr. J. M. de Andino, observed at San Juan. In view of the similarity in the from Guayama, P.R., Mr. Luis G. Veray, from Central tracksthis assumption is reasonable. The results were Constancia in Toa Baja, P.R., and Mr. Andres Gelabert, remarkably equal, station for station ; the average for all from Arecibo, P.R. proved to be extremely useful. The 36 stations was 0.28 inchper hurricane-hour in San authorities at Ramey AFB, and U.S. Naval Base, San Felipeand 0.26 in Betsy. In other words the average Juan, kindly made theirmeteorological observations avail- rainfall per hourwas about thesame in the two hurricanes able for this study; their valuablecooperation is sincerely in spite of the tremendous difference in their size and in- acknowledge,d. tensity. Apparently the inhensity of the hurricane is not) as importantin determining t.he total rainfall ata sta t‘Ion REFERENCES as the size and speed of motion, which in turn determines Gordon E. Dunn, Walter R. Davis, and Paul L. Moore, “Hurri- the length of t’heperiod it affects the station. This result, cane Season of 1956,’’,llontJ~I?/ 1YeatJtcr Reliew, vol. 84, No. 12, if it proves to be true in general, might, be useful in pre- DeC. 1956, pp. 436-443. dictingtotal rainfall and ensuing flood conditions in a 2. 0. L.Fassig, “San Felipe-The Hurricane of September 13, similar situation. 1928 atSan Juan, P.R.,” Jfontltly Weather Reuiew, vol. 56, So. 9, Sept., 1928, pp. 35C352. ACKNOWLEDGMENTS 3. John D. Horn, “On Irregular Movements of Tropical Cyclones in the Pacific,” Bulletin of the -4naericnn Jfeteorological Society, The writer wishes to express his appreciation to several ~ol.32, No. 9, Nov. 1931, pp. 34&346. people, who, directly or indirectly, contributed to the com- 4. WilliamllIalkin and George C. Holzworth,“Hurricane Edna, pletion of this report. Mr. Ralph L. Higgs assisted in 19.54,” Jlonthly Weather Review, vol. 82, No. 9, Sept. 1954, pp. 267-2iD. securing some of the original data, and gave valuable en- r,. Vaughn P. Rockney, “Hurricane Detection by Radar and Other couragement and advice. Messrs. David Smedley and Means,”Proceedings Tropical C2/clone Symposium, Brisbane, Americo Maldonado also contributed in assembling data. Attst1.irc7ia. Dec. 10-14, 1956, Melbourne, 1956, pp. 179-197. Messrs. Harry M. Hoose and D. C. McDowell readthe 6. U.S. Weather Bureau, “Characteristics of United States Hurri- manuscript and gave valuable suggestions. The interest, canes Pertinent to Levee Design for Lake Okeechobee, Florida,” Hy~7ro?11eteorological ReportNo. 32, March 1954, 106 pp. 7, and spirit of cooperation of amateur \Teather observers T. -C. Yeh, “The Motion of Tropical Storms under the Influence over the island who recorded observations and made them of a Superimposed Southerly Current,” Journal of Meteorology, available to the Weather Bureau is deeply appreciated. TO^. i,Xo. 2, April 19.50, pp. 10&113.

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