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U.S. DEPARTMENT OF COMMERCE ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATIQN INSTITUTES FOR ENVIRONMENTAL RESEARCH

Institutes for Environmental Research Technical Memorandum -NSSL 34

NOTE ON PROBING BALLOON MOTION BY DOPPLER

Roger M. Lhermitte property of NV>;JC Library

NATIONAL SEVERE STORMS LABORATORY TECHNICAL MEMORANDUM NO. 34

NORMAN, OKLAHOMA JULY 1967 TABLE OF CONTENTS

Page

Abstract 1

. 1 Introduction and Method 1

2 The Balloon Sel£-Induced Turbulence 2 3 Balloon Motion Variability and Its Relationship to 4 Balloon Size and Ascent Rate ,. 4 Conclusion 13 Acknowledgments 13 Re£erences 14

iii· LIST OF FIGURES

Page 1 Effect on balloon motion caused by balloon burst 3 2 Time variations of velocity compared to the motion 5 of surrounding ice cryst~ls 3 Time variations of the radial speed of small 5 balloon (D = 12")

4 Helicoidal trajectory parameters~ associated with 6 the sinusoidal variation of balloon speed 5 Change of the 100-gm balloon motion with the 8 vertical lift or Reynolds number

6 Time variations of Doppler speed for 30-gm~ 24 inch 9 diameters with different vertical ascent rate 7 Relationship between A and Rd for 42 flights 10 8 Relationship between A and balloon diameter D 10 9 Relationship between A and w (24 flights) 10 10 Relationship between orbit radius and balloon 11 diameter for 26 flights

11 Relationship of orbit radius~ r~ to balloon 11 diameter~ D~ as a function of Reynolds number Rd 12 Relationship between riD and the balloon relative 12 mass as proposed by MacCready (15 flights)

iv NOTE ON PROBING BALLOON MOTION BY DOPPLER RADARl ,2

RogerM. Lhermitte ~ National Severe Storms Laboratory5

ABSTRACT The motion of' free balloons has been analyzed with techniques. Balloons in the subcritical Reynolds regime exhibit periodic variations of' radial velocity. These vari­ ations are explained in terms of an helicoidal balloon motion with orbit radius approximately equal to the balloon diameter and vertical wavelength 10 to 15 times larger. Large irregular oscillations of balloon motion characterize flights with Reynolds number larger than 2 X 10? , 1. INTRODUCTION AND METHOD Free balloons, which can be tracked by optical methods or by radar, have been considered as acceptable tracers f'or air motion and are conventionally used f'or . wind measurement. Analysis of the small-scale variability of' the wind by use of very accurate automatic tracking have shown, however, that the wind measurements have a typical variability related to an erratic motion of' the balloons. Small-scale irregularities of the balloon motion can be conveniently analyzed with Doppler radar, which detects vari­ ations of' target range smaller than the radar wavelength. The Doppler-observed speed is the radial speed associated with the change of the balloon range as a function of.time. This paper reports investigations of' small-scale motions of balloons over a ran~e of' spherical balloon sizes (12" to 72") and ascent rates (0 to 6.5 m sec-I). Expandable neoprene balloons were used in experiments, but the data analysis was limited to altitudes where expansion of the balloon diameter was negligible.

1 In preliminary rorm, this paper was presented at the Conference of' the American Meteorological Society on Physical Processes in the Lower Atmosphere, March 20-22, ' 1967, Ann Arbor, Michigan. 2 The program of Doppler radar studies at the National Severe storms Laboratory has received substantial support from the Federal Aviation Agency. 3 Present aff'iliation: Institute for Telecoinmunicatlon Sciences and Aeronomy, ESSA, Boulder, Colorado. . A balloon's erratic motion seems to be controlled by a sel~-induced turbulence originated by its vertical speed with respect to the surrounding air [Scoggins, 1964]. Scoggins noticed that i~ the balloon sur~ace in contact with the air is roughenect,the amplitude o~ the erratic displacement o~ the balloon decreases and the oscillation of balloon motion becomes more regular.

The ~light trajectory characteristics ~or expandable neoprene balloons used in our experiments have also been analyzed by MacCready [1965]. In his work the average balloon speed was computed ~or time intervals of 4 seconds ~rom the position data provided by an automatic tracking FPS-16 radar.

To separate the contribution to the Reynolds number~rom balloon si~ or balloon vertical velocity, we in~lated the balloons with a mixture o~ air and helium designed to control the li~ting ~orce with relative independence ~rom the balloon diameter. Such a method allows choice o~ a large range o~ vertical speeds ~or the same balloon size. The Doppler radar method requires that the balloon be made a good isotropic target, i.e., a per~ect sphere with a uni~orm re~lective coating. When the balloon sur~ace re~lectivity is not uni~orm, balloon spinning and tumbling causes a smearing o~ the Doppler s~ectrum. In practice, .the balloon's inner wall was coated with 'cha~~" dipoles by placing a ~ew drops of oil with cha~f in the balloon, and agitating the inflated balloon. This method provides a uniform distribution o~ the chaff dipoles on the balloon wall and a Doppler smearing typically smaller than 0.5 m sec -1. . The radial velocity information is obtained by processing the Doppler signal through a f'requency analyzing ~ilter bank. The spacing between filters corresponds to a velocity interval of O.2"m sec -1. [Lhermitte and Kessler, 1964] . Similar experiments have been done by McVehill . et aI, [1965]. The experiments described in this report,; however, cover a larger range o~ balloon size and vertical ascent rate and are aimed to def'ine a flight parameter which effectively controls the balloon self-induced turbulence. 2. THE BALLOON SELF-INDUCED TURBULENCE Figures la and Ib clearly show that the balloon's variable motion is caused by the balloon's spherical shape and ascent rate.

2 15

I U III 10 II) E

o o 10 20 30 40 50 60 SECONDS Q. Oct. 3, 1966. Burst Altitude: 2,000 me 30 gm Balloon, starting Diameter: D=3211 o - -1 5 8=17.5 , w=4.25m.sec ,Rd=1.7XlO. 20

15

I U CLIen to E

5

20 30 40 50 60 SECONDS b. Oct. 11, 1966. Burst Altitude: 2,020 m. 100... gm Balloon, starting Diameter: D=48 11 0 l 9=31.0 , w=5. 0m.sec- , Rd=3e05XlO~

Figure 1. - E.f.fecton balloon motion caused by balloon burst. Note that the velocity oscillations disappear completely a.fter balloon burst at time 43/45 seconds.

3 In these two separate cases the radial velocity of the balloon is presented in the vicinity of the balloon burst. The characteristic variability of the balloon radlal motion completely disappears after the balloon burst pOint. The difference between the mean motion before burst and the steady motion after burst is the contribution from the target's vertical ascent rate to the Doppler-measured radial component of velocity. The vertical velocity changes from 4 or 5 m sec-l upward before burst to an estimated 2 m sec-l downward after burst. Steadi­ ness of the target motion after burst proves that the balloons moved in horizontal laminar flow and that the erratic component of the motion arose from a self-induced turbulence generated by the vertical motion relative to the air. The small-scale variability of the balloon motion is controlled by a self-induced turbulence; this is also apparent in figure 2, which shows data acquired when the balloon was . traced inside a cloud of ice crystals. Since the motion of the ice crystals was observed at the balloon location, such obser­ vations offer a basis for comparing steadiness of air motion and of balloon motion. The short-term time variations of the motion of ice . crystals are small (less than 0.5 tol m sec-I) and indicative of steady air flow. The balloon Doppler speed presents however, a large variability with peak-to-peak radial speed deviations reaching 5 to 6 m sec-I. The difference of approximately 3 m sec-l between the balloon's mean Doppler speed and the ice crystals' Doppler speed arises from the difference between their vertical motions and can be explained by a downward motion of ice crystals of 0.65 m sec-l and an upward balloon motion of 4.3 m sec-I. 3. BALLOON MOTION VARIABILITY AND ITS RELATIONSHIP TO BALLOON SIZE AND ASCENT RATE Figure 3 shows an example of the results obtained by Doppler tracking of a small balloon. Notice that the balloon radial velocity shows peak-to-peaktime variations of 1.8 m sec-l with a period of 7.8 seconds. Note also the narrow Doppler spectrum, which is indicative of a good isotropic target. This type o£ speed variability is frequently observed for Reynolds number Rct smaller than 1 X 105 • . According to experiments in wind tunnels, this characterizes a subcritical .flow regime around the balloon. The Reynolds number is ..computed from:

(1)

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I U C1I 10 \II E

5

SECONDS

Figure 2. - Time variations of velocity compared to the motion of surrounding ice crystals. The lower trace indicates steady ice crystal motion and the balloon's erratic motion is shown in the trace above.

20

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-I ' U :: 10 E

5

o

li'igure 3. - Time variation 'of the radial spe-ed of small balloon (D=12"). Note the sinusoidal shape indicative of the heli­ coidal traj ectory shol'm ,in Figure 4.

5 where V is the motion with respect to the air, D the balloon diameter" and -,) the kinematic viscosity of the air. Such sinusoidal variations of the balloon radial velocity would occur if the balloon's trajectory were helicoidal as shown in figure 4. ... The trajectory parameters such as the "orbital" radius, r, ~nd the vertical displacement for a cO~lete revolution in the horizontal plane (vertical wavelength AJ can be computed from the Doppler data and the balloon ascent rate in the following manner. Assume a circular motion in the horizontal plane; this implies a peak-to-peak variation of the speed, VH, that is twice the tangential speed. We·1mow the period of revolution from the period of Doppler oscillations, T, and (2a) and the radius" r, is then

TVH r = 1ftT""' • (2b) As noted above, this type of balloon oscillation is typically observed with a Reynolds number about 1 X 105 or less. For larger Reynolds number" the motion becomes more erratiC, but can still be analyzed in terms of average peak-to-peak variations of Doppler velocity and average oscillation period.

Figure 4.- - Helicoidal trajectory parameters, associated with the sj.nusoidal variation of' · balloon speed: VH is the tangential speed, T the period of' revolu­ tion, and ).. the vertical wave­ length. 6 \ Figures 5 and 6 show examples of the character change of the balloon motion with Reynolds number or balloon diameter. Note in figure 5f the drastic increase of the irregular component of speed owing to an increase of the balloon diameter and ascent rate. For most of the flights, the balloon motion exhibits quasi-regular oscillations, which can be explained on the basis of an helicoidal trajectory specified by the two parameters, r and A, discussed above. The Doppler data acquired during 45 flights have been processed according to the above assumptions, with r and A computed for some of them and presented as a function of balloon size, vertical ascent rate, and Reynolds number. The relation­ ship between .A and ~ is shown in figure 7. One sees that A increases wi~h Rd from A = 3 meters when ~ = 104 to A = 30 meters when R

where A is in meters. To separate the contribution to R

r = n3/2 , (4) where rand D are in meters. For Reynolds number larger than 2 X 105, the relationship appears to be different. This may 7 -I U III I II' E

SECONDS 1 - - 1 " 110 (0). Lift: 38gm, w=lm. sec- , G=l OO ( b ) . Lift: 74 gm, w=l. 7m. sec , 0 = vH=1.3m.sec-l, T=6.9sec, A=6. 9m. VH=1.65m.sec-1, T=5.7sec , t=9 . 6m. 4 r=O.71m., r/D=0.77, Rd=4. 5X 10 r=O.74m., r/D=0.8, Rd=7X10

I U III II' E

(c). Lift : 8 5 gm, w=- 2 . 6 m. sec -1 , (;, ==1 5 0 (d). Lift: 167 gm, w=- 3 . Om . sec -1 , tJ =20° VH=1.9m.sec-l, T=5. 2sec, )..== 13. 5m. V =2.4m.sec-1., T=4 . 4sec , )..=1 3 .2m. 5 H r=O.78m., r/D=O.8 5, Ro=1.18X1 0 r=O .84m., r/D=O. 91, Rd =l. ::rrxJ.Q5

15

- I 'u U III : 10 II' E

5

o

(e )' . Lift:26 Ogm,w=4.7m.- sec -1 , 8== 27 ° (f): . Lift: 670 gm, w=6 . 3m. sec -1, 0=35° m 1 VH=3.3 • s ec- , T=3.1se c, A=14. 7m . VH=9m.sec-1, T~se c , p~4m. r=0,.81m., r/D=0. 88, R =2 .15X105 d Rd=4Xl05, (D=48 II ) Figure 5. - Change of the lOO-gmballoon motion with the vertical lift or Reynolds number. Subfigures a, b, c, d, and e, were obtained with the same balloOn diameter of 36 inches. Sub­ figure 5f was obtained with a 48-inch diameter and full lift. 8 October 12, 1966 Time: 1356 to 1535 CST 30-gm balloons inflated with He + Nitrogen D=24" Mean altitude 1500 m.

15 15 - •u •U GI 10 (II 10 oil E E'"

5 5

o SECONDS SECONDS - -1 Lift: 7 gm, w=O.- 8 m.sec -1 Lift: 20 gm, w=1.3m.sec -1 4 . , V =1.2m.sec-l , T=5. 5sec , . ".=4 4m. VH=1.5m.sec , T= .Osee, r.=5.2m. H 4 X10 4 r=0.5m., r/D=0.83 , R (~ =2.4XIO r=0.47m., r/D=0.78, Rd=3.9

15 15

•U • GI ~ 10 oil oil E E

5 . 5

10 20 30 40 50 SECONDS SECONDS 1 Lift: 32 gm, w=1.8m.sec- -1 ' ~ift: 57 gm, w=2.25m.sec- -1 . V =2.8m.see-l , T=2.6sec, ).,=5.8Sm. VH=2.5m.see ,T=3.2see, ).,=~.7m. H . · 4 r=O.58m., r/D=O.93, R =6.8XIO r=o.6m., r/D=l.O, Rd=5.4XIO d

Figure 6. - Time variations. of Doppler speed for 30-gm, 24-inch diameter balloons with dif:rerentvertical ascent rate.

9 5 4 3 .' ' ••• @b 2 • c ... e••• 8 d /-e . iii 10 ffi 8 -d ~ Figure' 7. - Relationship ... 6 ! 5 between A and Rd for -< 4 42 flights. Circled dots refer to similar 3 data obtained by 2 Scoggins (a~, McVehill (b, c and d and Johnson (e). I 4 5 6 10 2 4 6 8 10 2 4 6 8 10 2 3 RD ..5 3

2 .

_10 III ffi 8 t; 6 , ~ 5 -< .. Figure 8. - Relationship 3 between A and balloon diameter D. The 2 equation A=14D has been suggested by MacCready [1965]. 1.0 0.01. 2 .. 6 . 80.1 2 .. 6 8 1.0 2 3 BALLOON DIAMETER (METERS) 5 .. , 3

2 ...... -10 III . . ffi 8 ~ 1&1 . 6 . . ~ 5 ,;( .. 3 • 2 Figure 9. - Relationship between A and if (24 flights). I 0.1 2 .. 6 8 1.0 2 .. 6 810;0 2 3 Vi (M SEC-I) 10 5r---.-~~-r~~r_--~~~~~~----~~ 4 3 NOTATION EXAMPLES': 4 7@ DENOTES RD' 7JC10 li 2 2@ DENOTES RD' iXlO en a: , ~ ~ 1.0 ~ 8 en ~ 6 a:'" 5 , ... 4 Figure 10. - Relationship ~3 between orbit radius and o , I balloon diameter for 26 2 !.'.5@ ,.::@ f'lights. The Reynolds number is indicated f'or I 0.1 '--__-'----'---' ...... '-- __-'---'---'--L--'-'-..L-I..Ii-- __-'---' each f'light. 0.01 2 4680.1 2 4681.0 2 3 BALLOON DIAMETER(METERSI 5~--~~~-r~~r---.-~~-rTT~r_--~_, 4 3

2 .' . 1.0 8 "Ie 6 5 4 Figure 11. - Relat~onship 3 of orbit radius~ r~ to 2 balloon diameter~ D~ as a fUnction of' Reynolds number Rd. 0.1 4 :0 2 2 3 indicate the passage £rom subcritical regime to supercritical regime which occurs f'orRd = 2 X 105 to 3 X 105 in wind tunnel experiments. Figure ll~ however~ shows that the ratio between rand D is very close to unity~ but inereases slightly with the Reynolds number. The ratio riD caQ also be expressed asa f'unction of' the balloon relative'mass~ def'ined by

HereL is the effective lif't f'orce in grams and Ma = where D is the sphere diameter in centimeters and Pa is the density of' the displaced air. 11 Following MacCready [1965], consider the total amplitude Ymax of the regular lateral motion of wavelength A. In the cases of helicoidal motion, Ymax is equal to twice the orbit radius r. MacCready's tests show that

Ymax = 2r = .2A (1 + RM)-l. (6) With A = 14D, also suggested by MacCready, we have

(7a)

In figure 12, riD is plotted as a function or 1 + 2 HM. Our experimental results indicate that riD is reasonably explained by the relationship

(7b)

The relationship (not shown) between riA and RM shows much more scattering. This leads to a conclusion that the erratic horizontal displacement of a rising balloon is effectively controlled by the balloon diameter and relative mass. The vertical wavelength A is not an essential factor.

2.8 2 .6 2.4 2 .2 2 .0 1.8 r -I 1.6 o=2.25(1+2RM) 0 ...... 1.4 1.2 1.0 0 .8 0.6 0.4 0.2 0 1.0 2 .0 3.0 · I+2RM

FigUre 12. - Relationship between riD and the balloon relative mass as proposed by MacCready (15 flights).

12 4. CONCLUSION The data acquired in these experiments show that at subcritical Reynolds number regime, Doppler data on balloon velocities can be interpreted in terms of' an helicoidal balloon motion with orbit radius, r, and vertical wavelength, A. With the same units f'or r and balloon diameter D the ratio riD is approximately unity and decreases slightly with increased relative mass of' the balloon. The smallest vertical wavelength observed in these experiments was 3 meters, with a .3-meter diameter balloon (12 inches). The wavelength, A, increases to approximately l4,meters f'or larger Reynolds nwnbers which are still in the subcritical regime where r .egula:;:· oscillations of' the Doppler speed are observed. For Reynolds number above 2 X 105, the balloon oscillations become more erratic and the peak-to-peak displacement of the balloon f'rom its mean trajectory reaches 1.6 to 1.8 times the · balloon diameter. This corresponds to instantaneous deviations of' the Doppler speed o'f' 5 m sec-l with an average period of 4 to 5 seconds. For large balloon size (D = 2.0 meters) the erratic displacement can reach ±6 meters or possibly ±8 meters. This requires that the wind be computed for average balloon displacements of' 100 meters or more, to achieve reasonable accuracy in the calculation of' wind estimates. The use of' veroy small balloons may ~educe this displacement to 30 meters. These limitations are not much worse than those imposed by the balloon size itself' in the probing of small-scale variability of air motion. This indicates that if wind soundings in the boundary layer are made by use of' small-diameter balloons, average wind can then be estimated in layers with a thickness of' apprOXimately 50 to 100 meters. If more detailed analysiS of' the wind variability is required, other tracers must be used. Tracers with very small inertia, such as chaf'f' dipoles, of'f'er interesting possibilities for analyzing small-scale air motion. Two Doppler radars at separate locations, observing the motions of' targets having low inertia and low fall speed, of'fer great promise f'or analysis of' space and time wind variability in the boundary layer.

ACKNOWLEDGMENTS Mr. Dale Sirmans assisted by Mr. Jesse Jennings acquired and recorded the Doppler data. Mr. J. T. Dooley assisted by Mr. Masud Faruqui reduced a large part of' the data.

13 REFERENCES

Johnson" Warren B. Jr." 1965: "Studies of the Effects of Vari­ ations in Boundary Conditions on the Atmospheric Boundary Layer,," Dept. of Meteor." Uni v. of Wis." Final Report" Contract DA-36-039-AMC-00878(E)" USAERDA, Ft. Huachuca, Arizona~ . Lhermitte, · Roger M. · and Edwin Kessler, 1964: "An Experimental Pulse Doppler Radar for Severe Storms Investigations," Pro­ ceedings of World Conf. on Radio Meteor., Boston, AIDer. Meteor. Soc., pp 304-309. MacCready, Paul B. Jr., 1965: "Comparison of Some Balloon Techniques," J. Applied Meteor., Vol. 4, pp 504-508. McVehill, G. E., Pilie, R. J. and Zigross, G. A., 1965: "Some Measurements of Balloon Motion with Doppler Radar," J. App1,ied Meteor., Vol. 4, No. 1, pp 146-148. Scoggins, James R., 1964: IIAerodynamics of Spherical Balloon Wind Sensors, II J. Geophys. Res., Vol. 69, No.4, pp 591-598. Scoggins, James R., 1965: IISpherica1 Balloon Wind Sensor Behavior.," J. Applied Meteor • ., Vol. 4., No.1, pp 139-145.

14 National Severe Storms Laboratory Technical Memoranda

The NSSL Technical Memoranda, beginning with No. 28, continue the sequence established by the U.S. Weather Bureau National Severe Storms Project, Kansas City, Missouri. Numbers 1-22 were designated NSSP Reports. Numbers 23-27 were NSSL Reports, and 24-27 appeared as subseries of Weather Bureau Technical Notes.

No.1 National Severe Storms Project Objectives and Basic Design. Staff, NSSP. March 1961. No.2 The Development of Aircraft Investigations of Squall lines from 1956 1960. B. B. Goddard. No.3 Instability Lines and Their Environments as Shown by Aircraft Soundings and Quasi-Horizontal Traverses. D. T. Williams. February 1962. No.4 On the Mechanics of the Tornado. J. R. Fulks. February 1962. No.5 A Summary of Field Operations and Data Collection by the National Severe Storms Project in Spring 1961. J. T. Lee. March 1962. No.6 Index to the NSSP Surface Network. T. Fujita. April 1962. No. 7 The Vertical Structure of Three Dry Lines as Revealed by Aircraft Traverses. E. L. McGuire. April 1962. No. 8 Radar· Observations of a Tornado Thunderstorm in Vertical Section. Ralph J. Donaldson, Jr., April 1962. No. 9 Dynamics of Severe Convective Storms. Chester W. Newton. July 1962. No. 10 Some Measured Characterist ics of Severe Storm Turbu lence. Roy Steiner and Richard H. Rhyne. July 1962. No. 11 A Stud.y of the Kinematic Properties of Certain Small-Scale Systems. D. T. Wi II iams . October 1962. No. 12 Analysis of the Severe Weather Factor in Automatic Control of Air Route Traffic. W. Boynton Beckwith. December 1962. No. 13 500-Kc./Sec. Sferics Studies in Severe Storms. Douglas A. Kohl and John E. Miller. April 1963. No. 14 Field Operations of the National Severe Storms Project in Spring 1962. L. D. Sanders. May 1963. · . No. 15 Penetrations of Thunderstorms by an Aircraft Flying at Supersonic Speeds. G. P. Roys. Radar Photographs and Gust Loads in Three Storms of 1961 Rough Rider. Paul W.J. Schumacher. May 1963. No. 16 Analysis of Selected Aircraft Data from NSSP Operations, 1962. T. Fujita. May 1963 . . No. 17 Analysis Methods for Small-Scale Surface Network Data. D. T. Williams. August 1963. No. 18 The Thunderstorm Wake of May 4, 1961. . D. T. Williams. August 1963. No. 19 Measurements by Aircraft of Condensed Water in Great Plains Thunderstorms. George P. Roys and Edw i n Kess ler • Ju Iy 1966. No. 20 Field Operations of the National Severe Storms Project in Spring 1963. J.T. Lee, L. D. Sanders, and D.T. Williams. January 1964. No.21 On the Motion and Predictability of Convective Systems as Related to the Upper Winds in a Case of Small Turning of Wind with Height. James C. Fankhauser. January 1964. No. 22 Movement and Development Patterns of Convective Storms and Forecasting the Probability of Storm Passage at a Given Location. Chester W. Newton and James C. Fankhauser. January 1964. No. 23 Purposes and Programs of the National Severe Storms Laboratory, Norman, Oklahoma. Edwin Kessler. December 1964. No. 24 Papers on , Atmospheric Tu rbu Ie nce, Sferi cs, and Data Process i ng . August 1965. No. 25 A Comparison of Kinematically Computed Precipitation with Observed Convective Rainfall. James C. Fankhauser. September 1965. No. 26 Probing Air Motion by Doppler Analysis of Radar Clear Air Returns. Roger M. Lhermitte. May 1966. No. 27 Statistical Properties of Radar Echo Patterns and the Radar Echo Process. Larry Armijo. May 1966. The Role of the Kutta-Joukowski Force in Cloud Systems with Circulation. J. L. Goldman. May 1966. No. 28 Movement and Predictability of Radar Echoes. James Warren Wilson. November 1966. No. 29 Notes on Thunderstorm Motions, Heights, and Circulations. T. W. Harrold, W. T. Roach, and Kenneth E. Wilk. November 1966. No. 30 Turbulence in Clear Air Near Thunderstorms. Anne Burns, Terence W. Harrold, Jack Burnham, and Clifford S. Spavins. December 1966. No. 31 Study of a Left-Moving Thunderstorm of 23 April 1964. George R. Hammond. April 1967. . No. 32 Thunderstorm Circulations and Turbulence from Aircraft and Radar Data. James C. Fankhauser and J. T. Lee. April 1967. No. 33 On the Continuity of WaterSubstance • Edwin Kessler. April 1967.

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