ELECTRIC ACTIVITY ASSOCIATED WITH THE BLACKWELL-UDALL TORNADO

B. Vonnegut University of Chicago

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C. B. Moore University of Chicago

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284JOURNAL OF METEOROLOGYCORRESPONDENCEVOLUME 14Electricd activity associated with the Blackwell-Udall tornado By B. VONNEGUT and C. B. MOOREArthur D. Little, Inc., 30 Memorial Drive, Cambridge 42, Mass. 18 July 1956 and 16 August 1956In his recent article' describing electrical fieldmeasurements made near the Blackwell-Udal1 tornado,Dr. Gunn concludes that "there is little evidencesuggesting that the electrical effects near the funneldiffer basically from ordinary thunderstorm electrification," and that he "does not see much evidencepointing to the conclusion that the tornado is primarilyan electrical phenomenon." As advocates of theclassical idea that tornadoes are a manifestation ofthunderstorm electricity 2, 3v 4. 6, we wish to callattention to other electrical pbservations that weremade on the same storm, to comment on Dr. Gunn'sconclusions, and to suggest alternative explanationsof his interesting data.First of all, we should emphasize that while webelieve that the electrical effects in tornado stormsare a great deal more severe than in ordinary thunderstorms, we do not believe that they are basicallydifferent. We believe that the electrical charging ratesin tornado storms are many times greater than inordinary ones, and that there is sufficient electricalenergy to power a tornado. Because of their extraordinarily intense electrification, we believe thatsuch storms can produce electrical discharges of anintensity and character unknown in common storms.One of us has suggested that thunderstorm electrification is produced by the convective movements oflarge volumes of air, and that the electrical energyproduced in this way can cause the locally intensewinds of the tornado.'j We, therefore, share Dr. Gunn'sopinion that "the electrical behavior exhibited infig. 2 is probably what would be expected in ordinarythunderstorms if the turbulent velocity were increasedby an order of magnitude."Although Dr. Gunn's data indicate that the1 R. Gum, "Electric field inAensity at the ground under activethunderstorms and tornadoes, J. Meteor., 13, 269-273, 1956.2 C. T. Lucretius, De resum natura. (circa 60 B.C., metricaltranslation by W. E. Leonard, vol. 6), New York, E. P. Dutton.Inc., 260 pp., 1950.3 F. Bacon, "Nafyral history of winds - Extraordinary windsand sudden blasts, The works of Francis Bacon. (Vol. 3), Philadelphia, Carey and Hart, 449 pp., 1844.R. Ha'f.e, "On the causes of the tornado or waterspout,"Amer. J. Sci. Arts, 32, 153-161, 1837,6 J. C. A. Peltier, -, (translation by R. Hare), Amer. J.Sci. Arts, 38, 73-86, 1840.B. Vonnegut, "Possible mechanism for the formation ofthunderstorm electricity," Geophys. Res. Pafi., No. 42, 169-181,1955.Blackwell-Udal1 storm had vigorous electrical activity,it is worth citing additional corroborative data.During the course of this storm, Jones' made sfericsmeasurements at Stillwater, Oklahoma, of the azimuthand repetition rate of the lightning strokes. He `reports that, on the bearing of the tornado duringits progress, lightning strokes were occurring at arate ranging from 2 to 25 per second. Dr. Gunns hascited, as examples of very active storms, one thatgave a stroke every 5 sec and another that gave astroke every 2 sec. If the frequency of lightningstrokes as determined by Jones is an index of electricalactivity, the Blackwell storm was from four to fortytimes as active as those that have beenipreviouslydescribed by Dr. Gunn.Eyewitnesses to the tornado in Blackwell alsohave reported evidence of intense electrical activity.Montgomery, who viewed the tornado from a distance of about 3000 ft, reports: "As the storm wasdirectly east of me, the fire up near the top of thefunnel looked like a: child's Fourth of July pin heel."^"There were rapidly rotating clouds passing in frontof the top of the funnel. These clouds were illuminatedonly by the luminous band of light. The light wouldgrow dim when these clouds were in front, and thenit would grow bright again as I could see betweenthe clouds. As near as I can explain, I would say thatthe light was the same color as an electric arc welderbut very much brighter. The light was so intense thatI had to look away when there were no clouds infront of it. The light and the clouds seemed to beturning to the right like a beacon in a lighthouse."Montgomery also tells of other eyewitnesses whohad different views of the phenomenon, such as Mrs.Carl Sjoberg whose house in the direct path wascompletely demolished : "She saw lightning comingup from the ground two or three feet high and about,half as wide as adding machine tape, It was a deepblue and forked on the end like a `Y' or like a snake'stongue."1°According to Montgomery", Lee Hunter, who was4 mi north of Blackwell, described the tornado asfollows: "The funnel from the cloud to the groundwas lit up. It was a steady, deep blue light - very7 H. L. Jones, Research on tornado identijication. (3rd quait.Prog. Rep., Contract No. DA 36-039 SC 64436), Stillwater, Okla.A. and M. College, 8-35, 1955.8 R. Gunn, "Electric-field regeneration in thunderstorms," J.Meteor., 2, 136-137, 1954;`0 F. C. Montgomery, Tornadoes at Blackwell, Oklahoma,"Mon. Wea. Rev., 83, 109,1955.10 F. C. Montgomery, "Some obseyptions on the tornado atBlackwell, Oklahoma, 25 May 1955, Weathenuise, 9, 97 and101, 1956.11 Personal communication.JUNE 1957 CORRESPONDENCE 285bright. It had an orange color fire in the center fromthe cloud to the ground. As it came along my field,it took a swath about 100 yards wide. As it swungfrom left to right, it looked like a giant neon tube inthe air, or a flagman at a railroad crossing. As itswung along the ground level, the orange fire orelectricity would gush out from the bottom of thefunnel, and the updraft would take it up in the aircausing a terrific light - and it was gone! As itswung to the other side, the orange fire would flareup and do the same." l3 of the luminousdischarges in tornadoes, it appears that large electricfields and current densities exist in the.interior of thefunnel and on the ground directly beneath it. Theintense electrical phenomena on the ground areapparently very localized and do not extend for anydistance from the funnel.From the map showing the position of Dr. Gum'sclosest stations, Nos. 1 and 2, and from the U. S.Weather Bureau map of the tornado path as determined by Staats and T~rrentine'~, (see fig. l), itappears that the tornado did not come within 2 mi ofthe network. Since the intense electrical fields on theground probably extend no more than a few hundredyards from the funnel, it is not surprising that onlyrelatively weak fields were observed at Dr. Gum'sclosest station.Dr. Gunn observes that the determination of theexact electrical behavior within the clouds is difficultbecause of the complexity of the electrical activity andthe magnitude of the field changes near the tornado.Interpretation of his data is further complicated bythe probability that the response of his measuringapparatus is far too slow to follow the rapid fieldFrom these and similarl2 C. Flammarion, The atmosphere. New York, Harper Bros.,l3 R. S. Hall, "Inside a Texas tornado," Weatherwise, 4, 54-57,l4 Accepted for publication in Bull. Amer. meteor. SOC.339-347, 1873.1951.AllanlaBelle e' Udal1N0 5 IOSTATUTE MILESfluctuations that occur in the tornado storm. Jones'sferics measurements indicate electric dischargesduring the Blackwell tornado occurred at a rate ofgreater than 20 sec-'. Dr. Gunn's apparatus, whichhe states has a response time of approximately 1 sec,would be incapable of responding to such rapid variations of the electric field.While it appears doubtful that Dr. Gunn's apparatus could resolve the rapid field changes in thestorms, it seems probable that his records mayprovide an average picture of the electric field. It ispossibly significant that his stations which werenearest to the tornado show appreciably lower andsmoother values of the field during the tornado'sclosest approach. Both Dr. Gunn's field record andthe reports of eyewitnesses indicate that there is areduction of cloud-to-ground discharges for somedistance around ' the tornado. Such observationssupport Peltier's suggestion5 that the tornado servesas a preferred conducting path for electricity. It ispossible that the continuous flow of charge in thetornado, indicated by its steady luminosity, maysuppress the electric field at the ground and cloudto-ground lightning for some distance from the funnel.Certainly at present we do not have sufficientscientific information to determine what is the roleof electricity in the tornado. The interesting observations supplied by Dr. Gunn should provide stimulusfor further studies of the electrical activity associatedwith these storms. In our opinion, such studiesshould include the following efforts :'1. Determination of the sign and magnitude of the electric fieldand point-discharge current directly beneath the tornado funnel ;2. Measurement of electric fields near the tornado with instruments having a time of response of the order of 0.01 sec or less ; and3. Determination of the nature of the electrical phenomenacommonly observed at night within and beneath the tornadofunnel.FIG. 1. Tornado path and station locations.

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