Edwin Kessler National Severe Storms Laboratory National Oceanic and Atmospheric Administration Norman, Okla.

Abstract sure within and carried off by the wind as their con- The prominent characteristics of tornadoes, their socio- nections are weakened. Only buildings of reinforced logical and meteorological importance, aspects of the concrete and rigidly connected structural steel character- national service that pertain to storm forecast- istically escape serious structural damage from violent ing and warning, observational and theoretical studies tornadoes, and windows, roofs, and sidings are always of tornadoes, and some prospects for modifying tor- vulnerable (Brooks, 1951; Flora, 1954; Melaragno, 1968; nadoes, are briefly surveyed. Some paths for future de- Somes et al., 1970). velopment of the warning service and of scientific in- During the last 15 years, about 125 persons have been vestigations are indicated. killed by tornadoes annually, and the annual property damage has averaged about $75 million. These figures 1. General description and significance may be compared with estimated losses caused by light- Tornadoes are among the smallest in horizontal extent ning, hail, and hurricanes as shown in Table 1. of the atmosphere's whirling winds, but they are the Although tornadoes hardly rank as a prominent cause most locally destructive. Although occasionally reported of death in the United States, the number of tornado from Australia, western Europe, India and Japan, it is deaths is highest in relation to property damage among only in the United States (Fig. 1) that very intense tor- the listed natural phenomena. This is attributable in nadoes occur frequently. A tornado typical of highly part to our inability to effectively warn everyone endan- destructive storms in the United States is marked by a gered by this very destructive phenomenon—a tornado characteristic funnel cloud, accompanies an otherwise usually comes and goes suddenly and affects only a thou- severe thunderstorm, is on the ground about 20 min, sandth part of a region covered by tornado-spawning and damages an area 1/4 mile wide along a path toward thunderstorms. Extreme variability is characteristic of the northeast about 10 mi long. While much damage is tornadoes and most tornado losses are associated with a probably caused by winds of about 125 mph, the maxi- few storms (Cressman, 1969) that utterly detroy the struc- mum winds of tornadoes (never accurately measured) tures in large parts of urban areas, or entire small com- are probably between 175 and 250 mph. Damage has munities (Fig. 2). Allen Pearson, Director of the ESSA also been attributed to the sudden drop of pressure ac- National Severe Storms Forecast Center, has noted that companying tornadoes. Values exceeding 0.1 of the total 85% of tornado fatalities from 1960 to May 1970 were atmospheric pressure or 200 lb ft-2 have been cited in produced by 1-1/2% of reported tornadoes. A severe tor- the literature, but the extreme observations made during nado event leaves a community momentarily stunned very unusual conditions appear questionable. During tor- nadoes, especially when structures are poorly vented, TABLE 1. U. S. losses attributed to some weather phenomena. roofs and walls may be moved outward by a higher pres- Average annual Average annual property Type of storm deaths in U. S. damage in U. S.

Tornado1 125 $ 75 million Lightning 1502 100 million3 Hail — 284 million4 Hurricane1 75 500 million

1 Based on data from the Environmental Data Service, ESSA; applicable to period 1955-1969. 2 Estimate based on data from National Center for Health Statistics applicable to period 1959-1965. See Zegel (1967). 3 Includes property damage by lightning-caused building fires, $30,600,000 in 1967, according to Accident Facts, National Safety Council, Chicago, 111. (1968 ed., 96 pp). Other property loss in- cludes forest fires, aircraft damage, disruption of electro-magnetic transmissions and casualties to livestock. See ESSA (1969). 4 Estimate for period 1958-1967 provided by Stanley Chagnon, FIG. 1. Distribution of tornadoes in the United States (taken Illinois State Water Survey, Urbana, 111.; about 10% of Illinois from Pautz, 1969). losses represents property, remainder is crop damage. 926 Vol. 51, No. 10, October 1970

Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Bulletin A?nerican Meteorological Society rather than to a description of the expected behavior of entities already developed. Forecasts of tornadoes are closely linked to forecasts of severe thunderstorms, and like other weather fore- casts, these must start from a description of the present state of the atmosphere. They are less specific than we would like, partly because of a lack of understanding, but partly because our observations are too sparse to describe atmospheric variability on the scale productive of the tornado or thunderstorm phenomena. Thus, the extent of a severe thunderstorm may be 10 to 25 mi, and the lifetime of a storm system can be considered about 6 hr. On the other hand, the distance between primary surface weather stations is about 100 mi, and between upper air stations over 200 mi. Observations are made hourly at the surface stations (more often under special conditions) but usually at only 12-hr intervals at the upper air stations. Therefore, even if our knowledge were otherwise adequate to the task, the weather observ- ing system we now have would limit us to indicating the probability of thunderstorms and accompanying tor- FIG. 2. Effects of a tornado at Udall, Kansas, 2235 CST, nadoes in regions much larger than the storms. 25 May 1955. Photo taken at 1600 on 26 May by Melvin Briefly stated, the tornado forecasting parameters are Haynes. warmth and moisture in a layer about 5000 ft deep near the Earth's surface, with a cool dry region at interme- and disorganized and draws a response of the magnitude diate levels, strong winds in the upper atmosphere, and demanded in war (Moore, 1958; Fritz, 1961). a trend toward intensification of related conditions. The The beneficial rains and relief from heat often brought prediction of both thunderstorms and tornadoes is de- by severe storms, as well as the deaths and property dam- rived from years of accumulated observations, evaluated age they inflict, justify man's rational interest in them. by statistical and dynamical methods, and controlled in But public attitudes probably express as well some psy- practice by the judgment of experienced forecasters chological and emotional traits instilled during the mil- (Winston, 1956). lennia of man's development while he was less insulated Present tornado forecasts usually refer to developments from wind and rain than he is in technologically ad- expected to begin from 1 to 7 hr after the forecast is vanced countries today. Certainly many of us regard a issued in regions of about 25,000 mi2. About one-third of severe thunderstorm with a mingled fear and fascination tornado forecasts are issued as the immediate conse- akin to that which probably gripped our ancestors as quence of a observation of an existing thunder- they fled the elements to the shelter of primitive dwell- storm in a suspicious area. About 40% of affirmative ings. predictions are correct, i.e., are followed by tornadoes Of course, the meteorologist's special interest in tor- somewhere in the forecast box during the forecast pe- nadoes is related to their intriguing dynamical properties riod. The incorrect affirmative forecasts divide about and to the idea that understanding of them should con- evenly between cases without tornadoes and cases with tribute to advancement of weather knowledge on a broad tornadoes outside but near the predicted regions. Since front. Some of the most important aspects of hourly and the climatological expectancy of tornadoes during 6 hr daily weather variations are represented in the great in a randomly selected 25,000 mi2 area in eastern and vertical transports of heat, moisture, and momentum in central United States is only about one in 400,1 it is thunderstorms and tornadoes, the conversion there of plain that the affirmative forecasts give evidence of con- large amounts of potential to kinetic energy, and the siderable skill in identifying the meteorological parame- dissipation of storm kinetic energy in the boundary layer. ters associated with development of severe storms and The atmosphere's larger scale features must also be in- tornadoes. fluenced by these storm processes, because of their in- Forecasts of severe storms and tornadoes 1 to 7 hr volvement with the vertical and global redistribution in advance are called "watches." In view of the wide area of energy originally absorbed as solar radiance at the covered by the forecast in relation to the area likely to Earth's surface. be affected, the public is only encouraged by a "watch" to remain alert to further advisories. The forecasts are 2. Tornado prediction i The expectancy of a tornado in a 25,000 mi2 area during As used here, a "prediction" or "forecast" refers to phe- six hours is about 1/20 near the seasonal and geographical nomena foreshadowed in advance of their development, maxima of tornadoes. 927

Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 sightings and damage reports and facilitates issuance of timely warnings to communities lying in the projected paths of storms. Warnings of several minutes to an hour or more have been provided in connection with practically all major tornadoes during the last 5 years. Usually, however, there are some endangered persons who cannot be reached by the warning system, or whose response to the warnings does not increase their safety (Fritz, 1961).

4. Tornado research Morton (1966) and some others listed at the end of this paper have additional references and comprehensive re- views of tornado research, and the reader is referred to FIG. 3. Radar display of tornadic storms recorded at Nor- them for more detailed information than can be fur- man, Okla., on 30 April 1970. Range marks denote intervals nished here. of 20 n mi. North is toward the top; the radar is located at the center of the range circles. The arrows indicate approxi- a. Observations mate locations of verified tornadoes. We observe tornadoes and atmospheric conditions nearby disseminated by teletype from the National Severe Storms to develop understanding of the physical processes at- Forecast Center in Kansas City, Mo., to local offices tendant to tornadoes, and to learn how they might be around the country. Occasionally, a local Weather Bu- better predicted and, perhaps, modified. reau office may issue a modified local forecast that takes Scientific observation of tornadoes is made very diffi- special account of peculiar local conditions. Since sub- cult because of their random occurrence, brief duration, scribers to the teletype service include most elements of small size, and great violence. During the past 6 years, the communication media, storm indications are quickly the National Severe Storms Laboratory has maintained a brought to the attention of the radio and television network of 30 to 60 conventionally equipped surface sta- public. tions in areas of 1500 to 7000 mi2 where tornadoes are expected at the rate of about 1 per 1000 mi2 per spring 3. Tornado warning season. Only four of the stations, however, have been As used here, a "warning" refers to an advice issued on strongly affected by a tornado vortex during this period. a severe phenomenon in progress. Severe storms and While these direct observations provide some valuable tornadoes are observed as they develop by Weather Bu- detailed information on the vortices, we obtain impor- reau personnel, by employees of local government, and tant additional data from engineering analysis of dam- by other persons. When the Weather Bureau, through aged areas, eyewitness accounts, and from photographs. its own action or receipt of a report becomes aware of Our information indicates that the tornado is char- a tornado in existence, a warning to communities in acterized by an inner region where the winds decrease the extrapolated path of the storm is immediately issued toward the center, as with the rotation of a solid, and by teletype, and directly by radio and television. In the an outer region where the winds fall off with increasing threatened communities, the public may also be warned distance. Many other tornado features are highly vari- by various actions of local authorities, including the able. The tornado cloud, presumed to be the locus of sounding of sirens. The warning of at least a few min- constant pressure at which the well-mixed subcloud air utes thus provided is credited with halving the loss of is cooled to saturation, varies in size and shape (Der- life. The greatest loss of life is commonly found in the garabedian and Fendell, 1970). In some photographs it first community struck by a tornado, downstream loca- appears smooth (Fig. 4a) suggesting laminar flow, in tions having the benefit of longer warning time. others as highly irregular suggesting strong turbulence As already suggested, observer reports these days are (Fig. 4b). Such differences are quite important from the valuably augmented by radar observations. The primary point of view of tornado dynamics. Since waterspouts radar network of the Weather Bureau has stations spaced are usually cylindrical and smooth walled (Fig. 4c), we 200-250 mi apart. When severe storms threaten, the are led to search for significant variability in surface radar screens are monitored continuously. The more roughness, or in atmospheric conditions, to account for intense echoes are associated with heavier precipitation the apparent variability of turbulence and shape of and a greater likelihood of hail, strong straight-line tornadoes. winds, and tornadoes. The echo from a tornadic storm There is no doubt that sferics and visible electrical within about 60 mi of the radar often displays a hook- phenomena generally accompany tornadic storms, but shaped appendage as shown in Fig. 3. Thus the fore- the dynamical role of electromagnetic phenomena in caster's observation of intense radar echoes and hook- tornadoes and the uniqueness of tornadoes' electromag- shaped echoes provides a continuous check on visual netic signature remain uncertain and controversial. The

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FIG. 4a. Smooth walled tornado cloud at Enid, Oklahoma, on 5 June 1966 (ESSA photo by Leo Ainsworth).

FIG. 4c. Waterspout over the Florida Keys on 10 September 1969 (ESSA photo by Joseph Golden).

FIG. 4b. Turbulent double-tornado on 11 April 1965 at magnetic fields over numerous waterspouts and found Elkhart, Indiana. The numerous storms of this date have little disturbance. Kinzer and Morgan (1968) located been studied in detail by Fujita et al. (1970) (photo by Paul the position of sferics sources in the tornadic storm in Huffman). Oklahoma on 10 June 1967 and reported no obvious apparent variability of tornado electricity is indicated by close connection between lightning areas and tornado Finley's report on 600 tornadoes (1882). He lists obser- locations. vations of thunder and lightning in 425 associated rain- We would be particularly urged to seek a fundamental storms. In 17 cases, luminosity of an apparent electrical role for electrical processes during genesis and mainte- origin was noted in the tornado funnel itself, while in nance of tornadoes, if tornadic winds much in excess of 49 cases the absence of any electrical indication in the 200 mph were proved. Winds up to about 200 mph can cloud was specifically reported. Interest in electrical be explained in terms of horizontal pressure differences theories rose when Jones reported unusual 160-kHz created as thermal properties of convectively unstable radiation from a tornadic storm (1951). Vonnegut (1960) air are redistributed during overturning.2 Perhaps elec- has presented an electrical theory of tornadoes and 2 The following analysis provides some support for this statement. We eliminate the acceleration term from the ver- Brook (1967) has reported on a magnetic variation ob- tical dynamical equation by integration along the central served during touchdown of a tornado near Tulsa. Weller axis of a model steady-state axisymmetric vortex, from its and Waite (1969) have proposed that tornadoes are asso- base to its top, i.e., between levels where the vertical veloci- ciated with intense electromagnetic radiation at tele- ties are zero. The pressure is assumed to be horizontally vision frequencies. On the other hand, Gunn (1956) uniform at height H. Then, measured the electrical activity of the tornadic storm which devastated Udall, Kans., on 25 May 1955, and g-^n-r,,] found it to be "more or less typical of exceptionally where T is an average overall pressure-weigh ted temperature, active storms." Wilkins' (1964) experiments cast doubt T1 and Tz are the average pressure-weighted temperatures in on a fundamental dynamical role for electrical phe- the interval 0 < z < H outside and inside the vortex, respec- nomena in tornadoes, and Rossow (1969) has measured (Footnote 2 cont'd next page)

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 trical phenomena supplement purely hydrodynamic pro- tions, while Barnes (1970) has discussed the implications cesses in an especially violent small percentage of of data provided by radiosonde ascents in storm cores, tornadoes. and the possible significance of boundary layer winds In a sense, the tornado itself is only an important de- for the development of storm rotation. tail of the circulation and energy balance of the larger thunderstorm. By virtue of their larger size and greater B. Mathematical modeling frequency, the parent thunderstorms as shown in Fig. 3, Current mathematical models of weather represent ex- lend themselves much more to detailed examination treme simplifications of the natural phenomena and are (Atlas, 1963). Present research is therefore concen- characterized by the same divisions that exist in our trated on details in atmospheric structure associated with observational systems. Thus, our models of atmospheric formation of tornadic and non-tornadic storms, on behavior on the scale of the global circulation and large variation of behavior among different storms that form weather systems are most adequate for their purpose. in the same general area, and on evaluation of the man- In use at the National Meteorological Center in Wash- ner in which forces manifested in the storm environ- ington, D. C. (Shuman and Hovermale, 1968), such mod- ment combine to produce major features of the in-storm els predict the general patterns of horizontal winds, motions. To this end, experimental networks include moisture, and vertical currents, and provide useful closely spaced surface and upper air stations, with use guidance to the thunderstorm forecaster who combines of quantitative radar and specially instrumented air- their indications with his knowledge of the distribu- craft (Kessler, 1965). Recently, rather detailed observa- tion of features specifically associated with local storms, tions of storm sferics have been undertaken by W. L. to help him forecast the probable location of storms. Taylor in conjunction with the field program at the Models that forecast directly the parameters known to NSSL. be important to thunderstorm and tornado development Within the last decade the emphasis on combinations are just now coming into operational use—some incor- of observations by many sensors at a few places has led porate both dynamical and statistical methodology and to significant new knowledge about major features of provide somewhat more detailed spatial distributions thunderstorm circulations. We have learned that severe over the United States than has been available hereto- and enduring tornadoes often form in small low-pres- fore (Klein, 1970). sure areas associated with the hook-shape marked by We are still far from simulating adequately the com- an arrow in Fig. 3. We have identified storms that split bination of the many factors associated with the devel- into separate severe entities moving distinctly to the opment of local storms. The horizontal winds accom- left and right of the mean-layer wind. Browning (1964) panying local convective phenomena are more complexly and Haglund (1969) have offered descriptions of the related to condensation and precipitation and vertical right-moving type, in which severe tornadoes are found, air motions than widespread weather is. The time scale while Hammond (1967) has given a detailed description of storm development precludes application of the geo- of one left-moving storm. Fujita and Grandoso (1968) strophic approximation and related simplified connec- and Kuo (1969) have further considered these observa- tions between pressure and wind that have proven use- (Footnote 2 cont'd) ful in models of global circulation. In spite of difficulties, tively, R is the gas constant for air, g is the acceleration of however, we can report significant progress. We have gravity, and Px/P2 is the ratio of the pressure outside the today some interesting models of rain showers that in- vortex base to that inside. If H = 15 km, T = 250C, and corporate simplified formulations of precipitation-related T2—Tx — 7C, an extreme value suggested by upper air soundings taken near tornadoes, then Px/P2^\ .06, corre- processes and of entrainment. These have shown some sponding to a pressure deficit in the base of the vortex of value in predicting the maximum height to which a about 60 mb. cloud tower may rise with specified ambient conditions Extreme wind conditions are suggested by a calculation involving this pressure deficit with Bernoulli's equation for and have provided guidance for the conduct of weather frictionless, adiabatic flow in a horizontal plane. The model modification experiments (Simpson and Wiggert, 1969). plane would contain a streamline and be quasi-horizontal The more comprehensive of today's shower models show near the lower boundary of the (highly idealized!) vortex. several important features that resemble observations of With v — 0 outside the vortex, Bernoulli's equation can be natural rain (Orville, 1970; Takeda, 1969). But even written the most advanced shower models are probably less de- 286 286 = V26CP(Pr - P2- ) 7 tailed by a factor of at least 100 than a storm model where Cp = 10 erg/gm deg is the specific heat of air at con- stant pressure, and b is a constant determined by the ambient that would illustrate the asymmetric horizontal and ver- conditions, about 6 for pressures near 1000 mb (106 dyn cm-2) tical structures shown in Fig. 3. 4 -1 and temperatures about 300K. Then v2—• 1.04 X 10 cm sec Today's mathematical models of a tornado vortex treat or 230 mph. Effects of friction should reduce this considerably. cylindrically symmetric cases. Among the most advanced A simple approximate equation that provides somewhat are the steady-state models of Kuo (1966), and Serrin lower wind speed estimates than the above analysis is (1970). Kuo's model involves the techniques of boundary layer theory, and appears to describe essential features of observed tornadoes in terms of an unstable vertical

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Bulletin A?nerican Meteorological Society stratification and an ambient field of rotation. The fact great variety and complexity of processes implicit in that these features are often present when tornadoes are tornado development and maintenance, and the rarity, absent, however, serves to emphasize that we still have relatively small scale, and great intensity of these natural very far to go in our modeling and in our observing to phenomena have been sources of great difficulty, there identify the factors responsible for concentrating angular has been significant progress nevertheless. Let us briefly momentum in the developing tornado. Serrin's vortex consider what further technological developments and model includes an exact treatment of the coupling of observational and theoretical studies should be encour- the vortex with the lower boundary, but this refinement aged. appears with the loss of a size parameter present in The theoretical and experimental models indicate the Kuo's model. Both models admit the possibility of de- importance of observations on even gross characteristics scending air motion in a tornado core and suggest the of tornado circulations. Are natural flows upward or importance of detailed observations of the airflows ac- downward in the funnel core? Measurements of the companying real tornadoes. temperature and pressure in the core would help to answer this question even if we did not have direct mea- C. Experiments surements of the wind. How is tornado behavior such The control of parameters afforded by laboratory con- as funnel-skipping related to the underlying terrain? ditions permits the experimental approach in identify- What is the wind inflow angle, air pressure and tempera- ing and analyzing factors responsible for the growth of ture at various distances from the visual funnel? How tornadoes. Such experiments have been conducted for does the wind vary with height in the vicinity of tor- many years, often in conjunction with theoretical inves- nadoes? If we could better answer these questions for tigations, and realistic appearing vortices have been pro- atmospheric cases we could better design our experiments duced in various liquids and in air, under a considerable and mathematical models and rationally extend our variety of experimental conditions. The very ease with search for influential parameters of the flow. which tornado-like vortices can be experimentally pro- duced has made it difficult to progress much beyond Waterspouts are very frequent near the Florida Keys. theoretical implications regarding the development of Since they are much more easily approached than tor- swirling motion in converging fluid at the base of a nadoes and appear to embody tornadoes' essential prop- rising column and the important influence of boundaries. erties, they should be the object of intensive observa- Concurrently with the recent development of numeri- tions. A promising use of aircraft involves the placing cal analysis of large-scale atmospheric circulations, how- of smoke flares on the sea surface around a moving ever, has come appreciation of the importance of simi- waterspout. Photographs of the smoke trajectory should larity both in theoretical and experimental modeling. indicate hitherto unknown details of boundary layer Similarity in flows on different scales is said to exist flow (Golden, 1970). Another proposal concerns mea- when the ratios of various quantities involving inertia, surement of vortex parameters from a sensor-equipped viscosity, rotation, and diffusion are the same. Considera- airfoil tethered to a control aircraft in such a way that tions of similarity, and increased attention to such nat- the airfoil could enter a waterspout core while the air- ural observations as are available, are leading to the craft remains at a safe distance (personally communi- design of models more revealing of the effects of natural cated by J. Kuettner, 1970). conditions. Dust whirls also are approachable and similar in Turner and Lilly (1963) and Turner (1966) have con- some important respects to tornadoes, though their structed physical models of vortices driven from above source of energy appears to reside in a statically unstable to stimulate the convection in a cloud, and have found boundary layer rather than in relatively warm clouds. rising motion in the vortex core with descending motion Interesting dust devil observations have been discussed in a surrounding annulus. Ward (1970) has ingeniously recently by Sinclair (1964, 1969) and by Ryan and Car- separated a fan from the vortex it creates in controlled roll (1970). The heuristic importance of empirical data inflow beneath. In Ward's experiment, control of the in- is illustrated by the latters' finding that the environ- flow angle and depth of the inflow layer represent the mental vorticity was uncorrelated with the maximum most important influences in the creation of a vortex, its wind speed in the dust devils they examined. intensity and diameter, and in contrast to models devel- With the stimulation of various severe storm study oped by Turner and Lilly, the development of a central programs, greater numbers of conventional observations downdraft. The complexities of measured pressure and and useful motion pictures should become available and velocity distributions in laboratory vortices have been we may reasonably expect an opportunity in the next investigated and discussed very recently by Ying and few years to extend the important study of the winds Chang (1970). in the Dallas tornado of 2 April 1957 (Hoecker et al., 1960). 5. Comments on the further development of It seems that great emphasis should continue to be investigations placed on observing the circulation in the neighborhood We have surveyed observational, theoretical, and experi- of a tornado since it seems likely that a tornado is largely mental aspects of tornado investigations. Although the controlled by its parent storm and the storm's environ-

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 ties revealed by observations and experiments. Of course, many techni-sociological forces are already encouraging the development of improved computers. We may em- phasize here that no computer will solve meteorological problems in such a way that careful scientists will not be an essential part of problem preparation; indeed, theoretical interpretation of data from observational and experimental programs will be increasingly required to develop mathematical formulations reasonably posed. With regard to physical modeling of thunderstorms and tornadoes, we expect that the difficulties inherent in modeling significant atmospheric processes such as condensation and precipitation, in diminishing the ef- fect of container sidewalls to levels consistent with the atmosphere's lack of sidewalls, and in simulating the vertical density gradient and diffusion processes charac- teristic of the atmosphere will continue to represent serious obstacles. We observe that these problems have been less serious with repect to interpretation of the FIG. 5. Installation of a paraboloidal 30-ft diameter antenna largely two-dimensional flows representative of atmo- at NSSL, May 1970. The antenna has since been completely spheric circulations on larger scales. In spite of diffi- enclosed in a radome. Doppler velocity measuring capability is being developed with an FPS-18 10-cm radar on the second culties, experimental methods should continue to be floor of the supporting structure. important for testing tornado hypotheses and for sug- gesting new lines of observational and theoretical study. ment. While encouraging existing programs having this 6. Comments on the general status of the objective, we emphasize two emerging tools. One is operational system for storm predicting and meteorological , which in units of two or warning three should map the distribution of precipitation ve- Present severe storm forecasts are very useful, but we locity with unprecedented detail (Lhermitte, 1966; Peace wish they were more precise and more accurate. Al- and Brown, 1968; Armijo, 1969). The development of though numerical methods have been used for forecast- improved Doppler techniques would have value for ing large scale weather patterns for over 10 years, the both fundamental research and research on an improved development of mathematical models for local storm warning system, the latter by providing bases for evalu- systems is still in its infancy. As previously indicated, ating the distinguishing features in a storm velocity field improved understanding and better numerical forecast- that may characteristically arise before a tornado be- ing models can be expected to evolve only as the in- comes active (Armstrong and Donaldson, 1969). Although sights provided by more detailed observations are as- some meteorological Doppler are presently in use sessed by careful scientists with the aid of more and other systems are under development (Fig. 5, for powerful computers. Eventually, methods will be de- example), the pace of work seems slow and could be veloped to combine such detailed data as that provided advanced in direct proportion to the funds and skilled by radar and satellites with other weather parameters personnel made available for the purpose. in dynamic storm models, and appropriate means for The second emerging technique is satellite infrared using such data in operational forecast preparation spectrometry, which is providing new detail on the ver- should become clearly indicated. tical thermal stratification of the atmosphere at intervals While studying new suggestions for tornado detection, of about 30 mi (Walk and Hilleary, 1969). Further de- such as acoustic sensing, for example (Chrzanowski et velopment of the satellite system should prove of great al, 1960; Cook, 1969), we should emphasize application value for the analysis of severe thunderstorm precursor of technology already available to hasten the prepara- conditions over the United States and refinement of our tion and distribution of forecasts. To this end, at the forecasting ability. National Severe Storms Forecast Center, computer tech- Mathematical modeling is very important and greater niques have largely supplanted the manual plotting of realism should be achieved as computers become larger observational data, and computers are also beginning to and faster, and as theoretical models are devised to in- be used there for analysis as well (Sangster, 1960; Pear- clude more of the static, dynamic, and electric proper- son et al, 1967; Inman, 1970a, b).

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FIG. 6. Digital data showing the hooked-echo in Fig. 2, 40 minutes before that PPI photo was made. Successive horizon- tal lines are 2° steps of azimuth and successive vertical columns are 1 mile range intervals. The PPI display begins with the twelfth column which corresponds with the range of 12 n mi. Computer-processed digital data could be a significant aid to human operators in an advanced national radar system.

We can also reduce the number of occasions, already 7. Comments on the prospects for a measure of rare, when timely warnings are not issued on storms in control of tornadoes progress, while making more effective use of human We have noted that tornadoes occur in air masses that skills. For example, the national radar network, which are relatively warm and moist near the ground, and cool is the backbone of the system used today for severe storm aloft. Other conditions, such as vertical shear of the warning, lends itself to significantly advanced automa- horizontal wind and large-scale ascending air motion, are tion. Contour-mapped echo representations such as the usually present also. Concerning the first of these condi- one shown in Fig. 3 could be supplemented by a cor- tions we can conceive of altering the atmosphere in a responding digital array as shown in Fig. 6, and extrapo- way to reduce the intensity of storms and the probabil- lation forecasts such as that illustrated in Fig. 7 could ity of tornadoes. This would involve heating the air in be prepared and disseminated with the aid of computers. mid-troposphere so as to reduce its overturning instabil- Mosaics of combined radar and satellite data covering ity. Imagine that we could eliminate tornadoes at a the whole nation could also be prepared automatically. particular time by raising the temperature by 1C in a The Weather Bureau is starting to develop in the Mid- middle layer comprising 25% of the mass of air over a west an operational test of advanced radar systems in particular 10-mile square. How much energy would be order to evaluate the probable costs and benefits of required? The astonishing answer is 1.6 X 1014 calories various system designs for nationwide application. or about 200 million kilowatt-hours, whose cut-rate cost

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 late the freezing of water drops. The artificial release of the latent heat of fusion can raise the air temperature enough to enhance significantly the growth of some clouds and to hasten the dissipation of others (Simpson and Wiggert, 1969). Conceivably, for example, this kind of process could be applied to alter nature's choice for rapid growth among a host of clouds. Eventually, we may learn how to alter the Earth's topography and roughness to change the paths by which natural ener- gies are used and to decrease the probability of tor- nadoes over inhabited areas. And we may learn how the direct application of a little heat at a particular point in time and place could beneficially modify the course of subsequent events. It must be plain from the foregoing discussion, however, that we are far from having a reasonable basis for estimating the ultimate success of such efforts. Acknowledgments. The author's knowledge of tor- nadoes and of what is being done about them has been greatly increased by discussions and correspondence with many persons. Particularly interesting and helpful have been discussions with several of NSSL staff, with Allen Pearson, Director of the National Severe Storms FIG. 7. Part of a conversation between a radar operator and Forecast Center, Kansas City, Mo., and with Prof. James a computer connected by telephone to the NSSL. The pre- Serrin of the School of Mathematics, University of Min- vious and extrapolated motions of radar echoes shown in Fig. 2 were the basis for real-time computer-aided prepara- nesota, Minneapolis. tion of experimental severe storm warnings and teletype tapes. The final set of columns lists communities threatened by echo No. 4, the storm's probable bearings and distances References at closest approach, and the period of greatest threat at Armijo, L., 1969: A theory for the determination of wind each community. and precipitation velocities with Doppler radars. J. Atmos. Sci., 26, 570-573. would be about $2 million. If delivered in an hour, the Armstrong, G. M., and R. J. Donaldson, 1969: Plan shear power would be about the same as the average combined indicator for real-time Doppler radar identification of output of all power generating facilities in the United hazardous storm winds. J. Appl. Meteor., 8, 376-383. States! 3 Atlas, D., editor, 1963: Severe local storms. Meteor. Monogr., Of course, having only the energy would not be 5, No. 27, 247 pp. enough. The nature of a system to identify the critical Barnes, S. L., 1970: Some aspects of a severe, right moving thunderstorm deduced from mesonetwork rawinsonde ob- air mass and to provide timely delivery of power to it servations. J. Atmos. Sci., 27, 634-648. taxes our imagination, and the cost of the whole, in Brook, M., 1967: Electric currents accompanying tornadic light of the loss figures presented in the beginning of activity. Science, 57, 1434-1436. this paper, would not be justified by any conventional Brooks, E. M., 1951: Tornadoes and related phenomena. cost-benefit analysis. Furthermore, this is a very con- Compendium of , Amer. Meteor. Soc., Boston, servative representation, since the storms of which tor- 673-680. nadoes are a part usually cover about 400 mi2 and en- Brown, R. A., and R. L. Peace, 1968: Mesoanalysis of con- train air from a region more than 10 times larger! vective storms utilizing observations from two Doppler Thus, it seems that even an ultimate ability to exer- radars. Proc. 13 th Radar Meteor. Conf., Amer. Meteor. cise appreciable control over storm phenomena depends Soc., Boston, 188-191. on means for modifying the processes by which nature's Browning, K. A., 1964: Airflow and precipitation trajectories supply is used. Such an approach is represented in the within severe local storms which travel to the right of use of silver iodide and a few other chemicals to stimu- the winds. J. Atmos. Sci., 21, 634-639. Chrzanowski, P., J. M. Young and H. L. Marrett, 1960: In- 3 The energy of motion of the typical tornado itself is only frasonic pressure waves from tornadic storms. National about 1/1000 of that calculated above (Kessler, 1966). One Bureau of Standards Report No. 7035, 6 pp. and 16 figs. concept for diminution of tornadoes after they have formed involves modification of inward flowing air with an energy Cook, R. K., 1969: Atmospheric sound propagation. Atmo- requirement comparable to that discussed in the text. Another spheric exploration by remote probes, Final Report of the suggests release of matching kinetic energy, with the prob- Panel on Remote Atmospheric Probing to the Committee lem of attendant man-made damages. on Atmos. Sci., National Research Council, Vol. 2, 633-669.

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Bulletin A?nerican Meteorological Society Cressman, G. P., 1969: Killer storms. Bull. Amer. Meteor. Melaragno, M. G., 1968: Tornado forces and their effects on Soc., 50, 850-855. buildings. Kansas State University, Manhattan, Kansas, Dergarabedian, P., and F. Fendell, 1970: A method for rapid 51 pp. estimation of maximum tangential wind speed in tor- Moore, H. E., 1958: Tornadoes over , a Study of Waco nadoes. Mon. Wea. Rev., in press. and San Angelo in Disaster. Austin, University of Texas ESSA, 1969: Lightning. PI 660024, Public Information Office, Press, 334 pp. Environmental Science Services Administration, U. S. Morton, B., 1966: Geophysical vortices. Progress in Aeronauti- Dept. of Commerce. cal Sciences, Vol. 7, Pergamon Press, , 145-193. Finley, J. P., 1882: Character of six hundred tornadoes. Prof. Orville, Harold D., and Lansing, J. Sloan, 1970: A numeri- Papers of the Signal Service, No. VII, Washington Office cal simulation of the life history of a rainstorm. J. Atmos. of the Chief Signal Officer. Sci., 27, 1148-1159. Flora, S. D., 1954: Tornadoes of the United States. Univer- Pautz, M. E., 1969: Severe local storm occurrences 1955-1967. sity of Oklahoma Press, 194 pp. ESSA Tech. Memo WBTM FCST 12, Office of Meteorologi- Fritz, C. E., 1961: Disaster. Contemporary Social Problems, cal Operations, Silver Spring, Md. Harcourt, Brace and World, Inc.; New York, 651-694. Peace, R. L., and R. A. Brown, 1968: Comparison of single Fujita, T., D. L. Bradbury and C. F. Van Thullenar, 1970: and double Doppler radar velocity measurements in con- Palm Sunday tornadoes of April 11, 1965. Mon. Wea. Rev., vective storms. Proc. 13 th Radar Meteor. Conf., Amer. 98, 29-69. Meteor. Soc., Boston, 464-473. (See also Brown and Peace, , and H. Grandoso, 1968: Split of a thunderstorm into 1968.) anticyclonic and cyclonic storms and their motions deter- Pearson, A. D., J. G. Galway and R. L. Inman, 1967: Rela- mined from numerical model expeiiments. J. Atmos. Sci., tionship of surface dew point and integrated moisture in 25, 416-439. the planetary boundary layer. Preprints, Fifth Conference Golden, J., 1970: The Lower Florida Keys waterspout project. on Severe Local Storms, Amer. Meteor. Soc., Boston, 135- Bull. Amer. Meteor. Soc., 51, 235-236. 139. Gunn, R., 1956: Electric field intensity at the ground under Rossow, V., 1969: Observations of waterspouts and their active thunderstorms and tornadoes. J. Meteor., 13, 269- parent clouds. NASA Tech. Note D-5854, National Aero- 273. nautics and Space Administration, Washington, D. C., Haglund, G. T., 1969: A study of a severe storm of 16 April 63 pp. 1967. NSSL Tech. Memo No. 44, U. S. Department of Com- Ryan, J. A., and J. J. Carroll, 1970: Dust devil wind veloci- merce, ESSA Research Laboratories, 54 pp. ties: mature state. J. Geophys. Res., 75, 531-541. Hammond, G. R., 1967: Study of a left-moving thunderstorm Sangster, W., 1960: A method of representing the horizontal of 23 April 1964. National Severe Storms Laboratory Tech. pressure force without reduction of station pressures to Memo, No. 31, 75 pp. sea level. J. Meteor., 17, 166-176. Hoecker, W. H., R. G. Beebe, D. T. Williams, J. T. Lee, S. Serrin, J., 1971: The swirling vortex. Report from the School G. Bigler and E. P. Segner, Jr., 1960: The tornadoes at of Mathematics, University of Minnesota. To appear in Dallas, Texas, April 2, 1957. U. S. Dept. of Commerce, Phil. Trans. Roy. Soc. London, Series A. Weather Bureau Research Paper No. 41, 175 pp. Shuman, F., and J. B. Hovermale, 1968: An operational six- Inman, R., 1970a: Operational objective analysis schemes at layer primitive equation model. J. Appl. Meteor., 7, 525- the National Severe Storms Forecast Center. Tech. Circular 547. No. 10, NSSL, 50 pp. Simpson, J., and V. Wiggert, 1969: Models of precipitating , 1970b: Objective analysis of mean moisture aloft utiliz- cumulus towers. Mon. Wea. Rev., 97, 471-489. ing surface and radiosonde data. Tech. Circular No. 11, Sinclair, P. C., 1964: Some preliminary dust devil measure- NSSL, 33 pp. ments. Mon. Wea. Rev., 92, 363-367. Jones, H. L., 1951: A sferic method of tornado identification , 1969: General characteristics of dust devils. J. Appl. and tracking. Bull. Amer. Meteor. Soc., 32, 380-385. Meteor., 8, 32-45. Kessler, E., 1965: Purposes and program of the U. S. Weather Bureau National Severe Storms Laboratory. Trans. Somes, N. F., R. D. Dikken and T. H. Boone, 1970: Lubbock Amer. Geophys. Union, 46, 389-397. tornado—a survey of building damage in an urban area. , 1966: A storms incalculable energy. Natural History, 75, NBS Report 10254, National Bureau of Standards, U. S. 12-17. Dept. of Commerce, 69 pp. Kinzer, G. D., and B. Morgan, 1968: Location and move- Takeda, T., 1969: Numerical simulation of large convective ment of lightning centers associated with a tornadic storm. clouds. Stormy Weather Group Scientific Report WM-64 Paper presented at the AGU-AMS Meeting in Washington, (in two volumes), McGill University, Montreal. D. C., 8-11 April. Turner, J. S., 1966: The constraints imposed on tornado- Klein, W. H., 1970: The forecast research program of the like vortices by the top and bottom boundary conditions. Techniques Development Laboratory. Bull. Amer. Meteor. J. Fluid Mech., 25, Part 2, 377-400. Soc., 51, 133-142. , and D. Lilly, 1963: The carbonated water tornado vor- Kuo, H. L., 1966: On the dynamics of convective atmospheric tex. J. Atmos. Sci., 20, 468-471. vortices. J. Atmos. Sci., 23, 25-42. Vonnegut, B., 1960: Electrical theory of tornadoes. J. Geo- , 1969: Motions of vortices and circulating cylinder in phys. Res., 65, 203-212. shear flow with friction. J. Atmos. Sci., 26, 390-398. Ward, N. B., 1970: The exploration of certain features of Lhermitte, R. M., 1966: Application of pulse Doppler radar tornado vortices using a laboratory model. Paper presented technique to meteorology. Bull. Amer. Meteor. Soc., 47, at the AMS Conference on the Motion and Dynamics of 703-711. the Atmosphere, Houston, Tex., 23-25 March 1970.

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 Wark, D. Q., and D. T. Hilleary, 1969: Atmospheric tempera- Winston, J. S., 1956: Forecasting tornadoes and severe thun- ture: successful tests of remote probing. Science, 165, 1256— derstorms. Forecasting Guide No. 1, U. S. Weather Bureau, 1258, Kansas City, Mo., 34 pp. Weller, N., and P. J. Waite, 1969: The Weller method: tor- Ying, S. J., and C. C. Chang, 1970: Exploratory model study nado detection by television. Preprints, Sixth Conf. Severe or tornado-like vortex dynamics. J. Atmos. Sci., 27, 3-14. Local Storms, Amer. Meteor. Soc., Boston, 169-171. Zegel, F. H., 1967: Lightning deaths in the United States: Wilkins, E. M., 1964: The role of electrical phenomena asso- a seven year survey from 1959 to 1965. Weatherwise, 20, ciated with tornadoes. J. Geophys. Res., 69, 2435-2447. 168-173, 179

news and nntes

Unusual twister damage brick in a watertower, and to drive partially open the heavily At 2:15 a.m. on the morning of 18 April of this year a tor- secured steel door to the storm cellar. nado hit Clarendon, Tex., and totally destroyed 4 homes in The Lands have donated the damaged section of gate, still this Panhandle town. One of these was the home of Mr. and containing the wood fragment, to ESSA's Environmental Data Mrs. Pete Land, who survived by taking shelter in their Service. The pipe, or one similar to it, will be used to deter- storm cellar. mine the energy required to penetrate the pipe by forcing a When they emerged, they discovered that the twister had similarly shaped tool into the pipe on a testing machine. By driven a piece of wood through a 1-1/2 in. (3.8 cm) steel pipe making computations based on various assumptions as to the which formed the frame of a fence gate. The winds were also physical characteristics of the wooden missile, EDS scientists powerful enough to split the metal baseplate of a refrigerator will then be able to determine the probable impulse and door handle with a sliver of wood, to knock out sections of the velocity of the original missile.

Tornado winds drove this piece of wood into the 1-1/2 in. The pipe, penetrated by the wood, serves as a gate frame steel pipe.—ESSA photograph before removal for study by ESSA.—ESSA photograph (More news and notes on page 942)

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Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Type "C" Wind Alarm

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Bulletin American Meteorological Society 937

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