techivical b~sis for interim regional tornado …tornado data. during 1971 and 1972. observers...

ASLBP Technical Lib ~

L " '( ~ /' WASH-1300 , c:/ rUe-11l

TECHIVICAL B~SIS FOR INTERIM REGIONAL TORNADO CRITERIA

I

U.S. ATOMIC ENERGY COMMISSION OFFICE OF REGULA T/ON MAY, 1974

TECHNICAL BASIS FOR INTERIM

REGIONAL TORNADO CRITERIA

E . H. Markee, Jr . :r. G. Beckerley

Directorate of Licensing

K. E. Sanders

Directorate of Regulatory Standards

u. S . Atomic Energy Commission Office of Regulation

Wash:1ngt"On. D. C. 20545

WASH-l300

(UC-llJ

For sale by the Superintende:nt of Documents. U. S . Government Printing Office, Washington, D.C. 20402.

•

1

ACKNOWLEDGMENT

The authors wish to express their appreciation to W. H. Hoecker qf the Air

Resources Laboratories, NOAA, for kindly providing his copy of the photo

graph originally taken by Bill Burkette of the April 2~ 1957, Dallas tornado •

•

11

ABSTRACT

The objective of this study was to determine from available information

a regionalized design basis tornado for the contiguous United States .

From an analysis of tornado classification data from the National Weather

Service Offices for 1971 and 1972 and tornado occurrence datal it was

concluded that three general regions with associated tornado severity and

frequency of occurrence could be determined. Probable windspeeds were

calculated, and corresponding pressure drops and rates of pressur e drop

were determined from the cyclostrophic equation. The numerical values

for these tornado properties are considered as conservative upper bounds •

The values should be lowered as may be justified when sufficient data

become available.

Section

Acknowledgement.

Abstract

Figures .

Tables .

1. Introduction

• •

2. Frequency of Tornadoes "

iii

TABLE OF CONTENTS

3. Path Length and Width . •

4. Tornado Strike Probabil:lty •

5. Tornado Wind Speed Prob;!lbility

6. Probability of Occurrenl:e of Intense Tornadoes

7. Regionalization of TOrtUldo Criteria.

8. Regional Variation in Maximum. Windspeed.

9. Derivation of Pressure Drop Due to Tornadoes

10. Conclusions.

11. References.

Page

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No.

1

2

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FIGURES

Title

Total Number of Tornadoes within One-Degree

Squares in the Contiguous United States for

1953-1962. . . . . . . . . . . . . . . .


Squares in the Contiguous United States for

1955-1967.


Squares in the W.~hington-oregon-ldaho Area.

Tornado Strike Probability within Five-Degree

Page

4

5

7

Squares in the Contiguous United States. . . . 10

Percent Probability of Exceeding Ordinate Value

of the Winds peed • ; .

Calculated Tornado Windspeed by Five-Degree -7

Squares for 10 Probability Per Year. • • •

Maximum Persisting 12-Hour looO-rob Dewpoints

of Record (OF) • •

13

15

17

Tornado Intensit~r Regions. . . . • . . . . . . .. 18

No.

1

2

J

4

5

v

TABLES

Title

Table of Fuji.ta-Pearson Tornado Scale.

Windspeed Distribution . . . . • .

Cumulative Wi.ndspeed Distribution.

Regional Tornado Winds

Regional Preflsure Drop and Pressure Drop Rate .

Page

J

12

12

19

20

- 1 -

1. Introduction

Tornadoes have been reported in the United States since the early nine

teenth century. The number of observed tornadoes increased with the

increase in population and the spread of population until, by the end of

the century. several hundred t ornadoes were r eported each year. The

annual frequency of tornadoes observed during the twentieth century in the

United States has increased until it is now over 600 per year. It should

be understood that the increase 1s almost entirely attributable to the

increased population density, greater public awareness through forecasts

on radio and television. imp:roved storm reporting techniques, more trained

storm observers, and establishment of warning networks (Ref. 1). In many

areas with a high population density and an efficient reporting system~

essentially all tornadoes are now observed and reported. In sparsely

populated areas such as the Southwestern United States, there a re undoubtedly

many tornadoes each year that are not observed or reported. I~ is believed

that most of the unreported tornadoes are small ones that cause little or

no observable damage.

While the frequency of occurlrence of tornadoes in a particular, region is

important in evaluating nucll!8r facility safety in that reg'ion. data on

tornado path size and intensity (i.e .• land a rea contacted and maximum , 1/

windspee<F and pressure transient) are at least of equal importance.

During the last two decades, area and intensity data in certain regions

have been accumulated to the extent that some statistically significant

inferences can be drawn. However. there are still too few observations

and too few trained observers to secure the data necessary for determina

tion of exact tornado intensfty characteristics. Exact knowledge of all

details 1s not requisite for developing tornado criteria; conservative

!/Unless designated as translational or rotational components. the phrase "windspeed" refers to the combined total of these two components.

•

- 2 -

limits of damage-related parameters can be based on observations and

theoretical analysis . From a design point of view, it 1s important to

emphasize consideration of the more severe tornadic events rather than

the weaker ones which are large in number, but less significant in effect

on the public.

One of the obstacles to accumulating reliable quantitative information

has been the lack of widely accepted methods for measuring and reporting

tornado data . During 1971 and 1972. observers participating 1n the

"Tornado 'Watch Experiment" sponsored by NOAA, NASA. and NSF used a uni

form system of oBserving and reporting. Each tornado was characterized

by a series of three numbers (a windspeed class number . a path length

class number. and a path width class number). the "Fuj i ta-Pearson Scale."

(Ref. 2). The classification system is described in Table 1. If this

system proves successful and is adopted for general use, reliable quanti

tative data could become avail able within a few years~ at which time more

detailed analyses can be undertaken.

2. Frequency of Tornadoes

The frequency of tornadoes in the contiguous United States has been re

ported by Thom (Ref. 3) and Pautz (Ref. 4). Thom's data for the period

1953-62 are shown in Figure 1. Pautz' data for the period 1955- 67 are

shown in Figure 2. In both figur es, the total number of tornadoes

observed is plotted for each "one-degree square" (an area enclosed by

successive one-degree longitude and latitude lines); the area of each

square in square miles is 4780 cos ~. where ~ is the latitude at the

center of the square . Examination of the figures shows that, where

tornadoes have been observed, the frequency can be as high as 8 per year

in a one-degree square (about 3800 square miles in Oklahoma). In many

of those areas where few or no tornadoes were reported, future increases

in population density may be accompanied by an i~crease in the number

reported.

-3-

Table 1

TA.BLE OF FUJfTA·PEARSON TORNADO SCALE. Characteristics of a tornado can be expressed as a combination of Fujita-scale: windspecd and Pearson-scale path length and width. This scale permits us to classify tornadoes between two extreme FPP scales, 0,0,0 and 5,5,5.

'F-scale Maximum Windspeed

Scale mph kIlO m/ s

'F 0 . 0 0 . 1 0 .2 0.1 0.' 0.' 0. ' o. , 0.' 0.'

r 1.0 1.1 1.2 1.1 ... 1.5 I.' U 1.. 1. ,

, 2.0 2.1 2.2

'.1 2.' 2.' 2.' U 2.' 2 ••

r l.O 1 . 1 1.2 1.1 , .. 1.' 1.' 1. , 1.' , ..

F 4.0 ' .1 '.2 .. , ••• .. , , .. .., , .. .. ,

r !i.O ,., '.2 ' .1 , .. ,., , .. u , .. ,. ,

" ., .. " " " " " " " " " 81 .. .. 92

" 100 10' 109

III m 121 126 110 m 119 1<. 1 ..

'" ISO 162 167 m m 182

'" 192

'" 202

'" 212 218 m '" m

'" '" '" m 261

'" m

'" 28. 28' m

'" '" 113

15

" 40

" .. .. " " " 60

" " " " " 80 .. " 91 94

98 102 10' 109 III m 121 m ". ll2

'" 141 1" 149

". '" '" '" m

'" 180 1 .. 189 194 198 20'

'" 212

'" m m 212

'" "I

'" 251 >S, 161

'" m

18

" 11 22 2l

" 26 28 10 II

II 14 16 18 40

" ., " ., 49

" " " " SO 60

" " " " " " " " " 81

" " .. " " " " ' 00

102 104 10' 109 112 11.

ll, 119 121 I" In 129 ll2 m In 140

P·scalc Path Length

Sc-.. t1c

p 0.0 0. 1 0.2 0 .3

0.' 0.' 0.' o. , !).B 0.9

P 1.0 :l.l 1 . 2 1.. ) :1.4

1.' 1.6 1 . 7 .1.8 1.9

P :1.0 :l .l :1. 2 2.3 2.4 2.5 2.6 2 .7

2.' 2.'

p ).0 3.1 :1. 2 :1. J :!.4 :1.5 :1 .6 :1. 7 :1.8 :1.9

P 4.0 4.1 4.2 4. 3 4. ' 'I. 5 '1 . 6 ':.7 '1 . 8 '1.9

P !>'O !i.l 5.2 S.3 5. ' 5.S 5. ' S.7 5.8 5.9

miles

0.3 0.' 0.' 0.' o. , 0.' 0.' .. , 0.' 0.'

I. ' I . J 1 .3 1.' I .' 1.' 1. 0 '.2 1. ,

2.' 3.2 3.' ' . 0 .. , , .. , .. .. , '.J , .. .. ,

10.0 11.2 12.6 14.l 1 5.9 17.8 20 . 0 22.4 15.1 28.2

31.6 35. S 39.8 44.7 SO.l 56.2 &3.1 70 .8 79.4 89 . l

100 III 126

'" 159

'" 200 n.;

'" 202

km .. , 0.' 0.' o. , 0.' 0.' 1.' 1.1 1.3 I..

I.' 1.. 2.0 2.1 2.' 2.' 3.2 J.' '.0 .. , ' .1 ' .7 ••• '.1 '.1 '.0

10.2 11.4 12.8 14 . 3

16.1 18.0 20.3 22.7 <IS . 6 28.6 32.2 36.0 40.4 45 . 4

50.9 57 . 1 64.1 71. 8 80.6 90.4

102 11. 120 14J

161 181 20J

'" m ". m 360

." ."

Seak

p 0.0 '.1 '.2 0.3 0.' o. , 0.' o. , 0.' .. ,

P 1 . 0 1.1 1.2 1.1 1.. 1. ,

I.' 1 . , 1.. 1..

p 2.0 2.1 '.2 '.3 2. ' 2.' 2 •• 2.7

2.' 2 ••

P 3 . 0 3.1 3.2 3.J 3.' 3.' 3.' 3.' 3.' 3 ••

P 4.0 '.1 <.1 '.3 ••• .. , ••• '.7 ••• •••

P 5 . 0

'. 1 .., ' .3 , .. '.5 ,., '.7 5.' 5.'

P·seale Path Width fl yds -

" 19 21

" " JO JJ

" " " " " .. " .. 94

lOS 118 lJ3 14.

1" 187 210 2J5

'" '" JJJ

'" '" 470

'" '" '" ". m

'" 1054 1183 1326 1489

1670 1874 2102 2354 2646 2967 3332 l738 4H4 4704

1.0 lII i 1.1 l.J

I.' I.' 1.. 2 .• 2.2

2.' 2.'

• • , 8

• 1. II 11 14 16

18 20 22 25

" Jl

" J9 .. .. .. " 7. " .. " III

'25 14. J57

no

'" m 24' n, III J5l 394 442 .,. '" '25 101

'" .82 ... 1111 1246 1.398 1568

1160 1971 2218 " 2482 2798 113.l 3520 3942 4418 496)

meters

5

• • , • •

10 11 13 14

16 18 20

" " 29 J2 36 <0

" " " " " 81 90

102 11' 128 143

16J 18. 203

'" m 2 .. J22 360

". ". ... m ." 71. .. , ''''' l.Oklll

1.1 1.1 I.'

1 •• 1.. 2. ' 2.J 2.' , .. 3.2 3. ,

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- 6 -

Tornado frequency in Northwestern United States has been investigated by

Fujita (Ref . 5) . Figure 3 shows the number of tornadoes in one-degree

squares for the tri state (Washington, Oregon, Idaho) area as set forth

in four different surveys . Fujit~I's survey of the period 1950-69 was

based on reports of 63 tornado stclrms, 49 funnel clouds, and 2 water spouts.

It is noted that the frequency is less than in Central United States.

Again, part of the difference may be attributed to sparseness of population .

3. Path Length and Width

The boundary of a tornado is usually defi ned as the region outside of which

the windspeed is less than 73 mph (hurricane force wind on Beaufort scale).

From visual and photographic evidence, 8S well as examination of damage ,

the path length and width of many tornadoes have been estimated.

Thom (Ref. 3) analyzed data on IOlira (1937-62) and Kansas (1953-62) torna

does and found that both the tornado path length and path width fit log

normal distribut.ions. For t.he observations analyzed by Thom, the mean

path area (path lengt.h times pat.h widt.h) in Iowa was found to be 2.821

square miles.

Fujit.a (Ref. 1) has also reviewed t.he report.ed lengths and widths given

in the 1965, 1967, and 1969 issues of Storm Data. He found the mean areas

of all t.ornadoes from reported 1etlgths and widths to be 2.0 square miles

in 1965, 1.0 square mile in 1967 BInd 2.0 square miles in 1969 . When tor

nadoes with an area of 0 . 1 square mile or less are excluded, the mean areas

increase t.o 3. 8 (1965), 1.7 (1967). and 3.4 (1969) square miles per tornado.

This shows the sensitivity of the area calculation to the inclusion or

exclusion of the smaller tornadoes:. The values report.ed by Fuj ita are all

of the same seneral magnitude as lhom's values. In fact, the average of

the three annual average values obtained by excluding those tornadoes ~th

.... ~ 1953.58 -.'-, 1955·67 ~ (REF.14) I (REF.4)

f-'j~+-H+-I'~2 ~~ 1 2 11\ 1 1 2 1 ,1 2 ,1 11 12

11 1 1 __ 11 111 ~

1 1 1 1,' lr ttHY-++++-l-2~++-t'-t21 1 ~ i3 1 ~ 6

'2 12 1 1212

'\. 1953.62 '\.. ~, 1950·69 (REF. 3) W (REF.5)

LJ"l r? 1

tittE~'tE~~~~~ iT .2 1 3 1 \3 a ~ ' ~ . ~ 1 1

" I 2 1 : '

tfH4...j.-I-++-~12~~·~~}-fa~. ~2;\ 2 \3 2 1 15 14 , 2. 2. ;' - 3 \3 "2

Figure 3. 'TOTAL NUMBER OF TORNADOES WITHIN ONE·DEGREE SOUARES IN THE WASHINGTOl'l-blmlON-iOAHCf AllUi. · . ..... . .. .

.... I

- B -

an area of 0.1 square mile or less is 3.0 square miles per tornado, or

approximately the same as Thom's 2.82 square miles per tornado.

In an analysis of tornadoes in the Washington-Oregan-Idaho area, J. L.

Jaech (Ref. 6) found the mean path area to be 0.48 square mile. The dif

fe~ence between this value and Thom's and Fujita's values appears to be

real and to mean that tornadoes in the Northwest (during the period 1950-

69) had a smaller average damage area than the average in the Midwestern

States (during the period 1953--62).

4. Tornado Strike Probability

The probability (Ref. 3), P , 1:hat a tornado will strike a particular s

location during one year is equal to the tornado frequency, ii, (average

number of tornadoes per year) times the ratio of the average individual

tornado area, 8, to the total urea, A, in which the tornado frequency

has been determined:

p - ~Ca/A) • (1)

As an example, consider (Figurt! 1) th.e one-d~gree square at Des. Moines,

Iowa. The tornado frequency al: this location is 13/10 or 1. 3 per year,

and the area of the one-degree square (latitude 3 400 30'N) is 3634 square

miles. USing Thom's value for the average areas the probab~ity of a

strike is 1.3 x (2.82/3634) or about 0.001 per year (i.e. s once per

thollsand years). Por the Pacific Northwest, the smaller average area

(Ref. 6) and the lower tornado frequency (Ref. 51 would result in a

probability substantially less. than

6) estimates a probability of 6.1 x

at Richland , Washington.

0.001 per year; in fact. Jaech (Ref. -6 10 per year for a plant located

- 9 -

The tornado strike probability for each five-degree square in the con

tiguous United States, based on the land surface area within each square,

was computed using Equation (1) and Pautz' (Ref. 4) I3-year frequency

data (Figure 2), Thom's (Ref. 3) mean path area (2.82 square miles), and

values of A corresponding to the average latitude of the five-degree

square. The results. shown in Figure 4. are generally conservative in

that the value assumed fo r the mean path area is probably larger than

the actual value for many locutious.

In the sparsely populated areas. the values shown in Figure 4 may not be

conservative because the actuul tornado frequency may be greater than

that recorded. Conversely. iIl the more populous areas, the values shown

in the figure may be inflated by multiple recording of the same event.

It is recognized that the computed strike probability is dependent on the

size of the area chosen for the computation and could vary widely because

of the large variation in the observed frequencies of occurrence, especially

in Western United States. The five-degree square was selected to represent

the gross probaBility variation and to minimize the effect of population

on tornado reporting. The derived values were not found to .be very sensi

tive to moderate changes in the size of the square selected for smoothing.

5. Tornado Wind Sp.,eed Probab:i11ty

In order to establish a design basis tornado, it is necessary to consider

the probability tliat a specifj:c nuclear plant will be struck during the

life of the plant and the l1kE!llhood that the windspeed w11l exceed a

given value. Available data eln the frequency of tornado occurrence pro

vide a reasonable. although preliminary, basis for estimating strike

probability, as noted above . Unfortunately, consistent and systematic

observations of the windspeed associated with tornadoes are limited. The

4 ,

4 40

J5 3

3 )

2 5

125 ... 120 ... 115 ... 110 ... 105 . -. 100 .•. 95 .. 90 . - 15 .. -- 7 OJ . - -

" ~o .

N \ Js

'l"~ 31 V.

/ r-1.$

~\$

l{"c-~~-\ ?,. .~~ " 8.5 8.5 40 ) ~

I,

~ ~3 , ~ _1...,f-'8,1 __ 5.!,/ _~

3.3 _ r-7.~

14 105 183

1 1.6-J- \/ Jc\}l {' 50~'

'I

\. >--j 39;:--=----"~ '\ y-128 ~

3.2 11 130_ i- 336- 185 I-'

8.0_ -"\) ,00

"'

~ 10 140

~ 196 ,00 \ \ 117 ,33 --r

'l..L:"-.. L <,E,t' L '5~ 126

-~J ~

~ .

110 10' 100 9' 90 os Figur.4.

TORNADO STRIKE PROBABILITY WITHIN FIVE·DEGREE SQUARES IN THE CONTIGUOUS UNITED STATES (UNITS ARE 10-5 PROBABILITY PER "EAR.)

80 75

4'

I 40

J5

30

2'

-o I

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best data on which estimates of tornado intensity probability can be

based ~ere developed by NOAA c~imatolog1sts in observing 1612 t ornadoes

during 1971 (Ref. 7) and 1972 (Ref. 8). For the entire contiguous United

States, the 16U tornadoes were graded according to their F values. (See

definition in Table 1.> The distribution with respect to windspeed is

presented in Table 2. These data can be considered in terms of the

cumulative number of tornadoes haVing an intensity greater than each F

value as presented in Table 3.

In Figure 5, the,values given in Table 3 are plotted on log-probability

paper. The approximate strair~t line indicates that the windspeed approxi

mately follows a log- normal distribution. Figure 5 also shows (broken

line) the winds peed distribution during 1971- 1972 for all tornadoes occur-21 ring in States west of 105 0 west longitude (121 tornadoes reported) .-

Although the points are indicative of the trend of lower tornado intensities

in the West, the number of st r onger tornadoes observed in 1971-1972 (7 of

F3 intensity, 18 of F2 intensity) is too small to establish a full distri

bution curve. As a consequence, it is reasonable to adopt the distribution

for the total United States (581 tornadoes of F2 intensity or higher) as

an interim {and conservative)windspeed distribution.

6. Probability of Occurrence of Intense Tornadoes

In order to adequately protect: public health and safety. the determination

of the design basis tornado is based on the premise that the probability

of occurrence of a tornado that exceeds the Design Basis Tornado (DBT) -7 should be on the order of 10 per year per nuclear power plant.

For a design basis tornado wit.h the intensity expected at the plant loca

tion, this means assuming the acceptable probability of occurrence for

any single nuclear plant to e~~erience no more than one design basis

1:..1 Four of the States included in the region were not completely west of 1050 west longitude, namely, MOntana, Wyoming, Colorado, and New Mexico.

- 12 -

TABLE 2

WINDS:PEED DISTRIBUTION

No. of Windspeed Classification Tornadoes

F5 (windspeed >260 mph) 2

F4 (207 to 260 mph) 34

F3 (158 to 206 mph) 115

F2 (113 to 157 mph) 430

Fl (73 to 112 mph 710

FO (40 to 72 mph) 321 I

TABLE 3

CUMULATIVE WINDSPEED DISTRIBUTION

No. of Windspeed Classification Tornadoes

F5 and above (windspeed >260 Dlph) 2

F4 and above (>206 mph) 36



Fl and above (>74 mph) 1291

FO and above (>40 mph) 1612

Percent of Total

0.12

2 . 1

7.2

26.6

44.0

19.9

Percent of Total

0.12

2.2

9 . 3

36.0

80.0

100.0

-:I: .. !. c w w .. '" c z s:

1000 9 8 7 6

5

4

3

2

100 9 8 7 6

5

4

3

2

FOR ENTI RE CONTIGUOUS U.S.

..... 0 .... " .... . ..........

FOR ALL STATES WEST OF 105° W LONGITUOE 0 ....

10 , ,

0.1 0.2 0.5 I 2

Figure 5.

5 10 20 30 40 50 60 70 80 90 95 98 99 PERCENT PROBABILITY

PERCENT PROBABILITY OF EXCEEOING ORDINATE VALUE OF THE WINDSPEED

~

w I

- 14 -

tornado strike per ten udllion years . Based on a probability of occur--7 renee of 10 per year and the tornado strike probability, Ps ' as shown

in Figure 4, it is possible to compute the acceptable intensity probability,

Pi' using the following rela tionship:

P P . < 10- 7 (2) S 1

Once Pi is determined, the appr opriate windspeed may be determined by use

of Figure 5 .

Using Figures 4 and 5 (for the entire contiguous U.S . )~ the average tornado

intensity with a 10-7 probability per year for each liS - degree square" was

calculated and is shown in Figure 6. Examination of Figure 6 shows that

the only region with calculated tornado windspeeds equal to or less than

300 mph at the 10- 7 per year probability level 15 west of 1050 west longI

tude. The approximate r egion with calculated ~ndspeeds between 300 and

350 mph is north of 35 0 north ~atitude and east of 85 0 west longitude (the

Northeastern United States). 1be calculated windspeeds for the four

5-degree squares north of the 45th parallel and between 85 0 and 105 0 wes t

longitude range from 310 to 360 mph. The entire remaining area of the

contiguous United States has c~tlculated tornado windspeeds greater than

350 mph.

7. Regionalization of Tornado Criteria

Since the tornado windspeed dat:a obtained using the probabi.li.ty method

a re based on only a bwo-year period of reco r d. which mayor may not be a

representative sample , at least: one independent method should be used to

verify the results so that regional tornado criteria can be developed .

The tOnlado frequency varies w:i.th location, as shown in Figures 1. 2. and

3. A good correlation between tornado occurrence,and the dewpoint (tempera

tur e at which the air is satur<:lted with water) at the ground level has been

12' 120 II, 110 10' 100 ., .0

4'

h~l ~ 8' 80 1S 70

1 \

.,

3\0 \ ~/~ . ~ ,

290 290 ~34~0-+----z ..--.-/

240 70 380 - 260 280 300 360 380 3 \

W7 350 A2 310 3~\l ~

40'tt-t--_-I-

\ -"t-_~ i80 -\ II

250 /J 260-u 3' \

/ 290 - ! 300 /370 301-1 290 370 380

n

~1 300 -I- !to d-- 400 ~ 380 )\;L 370 340 340

/:: \

" 360 361 ~

\...: Y

370 2S

370\ ",II

-"L I I I

115 110 105 100 .5 .0 8S 80 1S

Figura 6. CALCULATED TORNADO WINDSPEED BY FIVE·DEGREE SQUARES FOR 10-7 PROBABILITY PER YEAR

'5

,40

35 U; I

30

1-12 '

•

- 16 -

found (Ref. 9). The surface dewpoint indicates the water vapor content of

the troposphere when the air :i5 well mixed. Latent heat generated is

a primary source of energy contributing to tornado development and severity.

The correlation suggests the use of regional dewpoint temperature data to

refine the definition of tornado regions.

A map of maximum persisting 12-hour dewpoints at a constant pressure in

Figure 7 (Ref. 10) shows that high dewpolnts occur eastward of the Rocky

Mountains due to lower elevations and air trajectories from the Gulf of

Mexico. There also is a secondary lobe of maximum persisting dewpoints

in Arizona and New Mexico due principally to the advection of warm moist

air flaw from the Gulf of California and the portion of the Pacific Ocean

offshore from Mexico. The calculated tornado intensities (Figure 6)

generally tend to follow the same pattern as t he maximum persisting dew

points except in California. Tornadoes near the west coast are probably

initiated by different energy mechanisms.

The conclusion drawn from the maximum persisting 12-hour dewpoint and

tornado intensity analyses is that the contiguous Ubited States can be

divided into three basic regions with respect to gross tornado-producing

effects. Figure 8 shows these regions.

8. Regional Variation in Ma~imum Windspeed

Owing to the lack of actual wi.nd measurements, the maximum windspeed

associated with tornadoes has been determined by examination of damage

and displaced objects, by motion picture records when available. or by

theoretical estimates.

The average tornado windspeed. for Region I (Figure 8). as determined by

the pr obability method in Figure 6, is about 360 mph, which compares

exactly with the design basis tornado which has been in common use east

of the Rocky Mountains (Refs. II, 12) (i.e .• 300 mph rotational speed

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-18-

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- 19 -

plus 60 mph translational speed). The average windspeed value for Region

II, by the probability method, is near 290 mph, which appears to be

reasonably conservative. Fujita has estimated maximum tornado windspeeds

:In the Washington-oregon-Idaho area (Ref. 5) , which is a mountainous

region interspersed with plains. He concluded that the maximum speed

would be 200 to 225 mph (including both rotational and translational

speeds) in certain "alleys" in the tristate area and 175 elsewhere . He

has also estimated maximum tornado windspeeds in the southernmost Rockies

(Ref. 13) and concluded that the maximum windspeed in the mountains would

be ZOO mph. These areas bracket Region tIl in Figure 8. The average of

the calculated windspeeds fro'm Figure 6 in Region III is about 270 mph .

It may be concluded that the probability method produces conservative

results owing to the use of windspeed probabilities based on the entire

contiguous United States and that Fujita 1 s estimates may not be conserva

tive enough because of the small sample of data.

Based on these results . i t may be concluded that design corresponding to

the wiodspeeds in Table 4 produced by tornadoes provides an acceptably

law level of risk for nuclear power plant failure due to tornadoes.

TABLE 4

REGIONAL lURNAOO WINDS

Maximum Rotational Translational Region Speed (mph) Speed (mph) Speed (mph)

I 360 290 70

II 300 240 60

III 240 190 50

Since the data are in terms of maximum ~dspeed, the speed for each

region was divided into two components consisting of a rotational speed

and a translational speed. using the same proportion used in the original

290 mph plus 70 mph criterion. The 70 mph translational speed was esti

mated from maximum translatio:nal speeds recorded (Ref. 14) in Region 1.

- 20 -

9. Derivation of Pressure Drop, Due to Tornadoes

One of the simplest models in use is the well-known cyc1ostrophlc wind

equation (Ref. 15). This equation was used to determine maximum pressure

drop for the design basis toma~do fOT each region. It expresses the

balance between the inward radial pressure force and the Durward centrifugal

force present in a tornado. Generally, it. is accepted as a good approxi

mation, particularly outside the ground boundary layer. The cyclostrophic

equation in combination with a Rankine vortex velocity profile previously

has been used (Ref. 13) to calc:ulate the pressure prof tIes for tornadoes.

A form of this equation is

(3)

wher~ p is pressure, t is time ~ T 1s translational speed, p 1s density

of air. 'I'm 1s radius of maximwll rotational windspeed, and Vm 1s maximum

rotational windspeed. Assuming r is typically 150 feet for intense m

tornadoes and p is an average sea-level density, the values given in Table

5 for pressure drop and rate 0:1: pressure drop were calculated from equation

(3) •

TABLE 5

REGIONAL PRESSURE DROP AND PRESSURE DROP RATE

Rat. of Total Pressure Pressure Drop

Region Drop (psi) (psi/s.c)

I 3.0 2.0

II 2.25 1.2

III 1.5 0.6

-21-

10. Conclusions

An analysis of tornado data has been performed for the purpose of region

alization of tornado design criteria . It can be concluded that the con

tiguous United States can be subdivided into three tornado intensity

regions. While variations within each region are anticipated , the current

data do not permit definition of more than three general regions at this

time. When sufficient data become available , these regions will b e reex

amined . Other tornado models such as the suction vortex theory of Fujita

(Refs . 16 and 11) and the two-cell model theory of Kuo (Refs . 18 and 19)

also are being investigated .

Nuclear power pl ants designed to these regional criteria should have an

acceptably low level of risk of tornado-induced plant failures which may

be hazardous to the public.

11. References

1. T. T. Fujita, ''Estimates of Areal Probability of Tornadoes from

Inflationary Reporting of their Frequencies, " Satel lite and

Mesometeorology Research Project (University of Chicago) , SMRL

Research Paper No. 89 , October 1970 .

2. T. T. Fujita, "A Proposed Characterization of Tornadoes and

Hurricanes by Area and Intensity," SMRP Research Paper No . 91,

1970 .

3 . H. C. s. "nlom, " Tornado Probabilities, " Monthly Weather Review ,

October-December 1963, pp . 730- 736 .

4. M. E. Pautz (Ed .) , "Severe Local Storm Occurrences, 1955-1967,"

u. S. Dept. of Commerce, Environmental S.cience Services

Administration , ESSA Tech. Memo. WBTM FeST 12 , September 1969 .

- 22 -

S. T. T. Fujita, "Estimate of Maximum Wind Speeds of tornadoes in

Three Northwestern State:;," SMRP Research Paper No. 92, December

1970.

6. "Statistical Analysi.s of Tornado Data for three Northwestern

States," a report submitted by J . L. Jaech, Staff Consultant of

Jersey Nuclear Co. (Ric~land. Washington) to R. Nilson . Manager

Quality Assurance and Licensing Dept. of Jersey Nuclear, December 29,

1970.

7 . T. T. Fujita, "F-Scale C:lassi£ication of 1971 Tornadoes." SMRP

Research Paper No. 100, 1972.

B. T. T. Fujita and A. D. Pearson, "Results of FPP Classification

of 1971 and 1972 Tornadoes," Eighth Conference on Severe Local

Storms, Denver. Col . , October 15- 16, 1973, pp. 142-145.

9 . Personal communication between E. H. Markee, Jr. and Mr . J. Galway,

National Severe Storms F·orecast Center, NOAA, Kansas City~ Kansas.

1972.

10. " Climatic Atlas of the United States." Environmental Data Service.

ESSA. June 1968 .

11. ANSI Nl77 Draft Standard. "Plant Design Against Missiles." March.

1973 Draft.

12. "Tornado Considerations," USAEC Reactor Technology Memorandum

No . 1. (internal memorandum) April la, 1968.

- 23 -

13. T. T. Fujita, "Estimate of Maximum Wind Speeds of Tornadoes in

Southernmost Rockies,1I SMRP Research Paper No. 105, June 1972.

14. L. V. Wolford, "Tornado Occurrences in the United States,"

Technical Paper No. 20, U-. S. Weather Bureau, 1960.

15. J. D. Stevenson, !!Tornado Design of Class I Structures for

Nuclear Power Plants," Proceedings of Symposium on Structural

Design of Nuclear Power P'lant Facilities, pp . 409- 417 , December

1972.

16. T. T. Fuj ita, "The Lubbock Tornadoes: A Study of Suction Spots."

Weatherwise. 23, No . 4, p'p. 160-174, August 1970.

17. T. T. Fujita, "Proposed ~lechanism of Suction Spots Accompanied

by Tornadoes , " SMRP Reseclrch Paper No . 102, 1972 .

lB. g . L. KUD, "On the Dynamj~cs of Convective Atmospheric Vortices,"

J . Atmos. Sci., 23, pp . J~5-42s January 1966 .

19 . H. L. Kuo, "Axisymmetric Flows in the BOlmdary Layer of a

Maintained Vortex," J. Atmos . SeL, 28, pp . 20-41. January 1971.