MONTHLY WEATHER REVIEW Editor, EDGAR W. WOOLARD
VOL. 70, No. 4
W. B. No. 1364 APRIL 1942 CLOSED JUNE 3, 1942 IWUED JUNE 25,1942
THE DEVELOPMENT AND TRAJECTORIES OF TORNADOES
BY J. R. LLOYD
[n. 9. Weather Bureau, Wmhlngton, March, W 2 ]
An excellent summary of the characteristic phenomena of the tornado, and an account of its genesis and develop- ment, are given by W. J. Humphreys in his treatise Physics
of the Air, 3 ed., pp. 218-224. The main object of the present paper is to put on record an investigation of the
syno tic data for two periods in March 1938 that com-
posed by Humphreys. Figures 1, 2, and 3 show the surface synoptic situations at three different hours on March 15, 1938. The surface cold front associated with the LOW over central Kansas was accompanied by an upper-air cold front that pre- ceded the surface front by from 75 to 100 miles or more. This up er front resulted from Marine Polar air over- runnin barine Tropical air; see figure 4
As siown in figure 2, by 12:30 p. m., C. S. T., the cold front aloft had, at its northern end, begun to move forward over the cold air mass a t the surface to the north- ward and northeastward of the center of the LOW; that is, this cold air mass was shallow and was being overrun by the Marine Tropical air from t.he south, whde this over- running Marine Tropical air was in turn being overrun from the south-southwest by subsiding Marine Polar air that originally came from the Pacific. There is a dif- ference of around 30 percent between the dew oints in the
Missouri, Arkansas and eastern Texas, and the dewpoints in the Marine Tropical air mass to the eastward. Figure 5 shows the upper-air soundings from El Paso and Shreveport taken at 3 a. m., C. S. T. The sounding at Shreveport shows very moist Marine Tropical air with a stable lapse rate from the surface up to a marked tem- perature inversion at 5,200 feet, above which is found dry Superior air with a steep lapse rate. The air column at Shreve ort is convectionally quite unstable. The sound- ing at !& Paso indicates dry subsiding Marine Polar air
with evidence of considerable stratification in the lower portions of the ascent. This air mass is considerably colder p a n the Marine Tropical mass to the eastward at Shreveport. As this Marine Polar air mass moved east- ward and descended the eastern slopes of the Rocky Mountain plateau it heated adiabatically in the lower layers but remained considerably colder aloft than the Marine Tropical air to the eastward that it displaced. Figure 6 shows the hourly progressions of the upper-air cold front, and of the six tornadoes that occurred on it. The second tornado, which occurred about 1:30 p. m. near McPaul in extreme southwestern Iowa, is of particu- lar interest because it formed to the northward of the center of the LOW, in the surface cold sector, and moved from southeast to northwest. When its occurrence was reported to the Weather Bureau forecast center at Chi-
, the writer (then stationed at tha€ office) a t once
plete r y substantiates the conception of the tornado pro-
Marine Polar air over the western and centra Y portions of
r erred that it must have moved from southeast to
M-1
northwest; this deduction was based on the hypothesis that tornadoes develop on upper-air cold fronts and move up these fronts approximately at the speed of the wind in the warm sector of the cyclone j u s t ahead of the upper- air cold front. The cold front aloft on which the McPaul tornado occurred then lay slightly north of west by slightly south of east, and in that locality was moving slowly northward with winds in the warm air ahead of it blowing from the east-southeast. The writer’s deduction was later c o n h e d by the Weather Bureau section director of the State of Iowa in reply to a request for information on
the McPaul tornado, as follows: “One of the most inter- esting things about this tornado, and it appears that it was truly a tornado, is the fact that it moved from south- east to northwest. I cannot recall any other such direction of tornado movement in Iowa.” This section director had had many years of experience in that state prior to the occurrence of the McPaul tornado. The trajectories of all the other tornadoes that occurred in connection with the upper-air cold front of March 15
were from southwest to northeast; the portion of the cold front on which these five tornadoes occurred lay generally north-south; the winds behind the cold front blew from a
general westerly direction, and the winds ahead of the front blew from a general southerly direction. I t should be noted particularly that the trajectory of the first tornado curved from due northeastward to northward in Illinois toward the end of its existence, corresponding to a
change in the orientation of the upper-air cold front from a north-south direction, where the tornado originated, to a northwest-southeast direction. Figures 7 to 12, inclusive, are corresponding charts for the conditions on March 30, 1938, when no fewer than 12
tornadoes occurred. On this occasion, the LOW centered over central Kansas, was accompanied by a surface cold front extending southward through Oklahoma and thence southwestward through western Texas, with an upper-air cold front curving southeastward to near Wichita and thence south-southwestward through central Texas, some
100 to 200 miles in advance of the surface front. The difference between the dewpoints in the Marine Polar air mass to the west of the cold front and in the Marine Tropical air mass to the eastward is even more marked than on the weather maps of March 15, and the tornadoes that occurred in connection with this cold front were more numerous and more violent. It will be noted in figure 10 that the winds in the Polar Pacific air mass are for the most part blowing directly from the west, while the winds in the Marine Tropical a x mass in advance of the front are blowing from the south and southwest and at higher velocities. The sounding from Shreveport, figure 11, shows very moist Marine Tropical sir up to a tpmperature inversion at 5,400 feet, above which is found dry Superior air,
65
MONTHLY WEATHER REVIEW APRIL 1942 66
SCALE OF MILES 0 50 100 200 900 400 500
I
I, A J N
FIOWBE l.-8urlace aynoptic conditions, March 16,1938, et 6:30 a. m., 0. 8. T.
APRIL 1942 MONTHLY WEATHER REVIEW 67
SCALE OF MILES
0 5 0 1 0 0 200 300 400 500
I
WOWBE 2.--Burfaee smoutlc conditions. March 16. 1838. at 12% a. m.. 0. 8. T.
68 MONTHLY WEATHER REVIEW APRIL 1942
SCALE OF MILES
0 5 0 100 200 900 400 500
I
I
1
A. J.k
B~arr~r 3.-8urfaee synoptlc condlrlnm, March 15, IPS, at 630 p. m., C. 8. T.
APRIL 1942 MONTHLY WEATHER REVIEW 69
similar to the condition at Shreveport on March 15; and the temperature inversion at the top of the Marine Tropi- cal air mass is at approximately the same elevation. The sounding from El Paso shows dry subsiding Marine Polar air with a steep lapse rate in the lower half of the ascent, and the Marine Polar air was considerably colder than the
Fioorrr I.-Wind dlrecllons and spreds nlon at 4,000 fret, 4:OO a. m.. C. S. T.. March IS, 1838. Broken line indlcates approximate positlon of upper-air cold front.
Marine Tropical air at Shreveport, as was also true on March 15. The sounding from Oklahoma City shows
moist Marine Tropical air up to about the same height
as the temperature inversion a t Shreveport, above which is found dry air that is somewhat colder than the dry air above the temperature inversion at Shreveport. How- ever, a t Oklahoma City no temperature inversion is found between the Marine Tropical air mass and the dry air mass above it. On both March 1.5 and March 30 the Marine Tropical air mass was over-lain by a dry Superior air mass, an excellent setup for great convective instability under the action of a steep cold front. All of the 12 tornadoes that occurred moved from southwest to northeast; in each case the cold front aloft lay in a general southwest-northeast direction, with the winds aloft. in the Marine Polar air mass blowing from the west, and in the Marine Tropical air mass from the south or southwest. Figure 13 shows a cross section through an ideal air mass system that is typical of the situations that occurred on March 15 and March 30. A mass of Marine Tropical
air, domed up in the center, is overlain by a mass of
Superior air. The oastern ortion of the Marine Tropical
Polar air, and a mass of Marine Polar air that is changing air mass rests on the shou H der of a mass of transitional
FI~WE 5.-Upper-alr aoundings from El Paso and Ghrevewrt, 3:OO a. m., C. 8. T. March 15,1038.
to Superior air through the process of subsidence is encroaching on the Marine Tropical air mass from the west. The lower stratum of the subsiding Marine Polar
91- 90. I
FIGURE (I.-Development and progression of the upper-elr cold front and 01 the slx tornadoes that occurred on It, March IS, 1938.
mass is somewhat warmer than the Marine Tropical mass that i t is displacing, but. the intermediate and upper strata of the subsiding Marine Polar air mass are consider-
70 MONTHLY WEATHER REVIEW APRIL 1942
APRIL 1942 MONTHLY WEATHER REVIEW 71
SCALE OF MILES
0 50 100 200 300 400 500
DL B 1 -< . . .. ,^ .
FXQWE 8.--Burlam spoptic mndltiollr, March 30,1888, at 1’230 p. m., 0.8. T.
APRIL 1942 72 MONTHLY WEATHER REVIEW
I J. P. S.
FIOWE ~.--Sutleeespnoptlc condltlods, March a0,leaq at e311 p. m., C. 8. T.
APRIL 1942 MONTHLY WEATHER REVIEW 73
ably colder than the Marine Tropical mass that it is displacing. From the foregoing, and from close observation and study of many other weather situations in connection with which tornadoes have occurred, the writer has drawn the following conclusions:
1. Tornadoes appear to occur only in connection with upper-air cold fronts. These upper-air cold fronts may be of either of two t,ypes: (1) The convent'iond type caused by the interaction of Marine polar and Marine Tropical air masses; or (2) t.he precipitation-induced type, or pre-cold-frontal squall line l , caused by the inter- action of a precipitation-cooled mass of air with a Marine
FIGUEE IO.-Wind directions and speeds aloft at 4,000 ket, March 30, 19% at 4 p. m.. C. 5. T. Broken line indicates approximate position of upper-air cold front.
Tropical air mass. The latter type always occurs in connection with thunderstorms which appear to be set up in connection with pre-cold-frontal horizontal conver- gence in a Marine Tropical air mass, The rain and hail
that occur in the thunderstorms generated in the pre-cold- frontal zone cool the air through which they fall, particu- larly in the upper and middle levels, to considerably below the temperature of the air in the Marine Tropical air mass ahead, where no precipitation has et occurred.
ally moves along rapidly, developing a squall line that exhibits most of the characteristics of a true air mass
There is thus built up a mass of denser air d oft that usu-
I H T Harrison and W K. Orrendorf on the "Pre-Coldfrontal Squall Line" in Metebro&icsl Circular No. i6 of the U. A. L. T. C. Meteorology Department, March 1.
1w1.
upper-air cold front, often causing violent thunderstorms and an occasional tornado. A marked charactcristic of these precipitation-induced or " squall line" upper-air
PSEUWhIAEAnC DUORAM
FIGURE ll.--Upper-air soundings from El Paso. Oklahoma City and Shreveport, March 30,1938, at 3 .a a. m., C. 6. T.
cold fronts is that they dissipate in a few hours, whereas the true upper-air cold front usually persists for several hours.
Tornadoes that occur in connection with true mass upper-
FIGUBE 12.-Development and progresston of tornadoes on upper-air cold Imnt. March 30,1838. C. 8. T.
air cold fronts usually occur in groups or families number- ing anywhere from 2 or 3 to as many as 12 or 15 in connection with a single front; while tornadoes that occur
74 MONTHLY WEATHER REVIEW APRIL 1942
in connection with precipitation-induced cold fronts, or pre-cold-frontal squall lines, usually occur singly or occa- sionally in pairs. 2. Tornadoes occur by far most frequent,ly in the surface warm sector of the cyclone; but they may occasionally occur even north of the center of the cyclone, and even in the surface cold sector of the LOW, as in the case of the tornado at McPaul, Iowa, on March 15, 1938, which moved from southeast to northwest. 3. Tornadoes form on cold fronts aloft, remain on the upper-air cold front throughout their existence, and move
FIGWE 13.-Cross section of air-mass setup favorable for development of tornadoes.
up the front with approximately the speed of the wind in the warm air mass immediately ahead of the front, which usually blows approximately parallel to this front. In other words, the direction and speed of movement of the tornado is represented, to a first approximation at least, by the resultant of the directmion and speed of move- ment of the front (or the wind component pushing the upper-air cold front along) and the direction and speed of the wind immediately ahead of the upper-air cold front, as shown in figure 14. The heavy curved line in figure
14A indicates the usual relative position of a cold front aloft in the great central valleys, with winds behind the
I
. 1-1
3Smp.h. f -
50mph.
-A - FIGURE 14.-Determination of the rate and dlrectlon of motion of a tornado: (A), the usual conditions resulting in a trajectory irom southwest to northeast; (B), the con- ditions resulting’in the exceptional McPsul tornado.
front blowing from the west, and winds ahead of the front blowing from the south, always resulting in a tornado trajectory from southwest. to northeast. The heavy curved line in figure 14B indicates the relative position of the cold front aloft that occurred in connection with the McPaul, Iowa, tornado. The converging winds from the south-southwest behind the front and from the east- southeast winds ahead of the front resulted in a tornado trajectory from southeast to northwest. The winds in the colder air mass behind the upper air cold front may flow approximately normal to the front, or almost parallel to it over a portion of the front, but the winds in the warm-air mass ahead of the front on which a tornado will
develop usually flow approximately parallel to the upper air cold front along a considerable length of the front. 4. The lapse rate in the cold Marine Polar air mass to the westward of the warm, moist Marine Tropical mass is about the same as the la se rate in the Marine Tropical
up to some 1% kilometers, above which the la se rate is
Marine Tropical mass to the eastward. During the day the Polar mass, which has really become a Superior mass due to subsidence, becomes warmer in its lower levels up to 3,000 or 4,000 feet than the Marine Tropical air mass ahead of it. Above the level where the la se rate in the Marine Polar mass be the air be-
ahead of it, usually being much colder in the higher levels. This colder air from the intermediate and upper levels in the Marine Polar mass is usually dry, and flows out over and above the lower portion of the Marine Tropical mass; cutting off the top portion and lifting it very ra idly, due
derstorms and occasional tornadoes develop where vertical convection is strong enough. Such thunderstorm and tornadic action dies out abruptly as soon as the cold front aloft has “cut the top” off the dome of Marine Tropical air and has entered the mass of dry Superior air that usuall lies to the eastward of the Marine Tropical air in SUC
cases. 5. The seasonal migration of tornadoes from the deep South in the late winter and early spring to the Canadian border in midsummer is due to the fact that the Marine Polar masses from the Pacific become too warm aloft in the southerly latitudes for the production of u per-air
farther north as the season progresses, causing tornadoes to migrate slowly northward as the season advances. This migration moves back southward again in autumn, as evidenced by the fact that tornadoes occasionally occur in autumn in the Mississippi Valley. However, such tornadoes in autumn occur only infrequently, probably because the Marine Polar masses at that season of the year are usually too warm aloft for the production of over- running u per-air cold fronts-i. e., they do not, exce t
that are steep enough, compared with the lapse rates in the adjacent Marine Tropical masses to the eastward, to form a cold front aloft that in overflowing the lower layers of the Marine Tropical masses “clips off” their tops and sets up violent thunderstorm or tornado action. It appears that a steep lapse rate aloft in the Marine Polar mass is an absolute necessity for the production of tornadoes of the group or family type and that they occur only in connection with Marine Polar cold fronts. The writer has yet to find a tornado of the group or family type that has been caused by the interaction of Conti- nental Polar and Marine Tropical air masses.
6. The rotation of the winds in the funnel cloud of the tornado must always be counterclockwise in the Northern Hemisphere, not due to the fact that the rotation of the earth prevents tornado vortices from whirling clockwise (since the vortices of waterspouts and dust whirls may whirl either clockwise or counterclockwise), but due to the fact that the movement of the air in the Marine Tropical masses in the Northern Hemisphere is always from a
southerly direction, and to the right of the Marine Polar masses from the eastern sides of which the upper-air cold fronts originate and in which the air movement is usually from a westerly quarter.
mass to the eastward, an a has about the same temperature
considerably steeper in the Marine Polar mass t g m in the
s to stee en sharpy, 7 comes increasingly coder ff“ than t R e Marine Tropical air
to the steepness of the cold front aloft; and vi0 P ent thun-
K
-
cold fronts; such upper-air cold fronts form fart E er and
occasiona lr y, have lapse rates at middle and upper leve 7 s
APRIL 1942 MONTHLY WEATHER REVIEW 75
dtltude (meters) m. s. 1.
Burface .___.._.
600 _______..___
1,000 1,600 2,000 __.....____
2,600 .__.....__.
3.000 ___.._...._
4,000 __._______.
6,000 ___________ 6,000 ___________ 7,000 ___...___..
9,ooO ...________
10,ooO ._._______
11,ooO ..._______
12,000 ..._._____
13,000 .._...____
14,000 _.________
16,000 ..________
16,000 ..._______
18,000 ._._._____
19,000 .___...__.
8,000 ___________
17,000 ._.__
~
____
Under these conditions, the southeasterly components of the southerly winds in the Marine Tropical masses interact with the northwesterly components of the westerly winds in the Marine Polar masses, to form vortices that necessarily must always have counter- clockwise motion, as shown in figure 15A. Another way in which it is thought that tornado vortices may be- and, in the writer’s judgment, perhaps usually are- formed is shown in figure 15B. Here, as the winds from the west in the Marine Tropical air mass approach the cold front they are slowed down considerably and deflected to the left, due to frictional drag as they come in contact with the wall of Ma.rine Tropicnl air that is moving rapidly from a southerly direction. The winds in the Tropical air mass immediately in advance of the cold front usually move with higher speed, due to pre-cold frontal convergence, than do the westerly winds in the Polar mass convergihg on the front. The Marine Polar air would then flow alongside and adjacent to the edge of the Marine Tropical air mass, and in the same direction as the flow of the Marine Tropical mass, at a speed considerably less than the northward speed of the Tropical air. Swirls would therefore develop at points along the interface between the two air masses, and where these swirls occurred in connection with the rapidly ascending air currents on the edge of the Marine Tropical mass, tornado vortices could easily be set up. Similar1 the winds in tornadoes that occur in the Southern aemisphere always have clockwise motion. 7. The violence of the tornado will depend largely upon three factors: (1) The strength of the opposing winds immediately behind and immediately ahead of the front which set up the whirl around the vortex, (2) the area and degree of saturation of the uprushing mass of Marine Tropical air that is disturbed by strong local convection
airplanes and radiosondes during April 1948
Btntions with elevntions in meters above sea level
-
Brownsville, Tex. Charleston, 8. C. Albuquerque, N. Mex. Atlanta Ga. Bismarck, N. Dak. Boise, Idaho. BWnlo, N. Y. (1,620 m.) (300 L.) (506 m.) (864 m.) (6 m.) (221 m.) (14 m.)
b & B P 3 s 5
-3 0s 5
El g g a g g
!$! 5 g k?iz 5
z PI E 2 2 PI 6 2 2 PI
-------_-
--__L_-_______
--___--___
30 834 12.9 46 30 984 15.9 67 30 963 7.3 73 30 911 10.9 Bo 30 1,013 21.8 84 30 991 7.7 72 30 1,018 14.9 84 ___- _____. _______ ____ 30 962 17.6 51 ___. _____. .___... ___. - .. .-.... ___.___ ___. 30 966 19.4 82 30 969 9.8 66 30 962 17.1 Bo
.._________.____________________ 30 907 14.4 50 30 898 7.3 66 30 897 12.1 66 30 go3 18.0 65 30 902 7.5 63 30 907 1L1 M
.....______._______.___._______. 30 854 11.6 48 30 844 6 .3 63 30 844 9.8 64 30 862 16.8 56 30 819 4.8 66 29 864 10.9 62
30 787 12.0 45 30 804 8.7 48 30 794 3.6 62 30 786 6.8 66 30 802 15.2 46 30 798 2.1 68 29 804 7.7 60 30 7M) 8.4 45 30 757 6.8 48 30 746 1.1 62 30 747 1.7 Bo 30 766 13.0 37 30 760 -0.7 68 29 767 6.4 49 30 706 4.4 48 30 712 2.7 4 0 3 0 701 -1 .6 6 0 3 0 702 -2 .2 6 4 30 712 10.2 3 3 3 0 704 -3.2 6 6 2 9 712 2 7 45 30 623 -3.6 63 30 628 -3.1 40 29 618 -7.6 69 30 618 -9.2 66 30 631 3.7 33 30 620 -8.8 Bo 28 628 -3.3 41 28 648 -11.0 67 27 663 -9.6 36 29 643 -14.2 65 29 642 -16.9 64 28 657 -3.8 36 30 b14 -16.0 64 28 653 -9.4 41 28 481 -17.8 54 27 486 -16.6 33 29 474 -21.3 63 29 474 -22.8 60 28 4Bo -11.2 39 30 476 -21.1 60 !?8 486 -16.4 39 27 4 3 -24.4 60 27 424 -23.7 32 28 413 -28.8 60 29 413 -29.7 69 27 430 -18.1 37 30 415 -28.6 48 28 424 -23.7 38
26 316 -38.7 .___ 26 319 -38.6 30 27 310 -43.9 ..__ 29 301) -43.6 .___ 28 328 -32.0 33 30 311 -43.0 .-.. 27 320 -38.4 35 28 273 -45.5 .___ 25 276 -45.6 ____ 27 266 -51.2 ___. 29 288 -60.0 .--. 26 283 -39.2 ___. 30 287 -49.8 ___. 27 2 7 K -46.6 _.__
24 235 -51.9 .___ 26 237 -51.4 ____ 28 228 -56.4 .-.. 28 228 -55.7 .__. 26 244 -46.8 ____ 30 229 -65.0 ____ 27 238 -5 2 0 ____ 23 201 -56.3 ..-- 23 203 -56.8 ____ 26 196 -69.4 ___. 28 194 -68.9 ..__ 22 210 -64.1 30 196 -58.9 ____ 26 a03 -66.6 ____ 20 171 -58.9 .__. 23 173 -69.7 ____ 25 166 -60.2 ____ 28 166 -69.3 ..__ 19 179 -69.9 ____ 30 167 -60.4 ____ 26 173 -69.1 .__- 19 145 -69.8 ____ 22 147 -60.3 ____ 24 141 -57.8 .___ 26 141 -67.9 .___ 17 152 -64.1 .._. 28 142 -68.7 ____ !2d 148 -Bo.O ____ 19 124 -61.4 ____ 21 1% -61.6 .___ 21 120 -57.6 ____ 28 120 -67.3 ____ 17 129 -68.7 ____ 28 121 -68.4 .___ 23 126 -61.3 ____ 18 106 -62.0 ____ 18 106 -62.7 ___. 17 102 -67.8 ____ 23 103 -68.0 ____ 15 109 -71.4 ____ 28 103 -68.7 ____ 22 107 -6 2 6 ____
12 7 6 -6 1 .3 ____ 7 7 7 -6 3 .6 ____ 6 7 4 -6 8 .0 ____ 10 1 6 -6 7 .6 ____ 10 7 8 -7 3 .1 ____ 8 7 6 -6 9 .2 .-__ 14 7 8 -G t 2 _-__ 10 64 -61.6 -... ..-. ...... .~ _.__. ...- .-.. ______ _______ ____ __._ _____. ____. ____ 6 64 -70.1 --.. ____ ______ _______ ____ 6 66 -61.6 .___
27 364 -32.0 48 27 369 -31.1 31 n 358 -38.4 49 ZQ 3% -3.s.6 68 27 375 -25.0 35 30 380 -35.9 47 n 369 -31.0 37
16 8 9 -6 2 .1 ____ 16 go--83.2 ._.. io 8 7 -6 8 .1 ____ 20 8p--68.0 ____ 11 9 2 -7 3 .3 ____ 19 m -s a .s .-.. 16 ~i -a o ____
on the cold front aloft, and which is acted on by the op- posing frontal winds to induce the spiral, upward counter- clockwise motion in the funnel cloud; and (3) the steep- ness of the cold front aloft. It is believed that these conclusions rovide a sound
tornadoes can be forecast once they have been formed.
If, for example, a dense network of tornado-reporting stations were organized, by which a large percentage of
basis on which the trajectories and spee a of movement of
i
FIOUEE 16.-Illustration of theoretical wnCgDt of develoDment of tornado vortiar.
the tornadoes that occur during daylight hours could be observed and reported immediately by telephone to a forecast district center in the area, it would be possible to forecast the approximate trajectory and speed of move- ment of a tornado, once it had been observed. Warning could be given of the approach of tornadoes and severe squall line thunderstorms only for periods of 15 or 20
minutes to perhaps 2 or 3 hours in advance; but com- mercial air-line operators and other interests vitally inter- ested in such storms, and the general public would profit to that extent.