104 



NA TURE 



[June 4, 1896 



THE TORNADO. 



THE exceptionally disastrous and destructive tornado 

 which occurred at St. Louis, in the State of 

 Missouri, shortly after five in the afternoon of May 27, 

 draws more than ordinary attention to this class of dis- 

 turbance, and excites, for a time at least, an interest in 

 such phenomena. These disturbances are by no means of 

 uncommon occurrence in the United States, but it is 

 happily not often that a densely populated city falls 

 directly in the track of the full fury of the storm. 



Such well-known authorities as Ferrel, Finley, and 

 Hazen have devoted much attention to tornadoes, and 

 it is chiefly to the writings of these that we look for 

 information. Several years ago the United States Signal 

 Service published a report of the character of 600 tor- 

 nadoes, and this clearly shows that no season of the year 

 is exempt from their occurrence, but their greatest 

 frequency is in the spring and summer, whilst in winter 

 they are seldom experienced. Their occurrence is more 

 common in April, May, June and July, than in any other 

 months of the year. They almost always occur after the 

 hottest part of the day, the hour of greatest frequency 

 being between three and four in the afternoon, and they 

 seldom begin after six in the evening. The centre of the 

 disturbance is almost always formed in the southern 

 or south-eastern segment of an ordinary area of low 

 pressure, and a study of the weather charts, embracing a 

 large area of the United States, shows that they are 

 often several hundred miles from the centre of the parent 

 disturbance. Those familiar with the formation and 

 behaviour of our thunderstorm disturbances in England, 

 will recognise an analogy to the tornado in their origin 

 and motion with respect to the primary disturbance, of 

 which they are mere secondaries. According to Finley, 

 of the 600 tornadoes upon which he reported, the rotary 

 movement of the whirling cloud was invariably from right 

 to left, or the opposite movement of the hands of a watch. 

 Ferrel remarks that this indicates either that the earth's 

 rotation on its axis, as in cyclones, must determine the 

 direction, or that the atmosphere has numerous whirls in 

 this direction. The progressive motion of a tornado is 

 almost always in a north-easterly direction, and here again 

 there is a resemblance to the ordinary track followed by 

 low-pressure areas in middle latitudes. The velocity of 

 progression of the tornado cloud is said to vary from 7 

 to TOO miles an hour, the average rate being 44 miles. 

 .According to Finley the vortex wind velocities of the 

 tornado cloud vary from 100 to 500 miles an hour, as 

 deduced from actual measurements, and velocities of 800 

 to 1000 miles an hour have been reported. A wind 

 xelocity of 500 miles an hour is equal to about 750 lb. 

 on every square foot. The width of the path of 

 destruction, supposed to measure the distance of sensible 

 winds on the sides of the storm's centre, varies 

 from 40 to 10,000 feet, the average being 1085 feet, as 

 deduced by Finley from a discussion of a large number 

 of instances. The length of the tornado's track varies 

 from 300 yards to about 200 miles, the average being 25 

 miles. The tornado has many features in coinmon with 

 the cyclone, but as experienced in the United States it is 

 essentially different in many points, and in the interests 

 of science it should be kept distinct. The tornado cloud 

 assumes the form of a funnel, the small end drawing 

 near or resting upon the earth, whilst the cloud and the 

 air below it revolve about a central axis with inconceivable 

 rapidity. Tornadoes differ from cyclones mostly in their 

 extent, but both have vertical and gyratory circulations. 

 A cyclone may extend over a circular area of one or two 

 thousand miles in diameter, while a tornado rarely affects 

 sensibly at any one time so great an area as a mile in 

 diameter. In a cyclone the base is so great in compari- 

 son with the height, that the whole mass of gyrating air 

 may be regarded as a thin disc, and consequently a large 



NO. 1388, VOL. 54] 



amount of the force is spent in overcoming the frictional 

 resistances at the earth's surface. In a tornado the 

 height is so great in comparison with the base that the 

 gyratory velocity is almost wholly free from friction. The 

 late Prof Ferrel, who ranks probably higher than any 

 other authority on winds and storms, was of opinion that 

 a cyclone " requires, in addition to the state of unstable 

 equilibrium for saturated air, such a disturbance in the 

 general equality of temperature over a considerable area 

 that there is a central and somewhat circular area of 

 higher or lower temperature, from which arises a vertical, 

 and consequently a gyratory, circulation " ; while the 

 tornado "simply depends upon conditions which give 

 rise to very local disturbances merely." Without doubt 

 the conditions which characterise the tornado are alsO' 

 common to such phenomena as waterspouts, cloudbursts, 

 whirlwinds, wind-blasts, and others of a like nature 



.An excellent descriptive report of the St. Louii cata- 

 sti-ophe appeared in the Daily Telegraphy and is abridged 

 below. The report shows that tlie tornado had many 

 features common to such disturbances. The occur- 

 rence of "three separate and distinct storms,' which 

 subsequently became one, is especially alluded to by 

 Ferrel in his general description of tornadoes. He 

 says : " As the tornado originates in air in the un- 

 stable state, it often happens that there is about ar> 

 equal tendency in the air of the lower stratum to burst 

 up through those above at several places in the same 

 vicinity at the same time. Each of these gives rise to a 

 separate and independent gyration in the atmosphere, 

 and a small funnel where they are of sufficient violence • 

 but generally, as they increase in dimensions and violence 

 they interfere with one another and finally become unite 

 into one." The reported wind velocity of eighty miles an 

 hour appears to be an estimate formed outside of the central 

 area of the storm. In England the wind has attained a 

 velocity of 107 miles for a whole hour, registered at P'lcet- 

 wood in the gale of December 22, 1894, and at Holyhead 

 on February 20, 1877, the anemometer registered an hourly 

 rate of 200 miles for a short time in the gusts. 



The weather at St. Louis nearly the whole of Wetlnesday, 

 May 27, was unusually warm and oppressive. Theie was not 

 a breath of wind, and the people suffered greatly from the heat. 

 About four ii'clock in the afternoon the western horizon became 

 banked with clouds piled one on top of the other, with curlinj; 

 edges tinged with yellow. The sight was beautiful, but some- 

 what terrifying. Then a light wind sprang up, followed by 

 sudden and ominous darkness. 



The gloom deepened, and when the storm actitrlly burst 

 upon the city pitch darkness prevailed. These strange atmo- 

 spheric disturbances had created anxiety among the people abroad 

 in the streets, but not alarm. 



There seemed to be three separate and distinct storms. 

 They came from the north-west, from the west, and from the 

 south-west, but when these reached die river they had become 

 one. 



Before the great mass oT menacing clouds which were hang- 

 ing over the villages of Cliyton, Kernridge, Eden, and Central 

 gave forth their contents funnel-shaped formations shot out of 

 them. Some of these funnels seemed to be projected into the 

 air ; others leaped to the earth, twisting and turning like some 

 wounded monsters. Lightning played about them. There was, 

 in fact, a marvellous electrical display. Then came the stupen- 

 dous outburst. 



Frcmi the great black clouds came a strange, weird, crack- 

 ling sound, at times stronger than ihe incessant peals of thunder, 

 which had from the first been a terrifying feature of the storm. 

 The funnels enveloped the western side of the city, and within 

 thirty minutes of their fir.st appearance on the horizon they were 

 dealing out destruction. 



So irresistible was the storm in its power, and so much greater in 

 its magnitude than any other previously recorded in America, 

 that souie of the staunchest Inisiness blocks in St. Louis, con- 

 sidered absolutely tornado-proof, went down before it as though 

 they were mere barns. Iron girders were torn from their massive 

 fastenings and carried blocks distant. Roofs that were braced 



