Popular Science Monthly/Volume 28/January 1886/Progress in Tornado-Prediction
|PROGRESS IN TORNADO-PREDICTION.|
By WILLIAM A. EDDY.
DURING the first part of 1884 the United States Signal Service began to pay special attention to the question of tornado-prediction. The development of the science was rapid under the active supervision of Lieutenant John P. Finley, having charge of that department of the service. It was found that the public interest in the question was wide-spread, and that, with the aid of voluntary reporters of tornado-phenomena, the possibility of saving life and property had begun to crystallize into a practical scheme. The power to verify predictions could only be obtained from two sources—from the press, and from tornado-reporters, who would voluntarily report the phenomena with some approach to scientific accuracy. The distinctions between a cyclone, five hundred or a thousand miles across the storm-center, a hurricane of more limited extent, and a tornado with a path only one thousand feet in width—all these must be properly classified before any system for the prediction of tornadoes could reach even approximately verified results. The number of tornadoes occurring in the region east of the Rocky Mountains was one hundred and seventy-two in 1884. The average is more than a hundred yearly. Whenever a tornado occurred, blanks specifying the questions to be asked and the method of investigation to be followed were at once sent to the postmaster or to the principal city or town officers in the vicinity of the scene of destruction, asking for facts, and requesting that the observer would kindly volunteer to continue a series of easy meteorological observations for the Signal Service, involving no more outlay of time than the reporter could conveniently spare. The result of this clever device was that the Signal-Office now has upon its books the addresses of more than fifteen hundred reporters in all parts of the country east of the Rocky Mountains, and especially in those States most frequently stricken with destructive tornadoes. Having thus organized a system whereby something like verification of tornado predictions could be attained. Lieutenant Finley then divided the territory east of the Rocky Mountains into eighteen districts. lie at once began making predictions in these districts, thus reaching a series of recorded averages whereby the results became more definite and local until the close of 1884 and the summer of 1885. The predictions in the latter year have become so effective that during the summer of 1886 it is hoped that, by means of signals, hundreds of lives and much valuable property will be saved.
The most striking examples of the knowledge attained regarding tornadoes are found by reference to the following statistics: During 1884, 3,228 predictions unfavorable to tornadoes were made, and of these, 3,201 were verified by reports sent in by tornado-reporters, who are instructed to carefully scan the newspapers as well as note the meteorological phenomena in a given neighborhood. When the conditions are unfavorable for the development of tornadoes, there are no unusual contrasts of temperature, the areas of warm and cold air are neither great nor well defined northward and southward, the winds are variable and not very strong, and the distribution of pressure is about normal.
When we consider that more than a hundred tornadoes occur annually, it follows that the successful prediction of safety for eighteen districts is a very satisfactory indication of the advance made by this science. The result is certainly practical and valuable, as with proper signals shown at telegraph-stations the inhabitants of Kansas, Missouri, and other States, can go to their work free from anxiety and not subject to false alarms on the appearance of every harmless thunder-storm. The predictions of safety are therefore particularly valuable in States in which tornadoes are frequent. When, however, we examine the statistics regarding the actual occurrence of tornadoes, we find the certainty lessened by the fact that the present limited resources of the Signal Service result in defective reports or in none at all from sparsely settled regions. Lieutenant Finley found that of thirty-eight predictions that tornadoes would occur, made in April and June, 1884, eighteen wore verified, and that of nineteen predictions made in June and July, 1885, fifteen were generally verified. In all cases there were violent storms, either tornadoes, hurricanes, or hail. Owing to the extremely local nature of tornadoes, their tracks at times being only a mile or two in length and a few hundred feet in width, it is obvious that many predictions must apparently fail, owing to the fact that the effects are not seen until long afterward, or not at all where there are vast stretches of treeless prairie. It is doubtless true that this failure, due to the vagueness and unsatisfactory nature of the reports, induced Professor T. B. Maury to maintain, as late as 1882, that the prediction of a tornado was a triumph not yet attained by the science of meteorology, though doubtless he believed that success would be achieved at no very distant day. In order that the reader may see some of the reasons for expected progress in this science, let us examine, first, the methods in use by Lieutenant Finley for tracing the movement of air masses, and second, the movement of the air-currents in the tornado cloud, as seen by hundreds of observers.
It is well known that, owing to frequent telegraphic reports, the pressure, temperature, cloud-formation, extent, and movement of immense masses of air are permanently recorded. The conditions favorable to tornadoes are positive and noticeable. The areas of warm southerly and cold northerly winds are well defined, uniform, of large extent, and reach well to the north and south. High contrasts of humidity, abnormal variations in dew-point, the location of areas of barometric minima and maxima, with their lines of actual and probable progressive movement, and especially the velocity and direction of the wind, must be considered and mapped out on special charts. The temperatures are thrown out of their usual equilibrium and normal distribution over an extent at times of two thousand miles of territory. The cold air encroaches far into the Southern States, and the warm air of the South at such times may stream northward during a week or ten days. The movement in readjusting the equilibrium is like two pendulums thrown far apart which swing toward their common center with a force proportioned to the extent of their displacement. But this simple simile only fits the case roughly, because the questions of wind-direction, the location of the moving center of low pressure, and especially the inequality of the displacement of the air-masses north and south, make the problem very complex. Lieutenant Finley says that "the departure from normal conditions of temperature in case of tornado development is from 15° to 50°, but with this abnormal condition of temperature there must be abnormal conditions of humidity, abnormal conditions of pressure, of wind-direction, of cloud-formation and movement."
The most remarkable and interesting feature of the development of tornadoes is the fact that they nearly always form southeast of a moving center of low pressure, and their tracks, scattered here and there, conform closely to the progressive direction of the main storm. For example, on February 19, 1884, forty-four tornadoes occurred in Georgia, Alabama, and South Carolina, but principally in Georgia and Alabama. They developed at a distance of from five hundred to two thousand miles from a storm-center that moved across the northern part of the United States, beginning at the northern extremity of the Rocky Mountains in Montana, thence southeasterly through Dakota, Minnesota, and Wisconsin to Northern Illinois and Indiana, northward through Michigan across Lake Huron, disappearing north of Quebec. This sudden sharp turn of the storm-center southward into Illinois and Indiana seems to have relation to the unprecedentedly large number of tornadoes that developed not far from the South Atlantic coast, extending inland as far as Southern Illinois and Indiana. This southward lunge of a mass of cold, moist air seems to have caused the abnormal conditions of temperature and dew-point, and the high winds necessary to cause the most tremendous exhibition of destructive tornado-power ever recorded by the Signal Service. This invariable location southeast of the storm-center is one of the main peculiarities of tornado development upon which the predictions depend.
One of the best illustrations of the advance made in definiteness in prediction during 1885 occurred on August 3d, in the instance of the tornado at Camden, New Jersey, and at Philadelphia, Pennsylvania. In October, 1885, the writer had sent a short communication to the press of the country, advocating that tornado-signals of either safety or danger be shown during certain seasons of the year at all telegraph stations in States in which tornadoes are frequent. The gentlemen of the press had generally favored the scheme, and one of the editors wanted to know if the tornado at Camden had been predicted. It occurred at 3.20 p. m. (seventy-fifth meridian time), and was very destructive, involving a loss of about half a million dollars' worth of property. The chart used by Lieutenant Finley shows that tornadoes were predicted and their location marked upon the map for the States of Delaware, Southeastern Pennsylvania, and for New Jersey. The tornadoes actually occurred in these States about eight hours from the time of the prediction, which was made on the basis of the 7 a. m. (seventy-fifth meridian time) telegraphic reports. On that day heavy wind-storms were predicted for Vermont, New Hampshire, Massachusetts, New York, Northern New Jersey, and Eastern Pennsylvania. Nature carried out these predictions with as fair a degree of accuracy and with as definite a conformity to location as could be expected at the present primary stage of this science. The accumulations of great quantities of evidence concerning tornadoes have revealed some interesting facts. It is supposed that certain localities in the Eastern States are entirely free from tornadoes, but an examination of Lieutenant Finley's record from 1794 to 1881 shows that they at times approach dangerously near the most unexpected localities. One occurred in New York city, July 13, 1859, and this fact has been further emphasized by the appearance of another at Westwood, New Jersey, October 4, 1885, only twenty-one miles above the city, and not far from the Hudson River. Of the six hundred tornadoes recorded from 1794 to 1881, sixty-two occurred in Kansas, fifty-three in Illinois, forty-three in Missouri, thirty-five in New York, thirty-three in Georgia, thirty-two in Iowa, twenty-eight in Ohio, twenty-five in Indiana, twenty-two in Minnesota, eighteen in North Carolina, eighteen in Pennsylvania, eighteen in Texas, eighteen in Tennessee, fourteen in South Carolina, fifteen in Michigan, fourteen in Alabama, fourteen in Nebraska, fourteen in Mississippi, ten in Louisiana, ten in Wisconsin, nine in Massachusetts, nine in Dakota, nine in Virginia, eight in Arkansas, eight in Maryland, five in Connecticut, six in Kentucky, five in Florida, five in New Hampshire, six in New Jersey, three in Maine, two in Arizona, two in Vermont, and one each in Colorado, California, Indian Territory, Nevada, New Mexico, Montana, Rhode Island, West Virginia, and Wyoming Territory. The above figures are defective, owing to the absence of records in the past, but it may be accepted as an undoubted fact, soon to be demonstrated by the more careful system of investigation to be carried on by an army of tornado-reporters, that the proportion of tornadoes in Kansas, Missouri, Iowa, and Wisconsin is much greater than shown. The real prevalence in these States, as compared with others, is better represented by the record of unusually destructive tornadoes. Of this class Kansas leads with twenty-five, Illinois follows with fifteen, then come Iowa and Missouri with twelve each. The Eastern States disappear from the list with the exception of Pennsylvania three and Connecticut one—the well-known destruction of life and property at Wallingford. Of the six hundred tornadoes, three hundred and four moved from southwest to northeast, and the remainder, with marvelously few exceptions, kept very close to that direction. So comparatively certain is this movement that the tornado-track can be escaped by running southeast, depending, of course, upon the direction from which the storm is seen. Northwest is not so safe a direction to take, because so many move northward, veering very slightly eastward. The length of the track varies from one to one hundred and fifty miles, and the average is thirty miles. The average width of the storm-path is one thousand and eighty-five feet, and the velocity of progression is about thirty miles an hour. The form of the cloud is almost invariably funnel-shaped, varied at times with that of the hour-glass, cone, and inverted funnel, modifications caused by different altitudes and velocities of air-currents. It is estimated that in the center of the funnel the air sometimes attains the enormous speed of two thousand miles an hour. The whirling movement is almost invariably in an opposite direction from that taken by the hands of a clock.
The weather-predictions of the Signal Service are distinct from the tornado-predictions, which involve local treatment that severely tests the science of meteorology. It is true the tornado region follows the usual storm-center along parallel lines, but at a distance of several hundred miles. The tornadoes develop far from the storm-center, and generally under conditions of partial sunshine and cloudiness and high humidity or excess of moisture. The relation of tornado-prediction to the usual weather-service is only in regard to details of temperature, wind-direction, dew-point, etc., as furnished by the general weather reports. The prediction of the movement of the usual storm-center is by no means so difficult as the attempt to even approximately locate the general region where a series of tornadoes will occur, because of the narrow track in which the destructive power is manifested. The officers of the Signal Service are careful to make no rash promises. While knowledge of the phenomena is not entirely complete, yet the advancement of the science is so marked and positive that tornadoes can be predicted for certain parts of States with a degree of average certainty that will, if carried out by the establishment of a system of signals in 1886, prove of very great value to the people. Already insurance companies have been enabled to take millions of dollars of tornado risks, and the more complete knowledge of the average danger for given localities will set the questions of premiums and rates of insurance upon a basis that will be profitable for the people as well as for the companies. The danger in localities will be established by averages, and the amount of precaution necessary will be known, and may be expressed in trustworthy percentages. This will economize expenditure both for insurance and tornado-retreats underground. The protection to life will be a very marked feature of the results attained. The approach of the tornado along its almost inevitable path, of from southwest to northeast, can be seen for fully an hour above the surface of a flat prairie, thus enabling people to get far beyond the reach of its narrow but fearfully destructive path. With this we close the question of the prediction of tornadoes for certain parts of States.
Let us now examine the closer prediction made by the trained observer or tornado-reporter, as he sees the tornado-cloud in process of formation. The question as to whether the furious movement of the clouds is forming the funnel-shape so much dreaded can only be decided by careful study of the sights and sounds described by hundreds of observers. A very important characteristic of tornado air-currents is that the disturbance begins in the upper air. In the "North American Review" for September, 1882, Professor T. B. Maury, in an article entitled "Tornadoes and their Causes," attributes the peculiar movement of our tornadoes to an upper air-current, which at times has been seen to be "moving from the southwest at the rate of one hundred miles an hour." In addition to this, Lieutenant Finley's descriptions of the thirteen tornadoes that occurred in Kansas, May 29 and 30, 1879, give abundant evidence that the southwest air-current forces the contest. Innumerable descriptions show that the cloud in the northwest is heavy, black, and comparatively slow in its movement, until struck by a light, rather smoky, and more rapidly moving cloud from the southwest. Then the clouds rush to a common center, and there is a violent conflict of currents, driving clouds in every direction, up and down, round and round. Clouds like great sheets of white smoke dash about in a frightful manner, with such unnatural velocity that the observer is often panic-stricken, and flees to the nearest cellar for safety. Finally a black, threatening mass descends slowly toward the earth, whirling violently, but still manifesting confusion in form. This soon gives place to the peculiar funnel-like shape, with definite outline so well known. It appears intensely black, like coal-smoke issuing from a locomotive, and its trunk-like form sometimes has a wrenching, spiral motion, like a snake hung up by the head and writhing in agony. As white clouds approach and are drawn into the vortex, the funnel-shaped trunk sways like an elastic column. It sometimes rises, falls, and careens from side to side like a balloon. Branches and trunks of trees, rails, tree-tops, roofs, pieces of houses, straw, furniture, stoves, iron-work, lumber, and other débris are seen flying about in the central part of the cloud, but are gradually drawn upward and thrown out near the top, usually not until the storm has progressed a mile or two farther on from a given point. Dark masses of cloud are seen to shoot downward on either side of the funnel, to enter it just above the ground, and to apparently rush upward through the center and out at the top in a terrific manner. Sometimes the funnel pauses and whirls with apparently increased velocity, reducing everything to splinters, and leaving scarcely a vestige of a house or clump of trees, all being ground comparatively fine and carried away as chaff. At Westwood, New Jersey, October 4, 1885, fully three quarters of a school-house was carried away from the foundation. Its fragments were scattered along the storm's track for about half a mile, and the rest was seen no more. The people at Westwood describe the roar of the tornado as having a peculiar hollow, humming sound. It somewhat resembled the rumbling of cars, or the booming of the sea. The sound is indescribable and unlike any other in Nature. It is so loud that the falling of heavy trees against the side of a house and the crash of falling buildings are lost in the general roar. These facts attest the tremendous rapidity of the air-currents.
In addition to a downward movement of air, there is also a violent reactionary upward movement through the center of the funnel. This center is almost a vacuum surrounded by a cylindrical mass of air of great density and revolving force. Professor William M. Davis, of Harvard College, whose work, entitled "Whirlwinds, Cyclones, and Tornadoes, is well known for its merit and originality, maintains that the destructive power of a tornado is due to the rush of air along the earth's surface toward the vacuum center of the funnel. Some buildings have a stricken, pinched appearance at the top, as if the air had rushed under the edge of a huge cylinder, and swept upward with tremendous power. While it is true that the downward movement predominates, yet the upward movement in the center is equally marked. The iron grip of the tornado-funnel is relieved only by the escape of currents to the upper air through its center, and this again is doubtless due to the decrease of the contrasts of temperature between the opposing currents, thus gradually lessening the air-movement. In the Westwood tornado, when the funnel had gone about a mile northeast of the village, it became thinner, and the distance to the top of the revolving column did not seem more than one hundred feet. As its force still further weakened, it became only a shallow, whirling cloud of débris, six or seven feet above the ground, and about fifty feet in width. These facts present a problem of the relation of air-pressures in which we may look for destructive action m proportion to the height of the column of revolving air.
Lieutenant Finley's interesting studies will soon be of great service to the people. The advancement of the science of meteorology, as well as of other sciences, has always been made through those whose energy in the examination of these subjects has been manifested as an intrinsic liking, regardless of personal gain, a characteristic pointed out long ago by Jean Paul Richter, and reaffirmed by Emerson as the true aim of the scholar. It has been thought that the time will come when greater numbers of men of leisure and means will become steady workers along paths of unprofitable public usefulness. The people look for science to come to their rescue regarding certain evils in politics or in commerce, in over-legislation, in physical and mental life, and in the destruction of life and property by the elements. It does not follow that the service will be rewarded, yet the control or anticipation of any form of destructive action in Nature is a benefit that will live in the annals of the race for many a century.