PROGRESS IN TORNADO-PREDICTION. 309 



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 pre- 

 dictions that tornadoes would occur, made in April and June, 1884, 

 eighteen were 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 meteor- 

 ology, 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, temj)erature, cloud-formation, extent, and movement of im- 

 mense masses of air are permanently recorded. The conditions favor- 

 able 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 proba- 

 ble 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 dis- 

 tribution 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 oi temperature in case of 

 tornado development is from 15° to 50°, but with this abnormal con- 

 dition of temperature there must be abnormal conditions of humidity. 



