Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/140

Rh 130 METEOROLOGY [WHIRLWINDS AND humidity diminish with height at an abnormally rapid rate. Cyclones are thus phenomena resulting from a disturbance of the equilibrium of the atmosphere considered horizon tally, but whirlwinds and tornadoes have their origin in a vertical disturbance of atmospheric equilibrium. Among the most remarkable of the tornado-swept regions of the globe are certain portions of the United States ; and to the examina tion of these the meteorological service of the States has given special attention by a systematic, careful, and minute observation of the attendant phenomena and the destructive effects. The tornadoes of the last eighty-seven years, numbering about six hundred, have been classed under the different States where they are reported to have occurred, and fig. 5 shows this relative distribu tion over the States. The areas of greatest frequency are at long distances from each other. That part of the great basin lying west of the Mississippi, including the States of Iowa, Missouri, Kansas, and Nebraska, is the region in which tornadoes are most frequent. Tornadoes occur at all seasons, being most frequent, however, from April to September, and least frequent in December and January. The hour of occurrence of one hundred and sixty-two of the tornadoes is given in tire official report as follows : Midt. to 2 A.M. 2 2 A.M. ,, 4 5 4 ,, ,, 6 ,, 3 6 8 4 Thus the diurnal pei 8 A.M. to 10 A.M. 1 10 ,, ,, Noon 7 Noon ,, 2 P.M. 13 2P.M.,, 4 47 iod of tornadoes is an 4 P.M. to 6 P.M. 52 6 8 ,, 17 8 ,, ,, 10 ,, 7 10 ,, Midt. 4 ilogous to the period FIG. 5. Distribu tion of Tornadoes in the United States. for temperature, wind velocity, and thunderstorms. The atmo spheric conditions which appear invariably to precede the formation of the tornado are violent contrasts of temperature and humidity immediately to the north and south of the path to be traversed by the storm. It is highly interesting to observe from fig. 5 that the region of most frequent occurrence of tornadoes is the region where a large number of the cyclones of the United States appear to originate (and the same region Loomis has shown to be remark able for violent contrasts of temperature occurring within limited spaces and times), and that, as appears in the regions of the Alleghanies, they decrease in frequency with height. Fig. 6 shows the waterspout in different aspects. A black cloud covers the sky, from which a projection is let down in the form of an inverted cone, as at A, which continues to increase and extend downwards. The sea immediately beneath is soon thrown into violent agitation, showing that the whirling movement which began in the clouds has extended to the sea, and is doubtless continuous throughout, though the portion of the column from A downwards is not yet made apparent by the condensation of its contained vapour into cloud. As the whirling move ment of the column becomes more intensely developed, the increased rapidity of the gyrations brings about increased rarefaction of the air within, with the inevitable result of increased condensation of the vapour into cloud downward. The protrusion of the cloud and its extension downwards are thus not due to the descent of vapour from the clouds, but to the visible condensation of the vapour of the spirally ascending air-currents arising from an increasing rarefac tion due solely to the accelerated rate of the gyrations, the condensation being analogous to that of the cloud seen in exhausting an air-pump. Under each of the columns of fig. 6 the surface of the sea is seen to be more or less heaped up, as well as in violent agitation, showing that atmospheric pressure immediately under the gyrating columns is less than it is all round. On land, when the tornado passes directly over a dwelling house or other closed building, it often happens that the whole building, walls and roof, is thrown outward with great violence, the wreckage presenting the appearance of a sudden explosion, proving that atmospheric- pressure outside the building was instantaneously and largely reduced, and the building shattered to fragments by the expansion of the air within. It is in this way that the tornado does some of its most dreadful work. The wind of the tornado reaches a velocity probably never equalled in cyclones. During the Ohio tornado of February 4, 1842, large buildings were lifted entire from their foundations, carried several rods through the air, and then dashed to pieces, some of the fragments being carried distances of 7 and 8 miles ; and large oaks nearly 7 feet in girth were snapped across like reeds. This tornado swept on its course at the rate of 34 miles an hour, and at one