Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/141

Rh TORNADOES.] METEOROLOGY 131 place did its fearful work in the brief space of a minute. The tornado which passed over Mount Carmel (Illinois), June 4, 1877, swept off the spire, vane, and gilded ball of the Methodist church, and carried it bodily 15 miles to north-eastward. The velocity of the ascending currents which kept this heavy object suspended in the air for 15 or 20 miles must have been very great. Of the tornadoes the progressive courses of which were recorded, 310 advanced towards N.E., 38 towards S.E., 16 towards E.N.E., 14 towards E., 7 towards N.N.E., 5 towards E.S.E., and 3 towards S.S.E. The course is thus always FIG. 6. Forms of Waterspouts. towards some easterly direction, the great majority being towards the north-east. The velocity of their onward movement varied from 12 to 60 miles an hour, the average being 30 miles an hour. The time occupied in passing a particular spot varied from 10 seconds to half an hour, the mean time being nearly six minutes and a half. The width of the path of destruction marked with debris and other relics of the violence of the tornado varied from 40 to 10,000 feet, the average being 362 yards. The direction of the whirling movement of the tornado was invariably from right to left, or the opposite of the movement of the hands of a watch, resembling in this respect the vorticose move ment of cyclones in the northern hemisphere. The passage of the tornado cloud is often described as accompanied with remarkable noises, which observers variously charac terize as terrible, deafening, a terrific crash, the roar of a thousand trains of cars, or the uproarious din of innumer able pieces of machinery. The usual position of the gyrating columns of cloud is vertical ; but occasionally a curving form or slanting direction is assumed. It is probable that to these latter forms many stationary or slowly moving dangerous squalls are to be referred, which spring up with unexpected suddenness in lakes and arms of the sea in mountainous regions. The dust storm of India, Arabia, and Africa is a well- marked type of the whirlwind. Previous to the outbreak of a dust storm the air is unusually calm and sultry, just as happens in the case of the tornado. The simplest form of the dust storm is that of a tall aerial column of sand moving onwards, and drawing into itself, as it whirls round in its course, dust and other light bodies within the sweep of the strong air-currents which blow along the surface of the ground and converge vorticosely round the base of the column. A form commonly seen is shown in fig. 7, which represents several dust columns grouped together, each whirling independently round its own axis with incurving air-currents at the base, while the whole group of columns is borne bodily forward, and presents striking aspects as the forms and relative positions of the columns are changed. The importance of the observations made on dust storms as leading to a correct understanding of the whirlwind consists in this that it affords conclusive evidence that there is a strong inflow of the air along the surface of the ground all round vorticosely towards the base of the whirl wind, and that these same in flowing air-currents afterwards ascend through the air along the central axis of the whirl wind, carrying with them the evidence of their ascent in the visible solid particles of dust, sand, and other light objects they whirl up with them in their ascending course. Owing to the extreme dryness of the air-currents involved in the dust storm, the rarefaction generated by the rapidity of the gyrations is insufficient to pro duce condensation of the aqueous vapour in the interior of the column. Quite different, how ever, is it with waterspouts and tornadoes, where, in the great majority of cases, the air near the surface before being drawn into the ascending vortex is of a high temperature and near the point of saturation. From the extreme rarefaction to which these air-currents are subjected, owing to their sudden ascent in a rapidly gyrating column, excessive condensation follows, with an aqueous precipitation at times so astonishing that it can only be fittingly described as an aerial torrent of solid water, or an aerial avalanche of hail and ice. Certain tracts of the ocean in cluded within what may be called permanent anticyclones, or where atmo spheric pressure is higher than all round, are char acterized by an absence or com parative absence of rain. These regions are also remarkable for clear skies and strong sun heat. Similarly small anticyclonic areas occur ring between or in the vicinity of cyclones are characterized by dry air and clear skies, and it is under these conditions that the strongest sun heat is felt. When, as repeatedly FIG 7. Dust Storm.