Page:Popular Science Monthly Volume 9.djvu/327

Rh United States Signal Service for that year, as well as by those of other tornadoes.

The characteristic cloud of a tornado is the conus, which appears first above as a dense, dark disk, and is formed by the whirl of the tropical current rushing into the depression of the polar current which starts the tornado, and it is enlarged and lengthened by alternate and rapid condensations above and below, as the tropical air whirls and zigzags along the diagonal of conflict, until sometimes the conus above and below unite—as in the case of water-spouts at sea—and a rotating column of mingled air, dense cloud, dust, or water—as the case may be—is thus formed, and sweeps along the plane of meeting between the opposing currents, and beneath the bank of cumulus clouds which mark the area of a tornado's path of destruction.

The conus cloud, however, as above described, is only formed when the tornado has already commenced, and is therefore of no use to indicate its occurrence beforehand.

But when the dark and dense masses of cumulus clouds in a summer storm cease moving forward or laterally, but bank up higher and higher, and there is great commotion among them, and when there is an oppressive sultriness about the air, these phenomena always indicate that the suspended storm is in a crisis or condition to generate a tornado, in case some local obstruction or other cause disturb the equilibrium of resistance between the two conflicting currents.

—The condensed result of modern meteorological science is the general fact announced by Prof. Buys-Ballot, of Utrecht, that "the wind always blows from the place of highest to that of lowest barometer, turning by the rotation of the earth to the right on the northern hemisphere, and to the left on the southern hemisphere." This is known as "Ballot's Law," and is the chief basis of all scientific weather predictions at the present day.

The first part of this law, given in italics, is found to be universally correct. The second part, however, has many exceptions, and is as often "honored in the breach as in the observance;" for, in polar storms, the winds from the northern semicircle do not conform to it.

Among other definite results attained by barometric observations and deduced from Ballot's law, is the fact that the rain-area of a storm extends over that of lowest barometer and also surrounds it. The isobars, or elliptic lines, of equal barometer, surround the area of lowest barometer, and the most distant isobar marks the limit of the region of low barometer, and may be regarded as the boundary between the regions of high and low barometer. The gradients indicate the differences of pressure between the isobars on a line extending at right angles from that of highest to that of lowest barometer.

The shape of the area of lowest barometer in a progressive storm is that of an irregularly elongated ellipse, moving sideways, or in the direction of its shortest diameter; and the gradients are found to be