Cyclones.— The cyclone, or low pressure, is a system of spirally inflowing winds which ascend in the region of the centre, cool by ex pansion with gain in altitude and usually cause precipitation in the form of rain or snow. At or below the heights of the wispy cirrus clouds the air flows Outward, and at some more or less remote place moves downward in the form of an anti-cyclone. Storms move eastward in the middle latitudes and westward in the tropics, and rotate counter-clockwise in the northern hemisphere and in the opposite di rection in the southern hemisphere. While the velocity of gyration may be anywhere from zero to 100 miles per hour, depending upon the steepness of. the barometric gradient toward the centre, the velocity of translation of the whole whirling mass usually varies from 20 miles per hour in summer to 40 miles in winter.
In the anti-cyclone all the movements of the cyclone are reversed except the direction of translation. The air flows downward near the centre and spirally out ward along the surface of the earth. It usually is accompanied by clear, cool and settled weather, and this notwithstanding the fact that the air heats by compression as it descends, which apparent inconsistency will be explained in the paragraph on Cold Waves.
Origin of flowing together of the air between two anti-cyclones must produce a cyclone, and it has been shown that the un equal heating effects of land and water sur faces may be sufficient to initiate cyclonic and anti-cyclonic action.
A hypothesis to account for the beginning of storms that is attracting the attention of meteor ologists, assumes that the earth, at times, passes through shafts of extra solar heat, which ex pand the air at the equator more than they do at higher latitudes, causing the atmosphere at the equator to bulge up until masses at high levels gravitate toward the poles. These masses cool by expansion and by radiation, gain in specific gravity and sink to the earth in the middle latitudes or the north part of the tem perate zone. As they come down they rotate in an anti-cyclonic manner and constitute cold waves. Each descending mass forces the ascent of cyclonic whirls or storms on either side. The storms thus formed may move east ward for great distances, crossing continents and oceans before being disintegrated. Accord ing to theory these extra heat shafts from the sun may persist for months and return to affect our weather with each 25-day rotation of the luminary. An allowance of 18 days is made for the air to perform the overhead circula tion from the equator after each heat impulse before the cold wave appears on our western border. According to this hypothesis these cold
masses should settle over land in winter and over oceans in summer, which agrees with ob served fact.
Abbot's researches have shown much of the varied character of solar radiation. With a continuation of his work and of that of Kim ball, of the Weather Bureau, whose investiga tions were begun and continued for many years under the direction of the author, the time may come when the general character of seasons may be foretold.
Temperatures of Cyclones and Anti According to Ferrel and other early students temperature is supposed to be arranged systematically about the centres of cyclones and anti-cyclones. We now know that such is not the case, that the cyclone as a whole is not a warm area, nor is the anti-cyclone a cold area, but that the rising temperature occurs on the western side of the high area and on the eastern side of the low area, and that the falling tem perature is on the western side of the low area and on the eastern side of the high area.
Hurricanes.—The hurricane of the West Indies is the same as the typhoon of the Philip. pines and the China coast. The ordinary cyclonic storm that crosses the United States is about 1,000 miles in diameter. The hurricane is a cyclone usually of only about 100 to 300 miles in diameter, but with a gyratory velocity far in excess of the storms of the temperate latitudes. It occurs in the tropics, moves west. ward and northward at the rate of only '7 to 12 miles per hour, while the air inside of the storm may be whirling at the rate of over 100 miles. At latitude 26° its path recurves in the form of a parabola, and the storm passes to the northeast. They occur mainly in the four months July to October.
The tornado also is cyclonic in its movements, but instead of being 1,000 miles in diameter like the continental cyelone, or 100 miles like the tropical hurricane; it usually has a diameter of gyration of only about 100 to 300 yards, and its speed of rota tion is so terrific that no instruments have ever held together long enough to measure its veloc ity, which must equal or exceed that of a rifle bullet, as the writer has seen wheat straws that were shot by the wind of a tornado one-half inch into the tough body of an oak tree, and a two by four pine scantling driven through five eighths of an inch of solid iron. As a rule they occur in the spring of the year, in the southeast quadrant of a cyclone, when the tem perature and humidity are high. Their direc tion nearly always is toward the northeast. They occur with the greatest frequency in the States bordering on the Mississippi River.