Rate at Which Storms Travel.—The average rate of the progressive move ment of European storms is about 161/2 miles per hour. From an examination of extensive series of storms Professor Loomis has shown that the average rates of progress of storm centers are in miles per hour 28 for the United States, 18 for the middle latitudes of the Atlantic Ocean, 17 for Europe, 15 for the West Indies, and 9 for the Bay of Bengal and China Sea. On Jan. 7-8, 1877, a storm traveled in 24 hours from Indianola, Tex., to Eastport, Me.-1,782 miles, or 78 miles an hour. On the other hand, the rate of progress is, particularly in the tropics, sometimes so slow as to be virtually stationary; and, as already stated, they occasionally recurve on their paths.
Relations of Temperature, Rain, and Cloud to Storms.—Temperature increases at places toward which and over which the front part of the storm is advancing, and falls at those places over which the front part of the storm has already passed. In other words, the temperature rises as the barometer falls, and falls as the barometer rises. When the barometer has been falling for some time clouds begin to overspread the sky, and rain to fall at intervals; as the cen tral depression approaches the rain be comes more general, heavy, and continu ous. After the center of the storm ap proaches, or shortly before the barometer begins to rise, the rain becomes less heavy, falling more in showers than con tinuously; the clouds break up when the center has passed, and fine weather, ush ered in with cold breezes, ultimately pre vails. If the temperature begins to rise soon and markedly after the storm has passed, a second storm may be shortly expected.
Direction of the Wind.—If the winds are examined, they will be observed whirling round the area of low barom eter in a circular manner, and in a direc tion contrary to the motion of the hands of a watch, with a constant tendency to turn inward toward the center of lowest pressure, in the manner formulated in Buys-Ballot's law. The wind in storms neither blows round the center of lowest pressure in circles, nor does it blow di rectly toward that center, but takes a direction nearly intermediate, approach ing, however, nearer to the direction and course of the circular curves than of the radii to the center. In the front of the storm the winds blow more toward the center, but in the rear they blow more closely approximate to the circular isobaric lines. Where the direction of
the wind differs to any material degree from the above it is light, and conse quently more under local influences, which turn it from its true course. In the Southern Hemisphere a rotary mo tion is also observed round the center of storms, but it takes place in a contrary direction, or in the direction of the mo tion of the hands of a watch, instead of contrary to that direction, as prevails N. of the equator.
Force of the Wind.—The rule is sim ple, and without exception—viz., the wind blows from a high to a low barometer with a force proportioned to the barometric gradient or to the dif ference of the barometric pressures re duced to sea-level. Hence where the isobaric lines crowd together the violence of the storm is most felt, and where they are far asunder the winds are moderate, light, or nil. We thus see the importance of observations from a dis tance in forecasting the weather.
The progressive motion of storms, which may vary from zero to 78 miles an hour, measures the time taken in passing from one place to another, but it gives no indication of the violence of the storm. This is determined by the velocity of the wind round and inward on the center of the storm, which in Europe and the United States frequently amounts to 60 or 80 miles an hour con tinuously for some tune. In intermittent gusts a speed of 120 miles an hour has been several times observed in Great Britain—a velocity which is perhaps sometimes surpassed by storms within the tropics. On the top of Ben Nevis higher velocities, rising to upward of 150 miles, are of not infrequent occurrence.
Causes of Cyclones.—Dove, who did so much in this department of meteorology, held the view that cyclones are formed when two great atmospheric currents, called polar and equatorial, flow side by side, storms being the eddies, as it were, formed along the line of junction. It is to be kept in mind that the qualities of the atmosphere in the front portion of a cyclone are quite different from those in the rear—the former being warm and moist, while the latter are cold and dry. The conclusion is inevitable that the apparent rotation of winds in storms is simply a circulatory movement main tained between two currents, and that no mass of the same air makes the complete circuit of the cyclone.