Page:Collier's New Encyclopedia v. 09.djvu/141

LEFT STOBM 107 STORM rises as the barometer falls, and falls as the barometer rises. When the bai'ometer 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 stomi 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 blov/ more toward the center, but in the rear -^.^y 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 wat^-ii, 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 time. 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 atm.ospheric currents, called polar and equatorial, flew 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. Forecasting. — Valuable aid in fore- casting storms and weather is derived fl'om two important deductions from past observations: (1) A cyclone tends toward a path near to the anticyclone which lies immediately to the right of the progressive motion of the storm at the time. ^2^ When the rates of fall of the baromecer at stations in the W. of Europe are noted, it is found that the path taken by the coming storm is in- dicated by those stations at which the rates of fall of the barometer are greatest. See Meteorology. Relations of Stor'iTis to the Character of the Season. — This is vital and all- important. Thus, as regards the British Islands, when the general path pursued by storms in their E. course over Eu- rope lies to the S. the winter is severe, inasmuch as the British Islands are then on the N. side of the center of low pres- sure, and consequently in the stream of the N. and E. winds which there prevail. On the other hand, when the paths of storms lies to the N. the British Islands are on the S. side of the low pressure, and therefore in the stream of the warm, moist, S. and S. W. winds which there prevail. It is plain that the character of the weather of any particular day or season is wholly determined by the way in which areas of high and of low atmos- pheric pressure are distributed over the region during that day or season. Fur-