North America

by Israel C. Russell

The cyclonic storm of the Mississippi Valley, the Atlantic coast States, and southeastern Canada frequently originate in the Great plateau province, and are carried towards the Atlantic owing to the influence of the eastward-blowing winds of the upper atmosphere. At times these storms are of such magnitude and intensity that they cross the Atlantic and are observed in England and Scandinavia.

The courses they follow may be traced from day to day on the weather-maps issued by the United States Weather Bureau, and from the directions they are likely to follow and the atmospheric conditions pertaining to their various parts predictions of surprising accuracy as to the changes which the weather in a given locality will experience can be made one or two days before the changes occur.

Hurricanes.—Cyclonic storms of the general nature of the tornadoes, but of vastly greater extent and intensity, originate occasionally during the latter portion of the summer season over the tropical portion of the north Atlantic, move slowly westward to the vicinity of the Lesser Antilles, where normally their courses bend northward, and then skirt the Atlantic coast of the United States and drift eastward under the influence of the eastward-flowing upper air-currents, and not infrequently make their influence felt in the western portion of Europe. Occasionally, on account of the presence of an area of high barometric pressure to the north of Cuba, the course of one of these tropical hurricanes, as they are termed, is rendered irregular, and it passes over the Atlantic States or is deflected still more and crosses the Gulf of Mexico before reaching the border of the continent, as was the case in September, 1900, when a large part of Galveston was destroyed. The normal paths of the tropical hurricanes as they approach the coast of the United States and the exceptional course of the one which passed over Galveston, are indicated on the map forming Fig. 26.

Fig. 26.—Tracks of West Indian hurricanes. The circles on the tracks indicate the position of the storm centres at Greenwich mean noon on successive days. The date of the several storms is as follows:

1. Aug. 27-Sept. 1, 1890.
2. Aug. 19-Aug. 25, 1890.
3. Aug. 19-Aug. 31, 1891.
4. Sept. 4-Sept. 9, 1891.
5. Sept. 16-Sept. 25, 1891.
6. Sept. 28-Oct. 7, 1891.
7. Aug. 17-Aug. 22, 1892.
8. Aug. 15-Aug. 22, 1893.
9. Aug. 23-Aug. 28, 1893.
10. Sept. 6-Sept. 9, 1894.
11. Sept. 20-Oct. 4, 1894.
12. Oct. 5-Oct. 10, 1894.
13. Oct. 12-Oct. 18, 1894.
14. Oct. 24-Oct. 27, 1894.
15. Oct. 18-Oct. 25, 1895.
16. Sept. 5-Sept. 10, 1896.
17. Sept. 9-Sept. 25, 1896.
18. Sept. 26-Sept. 29, 1896.
19. Oct. 9-Oct. 14, 1896.
20. Oct. 23-Oct. 26, 1897.
21. Oct. 20-Oct. 23, 1897.
22. Sept. 11-Sept. 20, 1898.
23. Aug. 3-Aug. 25, 1899.
24. Aug. 30-Sept. 7, 1899.
25. Sept. 8-Sept. 14, 1899.
26. Sept. 1-Sept. 11, 1900.

The analogy of a tropical hurricane to a tornado has already been referred to, but while a tornado may lay waste a tract of country perhaps half a mile wide, and in exceptional cases 20 to 30 miles in length, a hurricane is from 200 to 300 miles in diameter, and may continue to be destructive, on account of the rapid inflow of air from the periphery towards the centre, for 2,000 or 3,000 miles. The velocity of the spirally blowing winds which are the characteristic feature of these great storms is frequently 100 miles or more per hour. In spite of their magnitude, however, the conditions leading to their origin and growth

are essentially the same as in the case of tornadoes, and even of the much smaller whirlwinds. They have their birth where the moist, still air above the ocean in the region of the doldrums at the season when the equatorial belt of calms is farthest north, becomes highly heated and rises on account of the pressing in of cooler and heavier air from adjacent regions. The ascending column is at first carried

slowly westward, in obedience to the general flow of the atmosphere in the intertropical belt, and at the same time the currents coming in from opposite directions give the ascending air a rotary motion. As the currents from the northeast are stronger than those from other directions, this whirling motion is from right to left, or opposite to the movement of the hands of a watch. The whirling air column extends into the upper atmosphere, and as it moves along past the West Indies becomes influenced by the prevalent flow of the upper air-currents, and is carried northwestward, and later eastward in a path which approximates to a parabolic curve. The inward-rushing spiral winds leave a calm centre, the "eye of the storm," which corresponds to the hollow core of a whirlwind and the calm centre sometimes noted in tornadoes. The upward ascent of warm, humid air is accompanied by a decrease of pressure and consequent expansion and cooling which leads to rapid condensation and a heavy downpour of rain; the change of the moisture from a vaporous to a liquid form liberates heat, which serves to perpetuate the upward flow of air, and thus prolongs the life of the storm. During the passage of the central area of low barometric pressure over a given locality the clouds frequently part and portions of the clear sky may be seen. Accompanying the rain are frequent lightning flashes, as during ordinary thunder-storms.