Page:Popular Science Monthly Volume 37.djvu/557

Rh Thunder-storms have been scientifically studied in various countries, and the broad fact has been elicited that they travel over the earth's surface like wind-storms, but at a higher velocity. To give an idea of this, I may quote some statements made before the Royal Meteorological Society last June, in relation to the storm of the 2d of that month. This storm progressed from Wiltshire to Edinburgh, over a distance of four hundred miles, at a nearly uniform speed, the rate of travel being about fifty miles an hour. This is an unusually rapid rate of advance for a windstorm over these islands. I am not speaking of the velocity of the wind in the storm, but of the velocity of the storm system as a whole. In this storm many of the hailstones which were collected weighed over an ounce. Some at Docking, near King's Lynn, were said to be three inches in diameter, and to weigh three and a half ounces. One was weighed at Barden Mill, near Tunbridge Wells, and was said to turn the scale at half a pound. As regards the incessant character of the lightning in London, one observer at Highgate counted twelve hundred and forty-four displays during the two hours ending at 11.10, giving an average of over ten per minute. Another observer, at Westgate-on-Sea, gave a much higher figure for frequency; his attempt to count breaking down at the very high number of one hundred and thirty-one per minute.

Thunder-storms are much more frequent in low latitudes than in high. In some tropical countries they are said to occur regularly every afternoon. At Rio the story was that at certain seasons, in issuing invitations to afternoon parties, it was usual to specify whether guests were to assemble before or after the thunder-storm. In Abyssinia, D'Abbadie gives, as the average of four years, 410·6 as the annual number of these storms. Many of these, however, consisted of only one or two flashes of lightning. It was formerly believed that such storms never were noticed in the arctic regions, but this is not the case, for one was experienced at Bell Sound, Spitsbergen, in 78° north latitude, in August, 1873; and a succession of thunder-storms was reported for several days in July, 1870, on the west coast of Nova Zembla. At any rate, in such high latitudes they are very rare.

Thunder-storms are generally divided into two groups—heat thunder-storms and cyclonic thunder-storms. The former are the summer type, while the latter occur principally in autumn and winter. We may also say that the former are essentially continental, while the latter are characteristic of the ocean or island climate. In Iceland all the thunder-storms are of this latter type, and occur in winter. The same conditions show themselves on the British Atlantic coasts, where there is a decided maximum of frequency of such storms in winter, even in the latitude of the