Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/139

THUNDERSTORMS.] METEOROLOGY 125) during the six months from October to March. The mean of the six hottest months shows the maximum to take place from 3 to 4 P.M. and the minimum from 4 to 5 A.M., these being the times of occurrence of the two minima of pressure. At this season, however, the morning minimum pressure is but faintly marked in such climates as those of Siberia. During the twelve hours from 9 A.M. to 9 P.M., when the temperature is above the daily mean, 717 of the whole number occurred, thus leaving only 139 for the twelve hours when the temperature is below the daily mean. The great majority of the thunderstorms occur during the part of the day when the ascensional movement of the air from the heated ground takes place, and they attain the maximum when the temperature and this upward movement are also at the maximum. Owing to the westerly winds from the Atlantic which prevail over Europe and western Siberia during summer, the maximum rainfall of the year occurs over this extensive region in this season ; and the importance and significance of the inquiry into this element of climate lie in the fact that the greater portion of the summer rains is discharged over these regions by the thunderstorm. The " Challenger " observations on the open sea show the maximum occurrence of thunder storms to be from 10 P.M. to 8 A.M., 22 being observed during these ten hours and 10 during the other fourteen hours of the day, a result which suggests that over the ocean terrestrial radiation is more powerful than solar radiation in causing vertical disturbances in the equilibrium of the atmosphere. Atmospheric vapour and ascending currents thus play an important part in the history of these thunderstorms. Where the climate is dry and rainless, like that of Jerusalem in summer, thunder is altogether unknown. On the other hand, where during a particular season an anticyclone with its vast descending current in the centre remains over a region, as happens over the centre of the old continent during the winter, over that region thunder is equally unknown during that season. Further, in such places as Lisbon and Coimbra, where the summer rainfall is small and ite occurrence infrequent, thunderstorms become less frequent, and the hours of their occurrence are later in the day than they are before and after the dry season. The thunderstorms at Mauritius call for special notice. There are two maxima in the diurnal curve, the larger from noon to 4 P.M. and the smaller from 3 to 6 A.M., which are near the times of the barometric minima ; and two minima, from 9 p. M. to 1 A. M. and from 8 to 10 A.M., these being near the times of the barometric maxima. But the important point as regards the thunderstorms of Mauritius is that for twelve years none were recorded in June and July, one only in August, one in September, and three in October. The annual period of the thunderstorms of this island extends from near the end of October to the middle of May, or during the time of the greatest rainfall, practically none occurring during the rest of the year. But rain continues to fall during the four months of no thunder, the mean monthly rainfall being then about 2 inches, falling, however, in September to 1 37 inches. During these four months, therefore, there is in the air the aqueous vapour, and, these being dry months, there is the condition of ascending currents. There appears, however, to be then wanting another element which seems essential to the electrical manifestations of the thunderstorm, viz., the con ditions which give masses of descending cold air along with the ascending _c in-rent of warm moist air. During the months when thunder is of no unusual occurrence the high pressure of Asia repeatedly advances, as Dr Meldrum has pointed out, close on Mauritius ; and so frequently is this the case that he considers the belt of calms between the two trade winds to stretch in a slanting direction from Madagascar to Ceylon. As long. as this state of things occurs with more or less frequency, the conditions of a descending cold current of large volume are provided, and thunder storms occur. But during June, July, August, and September, when atmospheric pressure is low in Central Asia, and there is an unbroken increase of pressure from Asia southwards to Mauritius, and while Mauritius remains in the heart of the south-east trades, the conditions of descending cold currents of any considerable volume are not present, and thunder is there unknown at that season. Now in situations which afford the three conditions of aqueous vapour, ascending currents, and descending cold currents, whilst the diurnal and annual periods are quite distinctly marked, the phenomena are more uniformly distributed through the hours of the day and months of the year than elsewhere. Tola and Fiumo, at the head of the Adriatic, being shut in and encompassed by lofty Alps, are illustrations. At Fiume the greater maximum occurs from 11 A.M. to 4 P.M. and the smaller from 2 to 4 A.M., and the minima from 10 P.M. to 1 A.M. and 5 to 9 A.M. While during the twelve hours the temperature is above the mean of the day from May to September the number of the thunderstorms here was 245 for the nine years ending 1879, the number during the twelve hours the temperature is under the mean was 185. The compara tively large number during the colder hours of the night is no doubt due to the warm moist atmosphere of this confined sea and the close proximity of the Alps. There is still another set of conditions favouring the develop ment of thunderstorms in certain climates which the observations made at Stykkisholm in the north-west of Iceland illustrate. During the fourteen years ending 1879 there occurred here twenty- three thunderstorms, but there was only one in the six warm months from April to September ; in other words, the thunder storms of this climate are essentially winter phenomena. Further, of the twenty-three hours in which they occurred, only three were at a time of the day when the sun was above the horizon, viz., twice in March and once in September ; in other words, the thunder storms of Stykkisholm are nocturnal phenomena. It is instructive to observe that in the north and north-west of Scotland thunder occurs most frequently during the night and in winter, whereas in central, southern, and eastern districts it occurs most frequently during the day and in summer, the thunderstorms in the former case approximating in type to those of Iceland and in the latter to those of Ekaterinburg. A little reflexion shows that in north western Europe it is during winter and during night that warm moist ascending and cold dry descending currents are most fre quently brought into close proximity during the great Atlantic storms of the season; and it is at the changes of wind, humidity, and temperature accompanying the passage of the centres of the cyclones that the thunder peals are heard. On the other hand, in the east and south of Scotland it is during the hot months of the year that these ascending columns of warm moist air and descending columns of cold dry air are most frequently brought together, and there, accordingly, thunder with the heavy rains which accompany it is of most frequent occurrence from 11 A.M. to 5 P.M. from May to September These essentially different types of thunderstorms have been classed by Mohn as heat thunderstorms and cyclonic thunderstorms. Given an initial difference of electric potential, it is easy to understand from the effects which follow the sudden extraordinary condensations of the aqueous vapour that take place how the most violent thunderstorms are pro duced. The difficulty is to account for the production of the initial difference of electric potential, how, for example, in the same great aerial current of the south-west monsoon, this difference of potential is produced in the molecules of aqueous vapour at Calcutta but not in the aqueous vapour at Mauritius. It is to the physicist that meteorologists still look for the explanation. Diurnal Period in the Occurrence of the Whirlwind, Waterspout, Dust Storm, and Tornado. Whirlwinds, waterspouts, dust storms, and tornadoes are essentially the same, differing from each other only in their dimensions, their intensity, or the degree in which the moisture is con densed into visible vapour, while the hailstorm and the rainstorm are simply the manner and degree of the pre cipitation accompanying them. In several important respects they differ widely and radically from cyclones (see ATMOSPHERE, vol. iii. p. 33). The largest tornadoes are of so decidedly smaller dimensions when compared with the smallest cyclones as to admit of no shading of the one into the other. Cyclones occur at all hours of the day and night, whereas whirlwinds and tornadoes show a diurnal period as distinctly marked as any in meteorology. Finally, cyclones take place under conditions which involve unequal atmospheric pressures or densities at the same heights of the atmosphere, due to inequalities in the geographical distribution of temperature and humidity; but whirlwinds occur where for the time the air is unusually warm or moist, and where consequently temperature and

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