Table XI.—Annual Variation of Thunderstorms.
| Jan. | Feb. | March. | April. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. | |
| Ediburgh | 1.8 | 1.4 | 1.4 | 3.8 | 12.3 | 20.8 | 28.2 | 19.1 | 7.0 | 2.3 | 1.1 | 0.8 |
| London | 0.6 | 0.5 | 1.6 | 6.6 | 12.7 | 18.3 | 25.5 | 19.2 | 9.3 | 3.1 | 1.7 | 0.9 |
| Paris | 0.2 | 0.4 | 2.3 | 7.5 | 14.9 | 21.6 | 22.0 | 17.0 | 9.9 | 3.5 | 0.4 | 0.4 |
| Netherlands | 2.2 | 1.8 | 3.7 | 6.5 | 14.0 | 14.7 | 15.6 | 14.7 | 10.3 | 10.1 | 3.8 | 2.5 |
| France | 2.2 | 2.8 | 4.1 | 8.4 | 13.8 | 18.7 | 14.6 | 13.5 | 10.0 | 6.3 | 3.1 | 2.4 |
| Switzerland | 0.2 | 0.3 | 0.5 | 4.9 | 11.9 | 22.9 | 29.9 | 18.0 | 9.8 | 1.1 | 0.3 | 0.2 |
| Hungary (a) | 0.0 | 0.1 | 1.6 | 5.7 | 20.9 | 25.0 | 23.2 | 15.9 | 5.7 | 1.3 | 0.4 | 0.2 |
| Hungary (b) | 0.0 | 0.0 | 1.0 | 3.2 | 11.8 | 20.6 | 30.7 | 25.3 | 6.9 | 0.5 | 0.0 | 0.0 |
| United States | 0.1 | 0.1 | 1.2 | 4.0 | 14.3 | 25.0 | 27.2 | 20.4 | 5.8 | 1.4 | 0.3 | 0.1 |
| Hong-Kong | 0.0 | 2.1 | 4.3 | 8.5 | 12.8 | 23.4 | 14.9 | 21.3 | 10.6 | 2.1 | 0.0 | 0.0 |
| Trevandrum | 3.2 | 3.8 | 13.1 | 20.9 | 18.6 | 4.9 | 1.2 | 3.5 | 2.5 | 12.9 | 12.0 | 3.3 |
| Batavia | 10.4 | 9.2 | 11.1 | 10.5 | 7.9 | 5.5 | 4.3 | 3.8 | 5.4 | 8.8 | 12.2 | 10.9 |
31. Even at the same place thunderstorms vary greatly in intensity and duration. Also the times of beginning and ending are difficult to define exactly, so that several elements of uncertainty exist in data as to the seasonal or diurnal variation. The monthly data in Table XI. are percentages of the total for the year. In most cases the figures are based on the number of days of thunder at a particular station, or at the average station of a country; but the second set for Hungary relates to the number of lightning strokes causing fire, and the figures for the United States relate to deaths by lightning. The data for Edinburgh, due to R.C. Mossman (64), refer to 130 years, 1771 to 1900. The data for London (1763-1896) are also due to Mossman (65); for Paris (1873-1893) to Renou (66); for the Netherlands (1882-1900) to A.J. Monné (69); for France(71) (1886-1899) to Frou and Hann; for Switzerland to K. Hess (72); for Hungary (67) (1896-1903) to L. von Szalay and others; for the United States (1890-1900) to A.J. Henry (68); for Hong-Kong (73) (1894-1903) to W. Doberck. The Trevandrum (74) data (1853-1864) were due originally to A. Broun; the Batavia data (1867-1895) are from the Batavia Observations, vol. xviii.
Most stations in the northern hemisphere have a conspicuous maximum at midsummer with little thunder in winter. Trevandrum (8° 31′ N.) and Batavia (6° 11′ S.), especially the former, show a double maximum and minimum.
Table XII.—Diurnal Variation of Thunderstorms.
| Hour. | 0-2. | 2-4. | 4-6. | 6-8. | 8-10. | 10-12. | 0′-2′. | 2′-4′. | 4′-6′. | 6′-8′. | 8′-10′. | 10′-12′. |
| Finland (76) | 2.3 | 2.0 | 2.2 | 3.0 | 4.6 | 12.1 | 18.9 | 19.2 | 16.1 | 10.1 | 6.1 | 3.4 |
| Edinburgh (64) | 1.7 | 2.0 | 1.4 | 1.7 | 4.7 | 14.2 | 22.4 | 23.7 | 11.9 | 9.2 | 5.1 | 2.0 |
| Belgium (77) | 3.0 | 2.9 | 1.7 | 1.8 | 2.0 | 6.4 | 12.9 | 21.6 | 19.4 | 15.8 | 8.4 | 4.1 |
| Brocken (78) | 1.6 | 2.5 | 1.3 | 1.3 | 4.2 | 3.1 | 12.1 | 28.6 | 22.4 | 10.1 | 7.2 | 5.6 |
| Switzerland (72) | 3.1 | 2.3 | 2.1 | 1.6 | 2.0 | 7.3 | 13.8 | 20.9 | 20.8 | 14.6 | 8.0 | 3.5 |
| Italy (77) | 1.3 | 1.6 | 1.4 | 2.0 | 3.0 | 8.5 | 19.5 | 26.5 | 16.6 | 9.8 | 8.3 | 1.5 |
| Hungary (i.) (67) | 2.1 | 1.9 | 1.9 | 2.1 | 2.9 | 11.5 | 18.1 | 22.0 | 17.9 | 10.7 | 6.2 | 2.8 |
| Hungary (ii.) (67) | 6.9 | 4.2 | 2.3 | 2.0 | 2.0 | 5.0 | 9.9 | 16.9 | 18.2 | 10.7 | 11.7 | 10.0 |
| Hungary (iii.) (75) | 2.3 | 1.9 | 2.0 | 2.4 | 2.7 | 7.9 | 16.1 | 22.1 | 19.1 | 12.7 | 7.6 | 3.2 |
| Hungary (iv.) (75) | 2.6 | 2.2 | 1.9 | 1.9 | 3.6 | 13.3 | 19.9 | 20.7 | 15.2 | 9.2 | 6.2 | 3.3 |
| Trevandrum (74) | 5.6 | 4.9 | 4.3 | 1.3 | 1.4 | 2.0 | 13.3 | 24.5 | 15.9 | 13.3 | 7.6 | 5.9 |
| Agustia (74) | 2.9 | 2.9 | 0.3 | 0.0 | 1.7 | 2.9 | 15.1 | 36.1 | 22.2 | 9.3 | 4.6 | 2.0 |
32. Daily Variation.—The figures in Table XII. are again percentages. They are mostly based on data as to the hour of commencement of thunderstorms. Data as to the hour when storms are most severe would throw the maximum later in the day. This is illustrated by the first two sets of figures for Hungary (67). The first set relate as usual to the hour of commencement, the second to the hours of occurrence of lightning causing fires. Of the two other sets of figures for Hungary (75), (iii.) relates to the central plain, (iv.) to the mountainous regions to north and south of this. The hour of maximum is earlier for the mountains, thunder being more frequent there than in the plains between 8 A.M. and 4 P.M., but less frequent between 2 and 10 P.M. Trevandrum (8° 31′ N., 76° 59′ E., 195 ft. above sea-level) and Agustia (8° 37′ N., 77° 20′ E., 6200 ft. above sea-level) afford a contrast between low ground and high ground in India. In this instance there seems little difference in the hour of maximum, the distinguishing feature being the great concentration of thunderstorm occurrence at Agustia between noon and 6 P.M.
Table XIII.
| Year. | Nether- lands. | France. | Hungary. | U.S.A. | Year. | Nether- lands. | France. | Hungary. | U.S.A. |
| 1882 | 98 | .. | 141 | .. | 1893 | 102 | 288 | 233 | 209 |
| 1883 | 117 | .. | 195 | .. | 1894 | 111 | 300 | 333 | 336 |
| 1884 | 95 | .. | 229 | .. | 1895 | 119 | 309 | 280 | 426 |
| 1885 | 93 | .. | 192 | .. | 1896 | 109 | 266 | 299 | 341 |
| 1886 | 102 | 251 | 319 | .. | 1897 | 119 | 297 | 350 | 362 |
| 1887 | 78 | 292 | 236 | .. | 1898 | 95 | 299 | 386 | 367 |
| 1888 | 94 | 286 | 232 | .. | 1899 | 112 | 299 | 368 | 563 |
| 1889 | 126 | 294 | 258 | .. | 1900 | 108 | .. | 401 | 713 |
| 1890 | 93 | 299 | 265 | .. | 1901 | .. | .. | 502 | .. |
| 1891 | 98 | 317 | 302 | 204 | 1902 | .. | .. | 322 | .. |
| 1892 | 86 | 324 | 350 | 251 | 1903 | .. | .. | 256 | .. |
33. Table XIII. gives some data as to the variability of thunder from year to year. The figures for the Netherlands (69) and France (71) are the number of days when thunder occurred somewhere in the country. Its larger area and more varied climate give a much larger number of days of thunder to France. Notwithstanding the proximity of the two countries, there is not much parallelism between the data. The figures for Hungary (67) give the number of lightning strokes causing fire; those for the United States (68) give the number of persons killed by lightning. The conspicuous maximum in 1901 and great drop in 1902 in Hungary are also shown by the statistics as to the number of days of thunder. This number at the average station of the country fell from 38.4 in 1901 to 23.1 in 1902. On the whole, however, the number of destructive lightning strokes and of days of thunder do not show a close parallelism.