Table I.—Annual Frequency (Relative).
| Place. | Latitude. | Jan. | Feb. | March. | April. | May. | June. | July. | Aug. | Sep. | Oct. | Nov. | Dec. |
| ° | |||||||||||||
| Hammerfest | 70½ | 20.9 | 17.6 | 8.8 | 0 | 0 | 0 | 0 | 0 | 4.4 | 9.9 | 17.6 | 20.9 |
| Jakobshavn | 69 | 14.6 | 3.0 | 9.2 | .5 | 0 | 0 | 0 | 0 | 9.2 | 15.1 | 18.4 | 20.0 |
| Godthaab | 64 | 15.5 | 12.4 | 9.7 | 4.9 | 0 | 0 | 0 | 1.2 | 8.7 | 13.3 | 17.0 | 17.4 |
| St Petersburg | 60 | 6.5 | 9.1 | 16.8 | 13.8 | 3.5 | 1.2 | 1.4 | 5.9 | 13.8 | 13.1 | 7.6 | 7.3 |
| Christiania | 60 | 8.6 | 11.4 | 14.0 | 11.2 | 0.6 | 0 | 0.2 | 6.5 | 14.6 | 12.2 | 10.3 | 10.3 |
| Upsala | 60 | 8.4 | 12.9 | 14.9 | 7.4 | 0.7 | 0.2 | 0.4 | 7.1 | 12.4 | 14.3 | 10.7 | 10.7 |
| Stockholm | 59 | 7.6 | 10.0 | 14.7 | 16.4 | 3.8 | 0.0 | 0.0 | 5.6 | 12.9 | 11.4 | 10.0 | 7.3 |
| Edinburgh | 56 | 9.6 | 12.6 | 14.0 | 9.5 | 3.4 | 0.0 | 1.7 | 6.0 | 12.6 | 13.5 | 11.8 | 5.2 |
| Berlin | 52½ | 7.6 | 10.8 | 16.4 | 15.5 | 11.4 | 0.6 | 2.9 | 2.9 | 6.5 | 13.2 | 8.5 | 4.1 |
| London | 51½ | 8.6 | 10.5 | 10.2 | 10.7 | 4.0 | 1.1 | 1.9 | 5.6 | 14.5 | 16.9 | 9.6 | 6.4 |
| Quebec | 47 | 3.6 | 14.8 | 8.3 | 14.2 | 4.1 | 5.9 | 7.7 | 5.9 | 11.2 | 12.4 | 7.7 | 4.1 |
| Toronto | 43½ | 5.4 | 9.5 | 8.7 | 11.8 | 9.0 | 6.2 | 8.0 | 6.4 | 8.5 | 11.1 | 8.7 | 6.7 |
| Cambridge, Mass. | 42½ | 5.1 | 8.2 | 11.8 | 10.2 | 6.4 | 5.1 | 10.3 | 8.5 | 13.3 | 9.2 | 6.8 | 5.1 |
| New Haven, Conn. | 41½ | 7.7 | 7.3 | 8.9 | 8.2 | 7.6 | 5.7 | 8.9 | 8.1 | 11.9 | 7.6 | 10.6 | 7.5 |
| Scandinavia | N. of 68½ | 16.4 | 13.8 | 14.8 | 1.6 | 0.0 | 0.0 | 0.0 | 0.4 | 7.8 | 15.1 | 14.4 | 15.7 |
| ” | 68½ to 65 | 15.3 | 14.6 | 13.7 | 2.9 | 0.0 | 0.0 | 0.0 | 1.1 | 9.7 | 14.6 | 14.0 | 14.1 |
| ” | 65 to 61½ | 13.2 | 12.3 | 14.5 | 5.4 | 0.2 | 0.0 | 0.0 | 2.8 | 13.1 | 14.2 | 12.8 | 11.5 |
| ” | 61½ to 58 | 9.5 | 11.2 | 13.5 | 10.9 | 1.3 | 0.1 | 0.4 | 5.7 | 13.6 | 13.8 | 10.4 | 9.6 |
| ” | S. of 58 | 8.2 | 11.9 | 12.6 | 13.3 | 1.5 | 0.1 | 0.6 | 4.9 | 14.9 | 13.5 | 10.3 | 8.2 |
| New York State | 45 to 40½ | 6.3 | 7.4 | 9.1 | 11.0 | 7.4 | 6.6 | 8.8 | 10.4 | 11.7 | 9.7 | 6.2 | 5.4 |
Table II.
| Station. | Annual Term. | 6-Month Term. | 4-Month Term. | |||
| Amp. | Phase. | Amp. | Phase. | Amp. | Phase. | |
| ° | ° | ° | ||||
| Jakobshavn | 10.40 | 123 | 1.13 | 206 | 1.41 | 333 |
| Godthaab | 8.21 | 111 | 1.54 | 316 | 0.64 | 335 |
| St Petersburg | 2.81 | 96 | 5.99 | 309 | 0.57 | 208 |
| Christiania | 4.83 | 116 | 4.99 | 317 | 0.76 | 189 |
| Upsala | 5.41 | 119 | 4.57 | 322 | 0.86 | 296 |
| Stockholm | 3.68 | 91 | 5.80 | 303 | 1.31 | 180 |
| Makerstown (Scotland) | 5.79 | 102 | 4.47 | 310 | 2.00 | 342 |
| Great Britain | 3.87 | 126 | 4.24 | 287 | 0.40 | 73 |
| Toronto | 0.18 | 12 | 2.13 | 260 | 0.52 | 305 |
| Cambridge, Mass. | 1.02 | 262 | 2.84 | 339 | 1.28 | 253 |
| New Haven, Conn. | 0.99 | 183 | 1.02 | 313 | 0.57 | 197 |
| New York State | 1.34 | 264 | 2.29 | 325 | 0.54 | 157 |
Speaking generally, the annual term diminishes in importance as we travel south. North of 55° in Europe its phase angle seems fairly constant, not differing very much from the value 110° in Lovering’s general formula. The 6-month term is small, in the two most northern stations, but south of 60° N. lat. it is on the whole the most important term. Excluding Jakobshavn, the phase angles in the 6-month term vary wonderfully little, and approach the value 309° in Lovering’s general formula. North of lat. 50° the 4-month term is, as a rule, comparatively unimportant, but in the American stations its relative importance is increased. The phase angle, however, varies so much as to suggest that the term mainly represents local causes or observational uncertainties. Lovering’s general formula suggests that the 4-month term is really less important than the 3-month term, but he gives no data for the latter at individual stations.
6. Sunlight is not the only disturbing cause in estimates of auroral frequency. An idea of the disturbing influence of cloud may be derived from some interesting results from the Cape Thorsden (7) observations. These show how the frequency of visible auroras diminished as cloud increased from 0 (sky quite clear) to 10 (sky wholly overcast).
Grouping the results, we have:
| Amount of cloud | 0 | 1 to 3 | 4 to 6 | 7 to 9 | 10 |
| Relative frequency | 100 | 82 | 57 | 46 | 8 |
Out of a total of 1714 hours during which the sky was wholly overcast the Swedish expedition saw auroras on 17, occurring on 14 separate days, whereas 226 hours of aurora would have occurred out of an equal number of hours with the sky quite clear. The figures being based on only one season’s observations are somewhat irregular. Smoothing them, Carlheim-Gyllensköld gives f = 100′ − 7.3c as the most probable linear relation between c, the amount of cloud, and f, the frequency, assuming the latter to be 100 when there is no cloud.
7. Diurnal Variation.—The apparent daily period at most stations is largely determined by the influence of daylight on the visibility. It is only during winter and in high latitudes that we can hope to ascertain anything directly as to the real diurnal variation of the causes whose influence is visible at night as aurora. Table III. gives particulars of the number of occasions when aurora was seen at each hour of the twenty-four during three expeditions in high latitudes when a special outlook was kept.