If we regard June, July, and August as the antarctic winter months, and December, January, and February as summer, we may take it that the mean winter temperature is -16.8° C. (1.8° F.), and the mean for summer, -1.5° C. (29.3° F.).
Table II. shows the minimum temperature for each month. The maximum temperatures are less interesting; the winter average is -1° to 0° C. (30° to 32° F.); the absolute maximum for the equinoctial months is 0° to 1° C. (32° to 34° F.), and for summer, 2° C. (36° F.).
These tables show that between the seventieth and seventy-first parallels of the southern hemisphere, and amid the ice of the Antarctic Ocean, first, the mean temperature is lower than that of the northern coast of Spitsbergen—Mossel Bay, 1872–73, -8.9° C. (16° F.); second, the minimum temperature is quite as low as the minima observed on the east side of Greenland (Sabine Island and Scoresby Sound); and third, that the mean temperature of the three summer months is lower than the corresponding mean in the ice of the Arctic Ocean—the observations of the Fram give a mean for June, July, and August of -1.2° C. (29.8° F.). Note that the calculations of Spitaler and Supan give a mean temperature for the parallel of 70° north latitude of -10.2° C. (13.6° F.). If we consider that a considerable fraction of the seventieth parallel of south latitude is land, we can suppose that it may have a mean temperature as low as the seventieth degree north, and include a pole of cold with lower temperature, as the Asiatic or North American poles of cold.
As in the case of the mean temperatures, the values I am able to give for mean barometric pressure must be regarded only as first approximations. During our drift in the pack-ice hourly observations were made with a marine barometer and with an aneroid. I have not yet been able to apply exact corrections to these observations, but if we bear in mind that while the temperature correction is negative, the correction for latitude is positive, and that for temperatures about 13° to 15° C. (55° to 60° F.) these corrections are numerically nearly equal, we can accept the uncorrected values as near enough for our present purpose. Table III. gives the averages of the aneroid observations, calculated to whole millimetres only. The mean for the year is 744.7 mm. (29.319 inches).
FIG. 1.
Tables IV. and V. give the principal minima and maxima of pressure observed; the values are reduced to the freezing-point and gravity at 45° latitude. The lowest pressure observed during our wintering was 711.74 mm. (28.022 inches), and the highest 772.14 mm. (30.400 inches), a range of 60.40 mm. (2.378 inches). Table VI. gives the monthly variations of the barometer, the mean value of which amounts to 34.30 mm. (1.350 inches), showing even more clearly than Table IV. that the cyclonic belt extends beyond the polar circle. From this table it appears, further, that the three months of almost continuous daylight (November, December, and January) are characterised by a very small variation of pressure—only 23.95 mm. (0.943 inch). The three corresponding months of winter have also a mean less than those for the intermediate or equinoctial months. Compare this with the mean pressures (Table III.). The differences between the annual and monthly means (Table VII.) show that February, March, and April form a negative group, in which the pressure is relatively low; the three months of polar night form another group of maximum barometric pressure; then follow August, September, and October, months of decreasing pressure, a group which, although not actually negative, forms a distinct secondary minimum; and lastly, three months of polar day forming a secondary maximum of pressure. The general result is illustrated in Fig. 1,—high pressure at the solstices, low pressure at the equinoxes,—and the existence of a direct simple relation between the barometric pressure and the progress of the sun is at once obvious.
Table VIII. gives the observed wind directions: the figures indicate the number of hours during which the wind blew from each direction during the twelve months, the sums constituting the “wind-rose” of the point of observation. Fig. 2 shows that winds blow from northerly and southerly points with almost equal frequency, and that easterly winds predominate over westerly. The directions of greatest frequency were west, east, and north-east.
FIG. 2.