For a better personal appreciation of the rate of evaporation, as well as for the purpose of comparison, I arranged an apparatus to tell the water-loss from a free water-surface. It was also desirable to determine the relative evaporation of the day and night. As employed, the apparatus consisted of a flat tin dish, with parallel sides, 10 cm. in diameter. To the side was attached, by means of a rubber stopper, a bent glass tube of small diameter. The water-loss was read on this tube. Observations were made at Laghouat and at Ghardaia. Following is a summary of the results obtained at Ghardaia: For a period of 7 days, after November 10, the daily water-loss between 7 a.m. and 7 p.m. was as follows: 5, 8, 8.5, 10.5, 13, 9, and 7.5 mm. The evaporation between 7 p.m. and 7 a.m. was so slight, 1 mm. more or less, that it could not be well determined by the apparatus used.
A good idea of the intensely arid character of the Algerian climate, taken as a whole, is to be obtained by a study of the relation between the total rainfall and the total evaporation, based on the official climatic reports. In tables 6 and 7 the report for the year 1908 is used. The figures represent the ratio (e⁄r) between evaporation and rainfall, in which the amount of evaporation is used as the numerator and the amount of precipitation as the denominator. In the monthly evaporation-rainfall table (table 6), in all cases where no rainfall was reported for the month it was called 1 mm. In table 7, however, which gives the seasonal evaporation-rainfall ratio, the actual figures of the government report were in all cases employed, since there was no season during the year 1908, even in southern Algeria, when no precipitation occurred.[6]
Table 6.—Evaporation-rainfall ratio, monthly, 1908.
| Station. | Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Nemours | 1.07 | 1.3 | 1.39 | 1.9 | 131 | 5.4 | 23.6 | 43.3 | 8.6 | 3.1 | 10.6 | 5.7 |
| Cape Falcon | 1.5 | 1.8 | 1.46 | 16.0 | 8.8 | 10.3 | 29.7 | 136.0 | 38.1 | 1.9 | 8.9 | 2.2 |
| Oran | 1.3 | 1.9 | 1.6 | 1.3 | 17.4 | 8.5 | 59.0 | 146.0 | 18.0 | 2.45 | 3.4 | 4.4 |
| Algiers | 0.63 | 1.3 | 0.5 | 1.1 | 22.3 | 11.7 | 219.0 | 28.8 | 5.07 | 1.06 | 1.2 | 1.1 |
| Bouzarea | 0.57 | 0.67 | 0.22 | 0.56 | 8.3 | 4.8 | 53.7 | 27.1 | 3.4 | 0.2 | 0.76 | 0.5 |
| Maison-Carée | 0.72 | 0.64 | 0.55 | 1.1 | 13.5 | 11.8 | 149.0 | 12.0 | 5.2 | 0.9 | 0.35 | 0.51 |
| Ft. National | 0.72 | 0.29 | 0.34 | 0.58 | 3.7 | 192 | 42.5 | 15.5 | 2.7 | 1.6 | 0.71 | 0.16 |
| Sidi-bel-Abbès | 0.63 | 0.76 | 0.6 | 1.1 | 18.8 | 18.5 | 77.8 | 37.8 | 14.8 | 1.4 | 1.9 | 0.8 |
| Saida | 0.67 | 0.64 | 0.36 | 0.83 | 2.2 | 6.9 | 86.7 | 12.1 | 4.7 | 5.8 | 1.2 | 0.78 |
| Batna | 1.6 | 2.3 | 1.5 | 2.5 | 46.5 | 33.4 | 24.4 | 7.6 | 3.5 | 4.9 | 2.6 | 1.6 |
| Tebessa | 4.6 | 6.05 | 21.2 | 3.9 | 252.0 | 2.8 | 9.8 | 14.8 | 1.1 | 3.9 | 3.5 | 3.7 |
| Bou Saada | 4.05 | 5.4 | 12.1 | 2.9 | 6.1 | 165 | 43.5 | 19.5 | 8.3 | 13.9 | 5.6 | 8.3 |
| Barika | 2.4 | 8.7 | 9.9 | 5.0 | 5.03 | 270 | 6.1 | 4.1 | 11.4 | 3.7 | 90.6 | 1.3 |
| Ain Sefra | 9.2 | 21.5 | 1.5 | 23.8 | 12.4 | 63.1 | 16.4 | 124.0 | 328.0 | 14.7 | 13.2 | 1.2 |
| Geryville | 6.3 | 1.9 | 2.1 | 23.9 | 2.5 | 16.7 | 22.5 | 23.4 | 3.4 | 2.1 | 4.1 | 3.9 |
| Laghouat | 3.09 | 4.7 | 9.8 | 203.0 | 7.0 | 373 | 421.0 | 21.0 | 25.9 | 14.2 | 154.0 | 104.0 |
| Ghardaia | 8.9 | 233.0 | 81.4 | 529.0 | 38.9 | 699 | 166.0 | 25.6 | 66.9 | 23.2 | 49.7 | 225.0 |
| El Oued | 53.9 | 27.7 | 67.0 | 629.0 | 369.0 | 465 | 509.0 | 482.0 | 68.6 | 46.1 | 215.0 | 123.0 |
Table 7.—Evaporation-rainfall ratio, seasonal, 1908.
| Station. | Winter. | Spring. | Summer. | Autumn. | Annual. |
|---|---|---|---|---|---|
| Littoral: | |||||
| Nemours | 2.69 | 44.09 | 24.1 | 7.4 | 3.0 |
| Cape Falcon | 1.83 | 8.75 | 58.6 | 16.3 | 3.7 |
| Oran | 2.54 | 3.7 | 71.0 | 7.95 | 4.2 |
| Algiers | 1.0 | 7.96 | 86.4 | 2.44 | 1.8 |
| Bouzarea | 0.58 | 3.0 | 28.5 | 1.45 | 0.93 |
| Maison-Carée | 0.62 | 5.0 | 117.4 | 1.48 | 1.5 |
| Tell: | |||||
| Ft. National | 0.39 | 1.4 | 83.2 | 1.67 | 1.1 |
| Sidi-bel-Abbès | 0.73 | 8.2 | 38.3 | 6.0 | 2.2 |
| Saida | 0.69 | 1.0 | 3.5 | 3.9 | 1.9 |
| Batna | 1.8 | 2.1 | 0.35 | 3.6 | 4.4 |
| Tebessa | 4.7 | 4.05 | 88.2 | 2.8 | 6.0 |
| High Plateau: | |||||
| Bou Saada | 5.9 | 7.0 | 76.0 | 9.2 | 11.0 |
| Barika | 4.1 | 6.6 | 93.5 | 35.2 | 12.2 |
| Ain Sefra | 1.4 | 12.5 | 67.9 | 18.5 | 11.1 |
| Geryville | 7.0 | 9.8 | 20.8 | 3.2 | 3.5 |
| Desert: | |||||
| Laghouat | 6.0 | 73.2 | 271.6 | 64.6 | 17.0 |
| Ghardaia | 154.9 | 416.3 | 293.7 | 195.9 | 59.7 |
| El Oued | 68.3 | 354.0 | 485.2 | 109.5 | 63.0 |
When we reduce the evaporation-rainfall ratios of the different physiographic provinces to simple expressions we gain a comprehensive view of this important climatic factor for the colony as a whole. Thus the annual evaporation-rainfall ratios are as follows: the littoral, 2.5; the Tell, 3.5; the High Plateau, 9.4; the desert, 46.5. The relation of these expressions may also be given thus: 1:1.4:3.7:18.6, for the several regions above given, by which we see how rapidly the aridity of the country increases as the desert is entered. The present custom of considering the southern base of the Sahara Atlas as the northern edge of the desert, in place of including the High Plateau, as was done earlier, is thus well grounded.
The temperatures of the air vary greatly for the different regions, and usually the variation is to be directly related to the positions of the stations as regards the coast and the altitude. Along the coast, however, the temperatures vary even if the stations are at approximately the same elevation. The mean annual temperature at Oran is 16.9° C. and at La Calle 17.7° C., while a study of the January temperatures shows that a similar relation holds good for the entire south coast of the Mediterranean, between Oran and Alexandria. Algiers furnishes one exception to this statement, in that the mean annual temperature is 18.1° C. On the High Plateau the mean temperature falls to 12.7° C. (Batna) and 13.5° (Setif). No records appear to have been made for the high mountains of eastern Algeria, although the fact that snow may remain in sheltered places as late as the latter part of July[7] would indicate that the mean temperature at 2,000 meters elevation is much lower than any above given. On the Algerian Sahara the mean annual temperature is usually higher than at any point nearer the coast, but even here there is considerable variation, depending, among other factors, on the altitude of the stations and their relation to the Atlas. The mean temperature is given by Engler as 20.5° C. for Biskra, 21.0° C. for Ghardaia, and 22.2° C. for El Golea. As increased temperature means increased evaporation, we have in this one factor a powerful agent making for aridity, and when this is associated with decreasing rainfall, as one leaves the coast region, its influence as a determinative factor in the environment of plants is thus seen to be of great importance.
The annual variations in the temperature of the air are very considerable throughout the colony and are especially great on the desert and the High Plateau. At Algiers the variation is 40.7° C., at Setif it is 48.2° C., and at Ghardaia it is 47.9° C. An annual absolute variation of 50° C. is not uncommon on the desert, and Engler cites a range of 57.0° C. at Ghardaia.
The daily range of the temperature is also considerable for all stations, but is especially marked in those of the High Plateau and the desert. The daily range as given by Engler for the former is 17.4° C., and for the latter 20.0° C., but the range on the High Plateau may be greater than 17.4° C., especially during the summer months. For example, at Batna, in 1904, a range of 19.4° C. was reported in April, 20.2° C. in June, 21.8° C. in July, and 20.2° C. in December. Except in December, the great ranges in temperature here cited were on days when the sirocco was blowing from the desert; hence, the usual daily range in temperature would be much less. In the Tell, and especially along the coast, the range in temperature recorded for any day is relatively little, although at the time of the sirocco, as well as during the season of drought, the daily range is not inconsiderable. Table 8 gives the maximum daily range for the months of 1904, for two stations, along the coast, including the Tell, and two stations on the High Plateau. The daily range reported for Batna in June, July, and August, 1902, was somewhat larger than the maxima given in table 8. In 1902 the maximum daily ranges in temperature for the three months at Batna were 23.9° C., 25.3° C., and 24.4° C.