The seasonal distribution of rainfall at four desert stations shows also marked irregularities. The distribution (in percentages) is given in table 3.
Table 3.—Seasonal distribution, in percentages, of rain.
| Station. | Winter. | Spring. | Summer. | Autumn. |
|---|---|---|---|---|
| Ouargla | 37 | 39 | 4 | 20 |
| Laghouat | 11 | 27 | 31 | 31 |
| Ghardaia | 15 | 23 | 13 | 49 |
| Touggourt | 13 | 30 | 3 | 53 |
The relative humidity at the desert stations is often very low, sometimes running in midsummer between 7 and 9 per cent for 6 days, and occasionally being too low to measure with the instruments employed. The mean relative humidity (table 4) shows the general very dry condition of the air of the desert as contrasted with a station in the Tell, and also indicates something of the monthly variations in this factor experienced among the desert stations themselves. The averages given are from 4 to 8 years, except the mean annual for In Salah, which is for 2 years only.
Table 4.—Mean relative humidity, in percentages.
| Station. | Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. | Year. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ouargla | 61.0 | 58.6 | 60.3 | 51.2 | 52.0 | 49.0 | 44.1 | 44.7 | 50.7 | 56.8 | 59.1 | 52.7 | 54.6 |
| El Golea | 60.8 | 56.8 | 41.3 | 41.3 | 41.0 | 31.3 | 28.5 | 31.8 | 34.8 | 37.1 | 58.1 | 65.1 | 45.5 |
| In Salah | 51.5 | 48.0 | 45.5 | 40.4 | 34.6 | 32.0 | 23.4 | 24.6 | 17.2 | 38.0 | 52.2 | 58.7 | 42.6 |
| Ft. National | 87.4 | 80.3 | 87.6 | 86.6 | 96.6 | 80.4 | 75.8 | 62.8 | 80.2 | 87.7 | 92.8 | 88.4 | 85.1 |
A consideration of the evaporation statistics of Algeria for the year 1908 shows some interesting relations. It has already been observed that the rainfall along the coast is less in the west than in the east, and it will appear below that as a rule the temperature of the western portion is lower than that of the corresponding regions lying to the east. In both rainfall and temperature, however, the greatest variation is to be found as one goes inland, when decreasing rainfall and higher temperatures are encountered. A similar relation obtains in evaporation, which becomes continuously greater as the distance from the coast increases; that is, the average evaporation for stations on the High Plateau is greater than for stations in the Tell, and the evaporation at desert stations is greater than the evaporation on the High Plateau. The total average evaporation for the year, in millimeters, for 5 stations on the littoral, was 1,365.3; for 7 stations in the Tell it was 1,378.6; for 4 stations on the High Plateau 2,352.2, and for 3 stations in the desert, 3,977.5. The least evaporation reported was at Bouzarea, which was 989.9 mm., and the greatest was at Ghardaia, 5,309.7 mm.[5]
Table 5 gives in detail the monthly as well as the total evaporation for the year at three desert stations and at Algiers for 1908.
Table 5.—Evaporation in millimeters, 1908.
| Station. | Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. | Year. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Laghouat | 88.9 | 102.3 | 143.4 | 203.1 | 289.2 | 373.9 | 421.2 | 379.6 | 264.8 | 173.9 | 153.8 | 159.2 | 2753 |
| Ghardaia | 172.4 | 233.4 | 340.7 | 528.7 | 611.7 | 699.1 | 749.2 | 693.0 | 468.8 | 329.4 | 257.7 | 225.5 | 5309 |
| Touggourt | 163.9 | 240.5 | 274.3 | 385.2 | 459.4 | 565.8 | 487.3 | 329.4 | 222.6 | 166.1 | 142.3 | ||
| Algiers | 84.2 | 96.8 | 90.5 | 118.3 | 151.8 | 158.6 | 175.0 | 205.0 | 165.5 | 129.0 | 136.1 | 143.2 | 1654 |