Climate of Algeria
The climate of Algeria is mild and temperate. This is due to several factors, among which are its situation relative to the Mediterranean on the north and to the Atlantic on the west, as well as to the great desert which constitutes its southern portion, the great variation in topography, and the fairly low latitude. Taking the colony as a whole, there is a great range in temperature, precipitation, relative humidity, and evaporation, to mention only such climatic features as have been reduced and are recorded; and the range in the intensity and in the quality of the light must also be great. The climate of the northern portion of Algeria is coastal, while that of the southern portion is continental.
The distribution in time and in space and the amount of precipitation are of the greatest importance as climatic features of Algeria. The rainfall is heaviest on the littoral, and especially heavy in the eastern portion of the littoral. An average of 1,000 mm. is reported from the immediate vicinity of the sea,2 and as one goes southward the amount of precipitation rapidly becomes less. In the Tell the average rainfall is 570 mm., while on the High Plateau it is 310 mm. On the desert the rainfall is uncertain both in amount and in time, except that when rains occur the time coincides with the rainy season of northern Algeria. At Biskra the annual precipitation is 199 mm., at Laghouat it is 198 mm., at Ghardaia it is 114 mm., and at El Golea it is 47 mm. In many places in the western Sahara, five years or more go by without fall of rain.
The differences in the geographical distribution of precipitation vary from year to year, as may be illustrated by referring to that for the year 1908, which may be compared with the normal usual distribution as given above. In the northern portion of the country more rain than usual was reported; for example, there was over 1,000 mm. on the littoral east of Algiers, and over 500 mm. on the High Plateau, but on the desert the amount was somewhat less. At Laghouat it was 161 mm., at Ghardaia it was 89.2 mm., and at Ouargla it was 28 mm.
Besides the differences in amount of yearly rainfall, well-marked seasonal amounts of precipitation are also to be noted. In the northern portion of the colony rains are likely to occur in winter and spring, the summer and early autumn being dry; and as one goes south of the Saharan Atlas nearly the same conditions obtain; that is, the rains usually fall during the rainy season of the coast. The seasonal distribution of rain for the Tell, including the stations of the littoral, the High Plateau, the Saharan Atlas, and the desert, for a series of years including 1908, is given in table 1.
Table 1 — Seasonal Distribution of Rain
| Season. | Tell (10). | High Plateau (8). | Saharan Atlas (10). | Desert (6). | Remarks. | |
|---|---|---|---|---|---|---|
| mm. | mm. | mm. | mm. | |||
| Winter | 469.9 | 119.3 | (50.8) | 113.5 | 37.4 | The Tell is represented by Fort National; the High Plateau by Geryville (with Ain Sefra, for 6 years, in parentheses); the Saharan Atlas by Djelfa; the desert by Ouargla. The number of yearly records on which averages are computed are given at the head of each column in parentheses. |
| Spring | 300.0 | 80.5 | (95.0) | 88.2 | 39.0 | |
| Summer | 38.1 | 66.5 | (35.8) | 50.4 | 3.7 | |
| Autumn | 287.2 | 139.5 | (58.5) | 123.1 | 19.6 | |
The seasonal percentages of precipitation give a more graphic conception of the rainfall conditions for the four physiographic provinces. In the Tell this percentage in winter is 42, in spring 27, in summer 4, and in autumn 27. On the High Plateau the percentages are 30, 20, 16, and 34 for the four seasons respectively. In the Saharan Atlas 30 per cent of the rain occurs in winter, 24 in spring, 13 in summer, and 33 in autumn. In the desert the percentages of rainfall are 37, 39, 4, and 203 for the four seasons.
It is of interest also to note the number of days on which the rain fell on an average each year over a period running from 7 to 12 years. Thus at two typical stations on the Tell rain was reported on 102 and 118 days; at two stations on the High Plateau it rained 65 and 83.8 days; at a station in the Saharan Atlas rain was reported on 70 days; at desert stations, at Ouargla rain fell on an average 14.2, and at Laghouat 49 days each year. As a comparison, it may be mentioned that for ten years at Wady Halfi, Egyptian Sudan, there were only 22 days on which rain-drops were seen to fall. (Engler, loc. cit.)
The amount of precipitation varies greatly for the different desert stations, usually becoming less as one goes south from the High Plateau. As has already been mentioned, the average rainfall at Laghouat, which lies at the southern base of the Saharan Atlas, is 198 mm., the average at Ghardaia is 114 mm., while that at El Golea is 47 mm. The latter station is about 225 miles south of Laghouat, in the midst of the Sahara. The amount of rainfall, however, is greatly influenced by altitude, although lack of adequate precipitation data for the desert makes impossible a detailed presentation of this phase of the subject. As the amount of the yearly precipitation is less in the extreme southern part of Algeria than it is nearer the Saharan Atlas, where the altitude also is greater, it might be expected that the number of rainy days would vary in a like manner. Such records as are at hand, however, do not show this to be the case. For instance, at Ouargla rain falls on an average 14.2 days, average of 7 years, while the rainfall is 90.2 mm.; yet at El Golea, with a rainfall of 47 mm., there are 23.4 rainy days each year.4
On the desert the rains are often of a torrential nature, as facts presented above would indicate, and sometimes as much rain falls within a few hours, or even a few minutes, as usually occurs in an entire year. How much of the annual precipitation is of this character and how much is of the non-torrential kind the usual summaries leave entirely out of the account. It is well known that the former type of storm is more destructive and less useful to plants than the latter type. To illustrate the irregularity of the rainfall in the northern Sahara the monthly precipitation at Ouargla for several years is presented in table 2.
Table 2 — Rainfall at Ouargla, in Millimeters
| Year | Amt. | No. of days rain | Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1900 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 1901 | 18 | |||||||||||||
| 1902 | 70.5 | 8 | 0 | 0 | 11.3 | 0 | 0 | 0 | 0 | 1.8 | 0 | 46.4 | 11.0 | 0 |
| 1903 | 135.6 | 14 | 55.0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 30.6 | 50.0 |
| 1904 | 35.0 | 26 | 20.0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 9.0 | 4.0 | 2.0 |
| 1905 | ||||||||||||||
| 1906 | 15.0 | 0 | 2.0 | 4.0 | 0 | 0 | 4.0 | 80.0 | 41.0 | 80.0 | ||||
| 1907 | 210.0 | 18 | 0 | 68.2 | 67 | 325.0 | 16.2 | 20.0 | 0 | 0 | 5.0 | 3.3 | 3.8 | 1.3 |
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 |
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 July7 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.
Table 8 — Maximum Daily Range in Temperature, 1904
| Station. | Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | |
| Algiers | 9.0 | 13.0 | 13.0 | 13.6 | 12.7 | 13.0 | 15.0 | 15.4 | 11.2 | 14.0 | 10.8 | 10.6 |
| Ft. National | 10.4 | 13.8 | 13.0 | 12.4 | 13.0 | 12.8 | 12.4 | 14.8 | 16.4 | 11.2 | 11.6 | 11.0 |
| Batna | 11.5 | 15.4 | 16.4 | 19.4 | 20.2 | 21.8 | 16.4 | 16.0 | 19.4 | 20.2 | ||
| Saida | 14.0 | 17.0 | 14.0 | 18.0 | 18.0 | 19.0 | 19.0 | 18.0 | 18.0 | 14.0 | 13.0 | 13.0 |
The temperature conditions for the year 1908 will serve very well to further illustrate this phase of the climate of Algeria. No freezing temperatures were reported from the coast stations, although at the northern base of the Atlas of the Tell several degrees of frost were recorded. At Blida, for example, the thermometer registered -4.0° C. as the minimum. Away from the littoral at every station whose records were seen the lowest temperatures were zero centigrade, or below. The coldest weather was felt at Aflou, at the north base of the Dj. Amours, of the Saharan Atlas, where -11.0° C. was reported. The summer was warm along the coast and hot in the interior. The records of temperature of 23 stations of the Tell were seen, and of this number, at only 8 was a temperature of 40° C. reached or exceeded. On the other hand, at 44 out of 56 stations on the High Plateau and the desert, the temperatures reported are as great or greater than 40° C. Where temperatures below this figure were recorded the stations were situated in the mountains.
The western portion of the Sahara is apparently not so hot as the eastern portion. The mean annual temperature for Biskra is 20.3° C.; for Ghardaia it is 21° C.; for El Golea it is 22.2° C.; and for Touggourt it is 23.4° C. The mean temperature at Cairo is 21.3° C.; for Suez it is 21.5° C.; and for Djedda, on the Red Sea, it is 27.3° C. The absolute maximum temperatures in southern Algeria for as many years and for two of the most southern army posts, El Golea and In Salah, are as follows: El Golea, 47°, 46.5°, 48°, and 49.2° C.; In Salah, 50.0°, 49.2°, 50.2°, and 48° C.; at Ouargla the maxima are 50.2°, 51°, 52°, 49°, and 48.4° C. On account of the fact that in the western Sahara at these stations in winter there are usually freezing temperatures, the yearly range of temperature is 50.0° C., or over. The greatest range reported for the western Sahara is that for Ghardaia as given by Engler, namely, 57°, from -7° to 50° C. At Timmimoun, 1904, the range was from -3.0° to 53.1° C., or one of 56.1° C. The extreme absolute daily range in temperature on the desert appears not to exceed that of certain stations on the High Plateau, although, as shown below, the temperature variations on the desert may be much larger than the records indicate. At Laghouat and at Ghardaia the thermometers which I exposed showed a daily range of from 10.5° to 12.5° C. only. The instruments were placed on the outside of buildings, and usually on the second story. As a contrast to this observed diurnal variation, an opportunity was given to take temperatures on the open desert at a time when the days were fairly warm and the nights were rather cold. The place was between Touggourt and Ouargla. At 3 o’clock on the afternoon of November 26, 1910, the shade temperature at the place in question was 23° C. During the night the thermometer registered -1.7° C. as the minimum, thus showing a drop of 24.7° C. in something over 12 hours. Table 9 gives the extreme daily range in temperature for three years observed at the stations named.
Table 9 — Absolute Daily Range in Temperature
| Station. | Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | |
| In Salah | 21.6 | 23.0 | 24.4 | 24.6 | 22.0 | 23.2 | 23.4 | 22.0 | 29.0 | 20.0 | 23.4 | 22.6 |
| Ouargla | 20.8 | 25.4 | 24.0 | 25.0 | 27.0 | 26.0 | 26.0 | 27.8 | 24.6 | 23.0 | 20.0 | 23.8 |
A further examination of the maximum temperatures shows certain climatic features of interest and of great importance as factors in the environment of the desert plants, especially the high average maxima and the large amount of heat received in the desert, as indicated by a summation of the maximal temperatures. That the greatest daily temperatures must be high is indicated by table 10, which gives the average maxima for three to five years at three typical desert stations.
Table 10 — Average Maximum Temperatures
| Station. | Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | °C. | |
| Ouargla | 21.9 | 18.3 | 30.9 | 37.8 | 41.1 | 47.1 | 49.4 | 47.7 | 44.3 | 40.0 | 31.0 | 25.0 |
| El Golea | 23.0 | 25.8 | 35.6 | 27.5 | 40.9 | 44.4 | 47.5 | 46.0 | 43.8 | 36.7 | 29.9 | 22.4 |
| In Salah | 27.4 | 30.1 | 37.2 | 41.0 | 43.4 | 47.6 | 44.3 | 47.1 | 45.9 | 41.3 | 37.0 | 27.1 |
The maximum temperatures show that the total amount of heat received on the desert, as compared to that received in the climatic provinces nearer the sea, is not only great, but also that the amount is variable on the desert itself, one station receiving much more heat at certain times of the year than another station. The total heat for each station is arrived at by adding up the daily maxima for the midseasonal months, January, April, July, and October, and dividing by the number of years whose records were consulted. The amounts given in table 11 are the averages for three to four years, for three desert stations and for one station in the Tell.
Table 11 — Sums of the Daily Maximum Temperatures
| Station. | January. | April. | July. | October. |
|---|---|---|---|---|
| °C. | °C. | °C. | °C. | |
| Laghouat | 439 | 476 | 1,135 | 668 |
| In Salah | 641 | 1.027 | 1.455 | 1.121 |
| Ouargla | 570 | 909 | 1.444 | 923 |
| Ft. National | 230 | 470 | 697 | 564 |
The variation in total heat received at the three desert stations may probably be explained partly at least by their differences in latitude and in elevation above the sea, as well as the different relations they hold to the highlands of the northern portion of the colony. Laghouat lies immediately south of the Saharan Atlas, latitude 33°48′, and at an elevation of 780 meters. Ouargla is approximately 200 miles south of the mountains, latitude 31°55′, and altitude 150 meters. In Salah is in the midst of the western Sahara, latitude 27°17′, and about 300 meters above the sea. Fort National lies in the Tell about 30 miles from the sea, and at an altitude of 916.3 meters. The latitude of Fort National is 36°38′.
There are no published records of the soil temperatures of southern Algeria. In the following table is given soil-temperature data taken at Ghardaia by M. Buret. Maximum and minimum standard chemical thermometers were used. They were placed in a horizontal position in fixed tubes which were about 30 cm. in length. Precautions were taken to properly insolate the instruments. The depth was 15 cm.
Table 12 — Soil and Air Temperatures, Ghardaia, July 2-11, 1911
| Date. | Soil. | Air. | |||
|---|---|---|---|---|---|
| Maximum. | Minimum. | Maximum. | Minimum. | ||
| °C. | °C. | °C. | °C. | ||
| July | 2 | 36.0 | 31.0 | 41.0 | 21.0 |
| July | 3 | 36.0 | 31.0 | 41.0 | 22.0 |
| July | 4 | 36.0 | 31.0 | 41.0 | 23.0 |
| July | 5 | 36.5 | 31.5 | 41.0 | 23.5 |
| July | 6 | 33.7 | 32.0 | 41.0 | 25.0 |
| July | 7 | 36.0 | 32.5 | 40.0 | 26.0 |
| July | 8 | 36.0 | 32.5 | 38.0 | 26.5 |
| July | 9 | 37.0 | 25.0 | ||
| July | 10 | 37.0 | 32.5 | 40.0 | 24.0 |
| July | 11 | 33.0 | 23.0 | ||
An examination of the meteorological reports for Algeria shows that the direction of the prevailing winds varies considerably, although the variation is possibly greater near the coast than in the interior. The winds undoubtedly play an important rôle in the environment of the vegetation of the country. At Algiers, in 1907, winds were reported from the north 288 times, from the east 203 times, from the west 224 times, and from the south 51 times. At Barika, on the High Plateau, the number of times and the directions of the winds for the same year are as follows: northeast, 203 times; east 242; south 21, west 89, and southwest 93. At El Golea, on the desert, the winds were as follows: north 192, northeast 614, southeast 107. The winds from the north, or northerly winds, are cool and laden with moisture, but they are most effective in regions near the coast or in the mountains. Winds from the south are dry winds, and are probably of great importance in limiting the distribution of plants through the increased aridity caused by them.
The most important of these desert winds is the sirocco, a wind that is most likely to blow in spring and summer, although occurring in autumn also, and to a very limited degree in the winter season. The sirocco crosses the Mediterranean and is felt in the southern portions of France, on the Italian Riviera, and in other parts of southern Europe. It is especially common on the High Plateau; for example, during five years it was reported on an average of 28.4 days each year. It does not generally last more than three days, but at Batna, in July, 1902, it was recorded on eight consecutive days. The sirocco operates to lower the relative humidity and to raise the temperature. For instance, on the first day of the eight-day sirocco above alluded to, the average relative humidity was 16 per cent, while the average on the preceding day was 25.6 per cent. It has already been remarked that on the days of the greatest temperature variation the sirocco was usually blowing. In the desert the winds often bring with them much dust and sometimes last during several days; for example, one is reported at In Salah, in May, 1904, for six consecutive days. Such winds on the desert may come from the north, or at least may be northerly, and when storms of this character are in progress, although possibly destructive of animal life or at least comfort, the effects as regards decreasing the relative humidity and raising the air temperature are not so marked as is the case when the sirocco is blowing.