Botanical features of the Algerian Sahara

by William Austin Cannon

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.