+--------------+------+------+------+------+------+------+------+
| Station. | Jan. | Feb. | Mar. | Apr. | May | June | July |
+--------------+------+------+------+------+------+------+------+
| Ouargla | 61.0 | 58.6 | 60.3 | 51.2 | 52.0 | 49.0 | 44.1 |
| El Golea | 60.8 | 56.8 | 41.3 | 41.3 | 41.0 | 31.3 | 28.5 |
| In Salah | 51.5 | 48.0 | 45.5 | 40.4 | 34.6 | 32.0 | 23.4 |
| Ft. National | 87.4 | 80.3 | 87.6 | 86.6 | 96.6 | 80.4 | 75.8 |
+--------------+------+------+------+------+------+------+------+
[**Continued]
+--------------+------+-------+------+------+------+-------+
| Station. | Aug. | Sept. | Oct. | Nov. | Dec. | Year. |
+--------------+------+-------+------+------+------+-------+
| Ouargla | 44.7 | 50.7 | 56.8 | 59.1 | 52.7 | 54.6 |
| El Golea | 31.8 | 34.8 | 37.1 | 58.1 | 65.1 | 45.5 |
| In Salah | 24.6 | 17.2 | 38.0 | 52.2 | 58.7 | 42.6 |
| Ft. National | 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 |
+---------+-----+-----+-----+-----+-----+-----+-----+
|Laghouat |88.9 |102.3|143.4|203.1|289.2|373.9|421.2|
|Ghardaia |172.4|233.4|340.7|528.7|611.7|699.1|749.2|
|Touggourt| |163.9|240.5|274.3|385.2|459.4|565.8|
|Algiers |84.2 |96.8 |90.5 |118.3|151.8|158.6|175.0|
+---------+-----+-----+-----+-----+-----+-----+-----+
[**Continued]
+---------+-----+-----+-----+-----+-----+-----+
|Station. |Aug. |Sept.|Oct. |Nov. |Dec. |Year.|
+---------+-----+-----+-----+-----+-----+-----+
|Laghouat |379.6|264.8|173.9|153.8|159.2|2753 |
|Ghardaia |693.0|468.8|329.4|257.7|225.5|5309 |
|Touggourt|487.3|329.4|222.6|166.1|142.3| |
|Algiers |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. |
+----------------+------+-------+-------+-------+-------+-------+
| Nemours | 1.07| 1.3 | 1.39 | 1.9 | 131 | 5.4 |
| Cape Falcon | 1.5 | 1.8 | 1.46 | 16.0 | 8.8 | 10.3 |
| Oran | 1.3 | 1.9 | 1.6 | 1.3 | 17.4 | 8.5 |
| Algiers | 0.63| 1.3 | 0.5 | 1.1 | 22.3 | 11.7 |
| Bouzarea | 0.57| 0.67| 0.22 | 0.56 | 8.3 | 4.8 |
| Maison-Carée | 0.72| 0.64| 0.55 | 1.1 | 13.5 | 11.8 |
| Ft. National | 0.72| 0.29| 0.34 | 0.58 | 3.7 | 192 |
| Sidi-bel-Abbès | 0.63| 0.76| 0.6 | 1.1 | 18.8 | 18.5 |
| Saida | 0.67| 0.64| 0.36 | 0.83 | 2.2 | 6.9 |
| Batna | 1.6 | 2.3 | 1.5 | 2.5 | 46.5 | 33.4 |
| Tebessa | 4.6 | 6.05| 21.2 | 3.9 | 252.0 | 2.8 |
| Bou Saada | 4.05| 5.4 | 12.1 | 2.9 | 6.1 | 165 |
| Barika | 2.4 | 8.7 | 9.9 | 5.0 | 5.03| 270 |
| Ain Sefra | 9.2 | 21.5 | 1.5 | 23.8 | 12.4 | 63.1 |
| Geryville | 6.3 | 1.9 | 2.1 | 23.9 | 2.5 | 16.7 |
| Laghouat | 3.09| 4.7 | 9.8 |203.0 | 7.0 | 373 |
| Ghardaia | 8.9 | 233.0 | 81.4 |529.0 | 38.9 | 699 |
| El Oued | 53.9 | 27.7 | 67.0 |629.0 | 369.0 | 465 |
+----------------+------+-------+-------+-------+-------+-------+
[**Continued]
+----------------+-------+-------+-------+------+-------+-------+
| Station. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
+----------------+-------+-------+-------+------+-------+-------+
| Nemours | 23.6 | 43.3 | 8.6 | 3.1 | 10.6 | 5.7 |
| Cape Falcon | 29.7 | 136.0 | 38.1 | 1.9 | 8.9 | 2.2 |
| Oran | 59.0 | 146.0 | 18.0 | 2.45| 3.4 | 4.4 |
| Algiers | 219.0 | 28.8 | 5.07| 1.06| 1.2 | 1.1 |
| Bouzarea | 53.7 | 27.1 | 3.4 | 0.2 | 0.76| 0.5 |
| Maison-Carée | 149.0 | 12.0 | 5.2 | 0.9 | 0.35| 0.51|
| Ft. National | 42.5 | 15.5 | 2.7 | 1.6 | 0.71| 0.16|
| Sidi-bel-Abbès | 77.8 | 37.8 | 14.8 | 1.4 | 1.9 | 0.8 |
| Saida | 86.7 | 12.1 | 4.7 | 5.8 | 1.2 | 0.78|
| Batna | 24.4 | 7.6 | 3.5 | 4.9 | 2.6 | 1.6 |
| Tebessa | 9.8 | 14.8 | 1.1 | 3.9 | 3.5 | 3.7 |
| Bou Saada | 43.5 | 19.5 | 8.3 | 13.9 | 5.6 | 8.3 |
| Barika | 6.1 | 4.1 | 11.4 | 3.7 | 90.6 | 1.3 |
| Ain Sefra | 16.4 | 124.0 | 328.0 | 14.7 | 13.2 | 1.2 |
| Geryville | 22.5 | 23.4 | 3.4 | 2.1 | 4.1 | 3.9 |
| Laghouat | 421.0 | 21.0 | 25.9 | 14.2 | 154.0 | 104.0 |
| Ghardaia | 166.0 | 25.6 | 66.9 | 23.2 | 49.7 | 225.0 |
| El Oued | 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.
TABLE 8.—_Maximum daily range in temperature_, 1904.
+--------------+-------+-------+-------+-------+-------+-------+
| Station. | Jan. | Feb. | Mar. | Apr. | May | June |
+--------------+-------+-------+-------+-------+-------+-------+
| | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
| Algiers | 9.0 | 13.0 | 13.0 | 13.6 | 12.7 | 13.0 |
| Ft. National | 10.4 | 13.8 | 13.0 | 12.4 | 13.0 | 12.8 |
| Batna | 11.5 | 15.4 | 16.4 | 19.4 | | 20.2 |
| Saida | 14.0 | 17.0 | 14.0 | 18.0 | 18.0 | 19.0 |
+--------------+-------+-------+-------+-------+-------+-------+
[**Continued]
+--------------+-------+-------+-------+-------+-------+-------+
| Station. | July | Aug. | Sept. | Oct. | Nov. | Dec. |
+--------------+-------+-------+-------+-------+-------+-------+
| | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ | _°C._ |
| Algiers | 15.0 | 15.4 | 11.2 | 14.0 | 10.8 | 10.6 |
| Ft. National | 12.4 | 14.8 | 16.4 | 11.2 | 11.6 | 11.0 |
| Batna | 21.8 | 16.4 | 16.0 | 19.4 | | 20.2 |
| Saida | 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.