The physical environment of the plants of southern Algeria is, in
a few broad features, similar to that of the southwestern portion
of the United States. These regions have about the same latitude,
both are separated from a large sea by mountains, and the range in
altitude is similar. There are other features, however, particularly
as regards the amount and the distribution of the precipitation,
in which the two widely separated regions are very unlike, and a
correlated difference in the habit and composition of the floras
of the two regions is apparent.
The Algerian climate as a whole is a mild, temperate one, but very
diverse. The latitude and topography taken in connection with the
presence of large seas to the north and west, and a large continent
leading away to the south are its chief determinants. The climate,
therefore, of the northern portion is coastal; that of the southern
portion continental.
Probably the most important of the secondary factors which modify
the climate of Algeria is its highly varied topography. An important
mountain system, the Atlas, made up of many more or less detached
groups and secondary systems, a plateau or steppe lying 3,000 feet
more or less above the sea, and finally the northern edge of the
Sahara, which has a very diverse topography of its own—such is
the surface of Algeria.
Algeria is divided into three climatic provinces corresponding to the
leading topographical differences: the Tell, including the littoral,
or portion between the maritime Atlas and the Mediterranean; the
High Plateau, or steppe, which lies between the Tellian Atlas and the
Saharan Atlas; and the desert. These provinces have marked individual
differences in rainfall, temperature, and other climatic features.
In the Tell and the High Plateau the winds from the sea deposit most
of their moisture. Along the coast as much as 700 mm. of rain is
recorded, while in other parts of the Tell it is about 570 mm. On
the High Plateau the yearly precipitation sinks to 310 mm. In the
desert south of the Saharan Atlas, however, where the altitude
is lower and the temperature greater than in either of the other
provinces, the yearly rainfall is 200 mm. and less. In some years,
in fact, no precipitation whatever is reported in the desert.
The seasonal distribution of the rains in any arid or semi-arid
region is of great importance as a factor in shaping the character
of the vegetation. For example, in the semi-arid region of the
southwestern part of the United States, in the Tucson region, there
are two distinct rainy seasons—the rains of winter and those of
summer—and here the plants with a water-balance are an important
feature; but farther to the west, where there are no summer rains,
there are no succulents. In Algeria, also, there is but one rainy
season and it has already been noted that the absence of plants with
water-storage facilities is one of the leading characteristics of
its vegetation. The seasonal distribution of rains is as follows:
In the Tell, in winter it is 42 per cent, in spring 27 per cent,
in autumn 27 per cent, and in summer only 4 per cent. On the High
Plateau the percentages are 30, 20, and 34 for winter, spring,
and autumn, and 16 for summer. In the desert the percentages for
winter, spring, and autumn are 37, 39, and 20 respectively, while
in summer 4 per cent of the entire rainfall occurs. On the High
Plateau, however, and in the Saharan Atlas the distribution of rain
is much more equable, since 16 per cent falls on the High Plateau in
summer and 13 per cent in the Saharan Atlas the same season. We
have, therefore, the interesting result that both in the Tell and
on the desert there is a long, dry summer season, but in the
intervening country more or less rain falls at this time of year.
It seems very probable that a careful study of the plants of these
regions would show reflected in the vegetation this peculiar
character of climate.[33]
The mean relative humidity changes in a marked manner as one passes
from the Tell, across the High Plateau, and enters the desert. For
example, at Fort National the mean relative humidity is 85 per
cent. On the desert it varies from 54.6 per cent at Ouargla to 42.6
per cent at In Salah. At times in midsummer the humidity in the
desert is too low to measure with instruments; it is often 7 or 9
per cent. On the other hand, the humidity in autumn is surprisingly
high, owing in part to the lower temperature and in part to the
northerly winds. However, no dew is reported and probably its
occurrence is rare.
With so great difference between the Tell and the desert in
relative humidity is associated marked variation in the rate of
evaporation. For example, at Algiers the total annual evaporation
is 1,654 mm., while at Ghardaia it is 5,309 mm., which is possibly
the greatest amount of evaporation thus far reported. Thus, the
difference in evaporation between the Tell and the desert is nearly
as the ratio 4 to 1.
The evaporation-rainfall ratios for the Tell, High Plateau, and
the desert are of great interest. The seasonal evaporation-rainfall
ratio for the littoral is 2.5 to 1; that of the Tell is 3.5 to 1;
that of the High Plateau is 9.4 to 1; and that of the desert is
46.5 to 1. If we represent the evaporation-rainfall ratio as unity,
the ratio for the Tell becomes 1.4 and the ratio of the High Plateau
becomes 9 to 3.7, while the desert ratio is 18.6.
The annual and daily variations of temperatures in the desert are
naturally relatively great. At Algiers the annual variation is
approximately 40.7° C., while at Ghardaia it is 47.9° C. As great
an annual variation as 57° C. has been observed at Ghardaia. The
daily variation of temperature is especially marked on the High
Plateau and the desert, ranging 17° C. more or less on the High
Plateau, and 20° C. more or less on the desert. These figures are
occasionally overstepped: for example, at Batna in July, 1904, when
the maximum daily range was 21.8° C.; and an observation made by the
writer in the open desert between Ouargla and Touggourt, in November,
1910, showed a variation of temperature of 24.7° C. between 3
o’clock in the afternoon and 6 o’clock the following morning.
The absolute maximum temperatures in southern Algeria are fairly
high. At El Golea, for example, they are 47°, 46.5°, 48°,
and 49.2° C., while at In Salah, about 700 miles from Algiers,
the maximum temperatures for four years have been found to be
50°, 49.2°, 50°, and 48° C. It is interesting to note that at
Ouargla, which is much nearer the coast than In Salah, even higher
temperatures have been recorded. The maxima for as many years are
as follows: 50.2°, 51.0°, 52°, 49°, and 48.4° C. Usually in
winter freezing temperatures are experienced at all stations in
southern Algeria.
Very little has been done on the soil temperatures in the desert
region, but at Ghardaia, in July, 1911, the temperature of the
soil 15 cm. beneath the surface of the soil ranged, maximum from
36° to 37° C., minimum from 31° to 33° C., giving an absolute
range of 6° C.
In addition to the rainfall, the evaporation, and the temperature,
there is another important climatic factor which should be taken
into account, but which can not be stated in accurate terms, _i.e._,
air-currents. It is a matter of common experience that one rarely
observes a calm day on the desert, but that usually the wind,
which is often of considerable force, is found to be blowing. This
is unquestionably an important factor in raising the total of
evaporation and therefore in increasing the arid conditions of
this region. The winds which are most effective in the direction
mentioned are those which come from the desert and are known as the
“sirocco;” these are most likely to blow in spring and summer,
although they occur in autumn also, and to a very limited degree in
winter. When the winds blow from the north cooler conditions occur,
the relative humidity is lower, and therefore the evaporation rate
is less. The sirocco, or desert wind, crosses the Mediterranean and
is sometimes felt in southern Europe. It does not generally last
more than three days at one time, but at Batna, in July, 1902, it
was reported for eight consecutive days. When the sirocco blows the
humidity is likely to be markedly affected; for example, at Batna
during the eight-day sirocco alluded to, the relative humidity fell
from 25.6 to 16 per cent on the first day.