114
CLAUDE J. GRENOT
ephemer als. Consequently, rainfalls providing only a small quantity of
water ranging from 1 to 5 mm are totally ineffective, since the water
evaporates before it can be accumulated. The hygrométrie degree of the
air remains low (Table III).
2. Temperature
Desert temperatures are characterized by high maxima and by great
variations between winter and summer as well as between day and night
(Fig. 3a,b and Tables IV and V) The dryness of the air is the factor which
is responsible for causing these sudden variations of great amplitude
(Schirmer, 1893).
At Reggan, in 1960-1961, the mean maximum air temperature was recorded as 45.5°C and the mean minimum as 31.0°C during the warmest
month (July). During the coldest month (January), the mean maximum
was 20.0°C, and the mean minimum 5.9°C (Table IV). That same year,
the absolute temperature extremes were 48.1° and 1.2°C. An absolute temperature maximum of 59.1°C was recorded at Tindouf (Pierre, 1958) and
54.4°C at Timimoun (Planhol and Rognon, 1970).
Freezing temperatures in the northern and central Sahara are not exceptional; Béni-Abbès, Adrar, In Salah and Reggan, can get such temperatures for more than 1 week/year, but it rarely gets lower than —3°C.
However, in other regions such as Tibesti, minimal temperatures might
sometimes reach —22°C.
Grenot and Niaussat (1967) have observed in a reg near Reggan, temperatures below freezing on the ground surface during several nights in
December, 1959 and January, 1960.
In the daytime, the soil warms up faster than the air, and as a consequence, the soil temperature is notably higher. However, at night, the
sandy surface (erg) cools off faster than the air, while the rock surfaces
(hamada) remain warmer (Fig. 3a,b and Table VI) (Grenot, 1968a).
The annual range of thermic variations can be greater than 60°C. The
diurnal thermic amplitude, which is the difference between maximal temperature during the day and minimal temperature during the night, is quite
often greater than 35°C. Temperature variations occur more abruptly in
an erg than in any other type of surface formations. In the spring, the
temperature of the sand surface in midday can be as high as 60°C, while
the temperature of the air, directly adjacent to it, remains below 40°C.
This temperature rise on the surface is not propagated deeply into the soil.
At 20 cm, diurnal variations are quite diminished; this is true especially
in rock formations, where temperature is seldom higher than 30°C, allowing the survival of various species of animals. During the summer, the sand
CLAUDE J. GRENOT
ephemer als. Consequently, rainfalls providing only a small quantity of
water ranging from 1 to 5 mm are totally ineffective, since the water
evaporates before it can be accumulated. The hygrométrie degree of the
air remains low (Table III).
2. Temperature
Desert temperatures are characterized by high maxima and by great
variations between winter and summer as well as between day and night
(Fig. 3a,b and Tables IV and V) The dryness of the air is the factor which
is responsible for causing these sudden variations of great amplitude
(Schirmer, 1893).
At Reggan, in 1960-1961, the mean maximum air temperature was recorded as 45.5°C and the mean minimum as 31.0°C during the warmest
month (July). During the coldest month (January), the mean maximum
was 20.0°C, and the mean minimum 5.9°C (Table IV). That same year,
the absolute temperature extremes were 48.1° and 1.2°C. An absolute temperature maximum of 59.1°C was recorded at Tindouf (Pierre, 1958) and
54.4°C at Timimoun (Planhol and Rognon, 1970).
Freezing temperatures in the northern and central Sahara are not exceptional; Béni-Abbès, Adrar, In Salah and Reggan, can get such temperatures for more than 1 week/year, but it rarely gets lower than —3°C.
However, in other regions such as Tibesti, minimal temperatures might
sometimes reach —22°C.
Grenot and Niaussat (1967) have observed in a reg near Reggan, temperatures below freezing on the ground surface during several nights in
December, 1959 and January, 1960.
In the daytime, the soil warms up faster than the air, and as a consequence, the soil temperature is notably higher. However, at night, the
sandy surface (erg) cools off faster than the air, while the rock surfaces
(hamada) remain warmer (Fig. 3a,b and Table VI) (Grenot, 1968a).
The annual range of thermic variations can be greater than 60°C. The
diurnal thermic amplitude, which is the difference between maximal temperature during the day and minimal temperature during the night, is quite
often greater than 35°C. Temperature variations occur more abruptly in
an erg than in any other type of surface formations. In the spring, the
temperature of the sand surface in midday can be as high as 60°C, while
the temperature of the air, directly adjacent to it, remains below 40°C.
This temperature rise on the surface is not propagated deeply into the soil.
At 20 cm, diurnal variations are quite diminished; this is true especially
in rock formations, where temperature is seldom higher than 30°C, allowing the survival of various species of animals. During the summer, the sand