122
CLAUDE J. GRENOT
reach 4.80 meters in Béni-Abbès, while in California or in the Australian
desert, for example, it is generally less than 3 meters (Dubief, 1950).
Air hygrométrie degree, or relative humidity, remains very low. "Nowhere else on the earth has air been found as dry as in the Sahara region"
(Savornin, 1947). In the arid Sahara, delimited by the isohyet of 2 or
5 cm annually, the relative humidity varies between 4 and 20% (Table
III). But in the septentrional Sahara it reaches 20 to 30% during the summer, and goes up to 50% during the winter.
Vegetation, a relay station between the water contents of the ground
and of the atmosphere, plays an important role in contributing to humidity
of the air. For example, in the same area and at the same moment, the
relative humidity in a reg or hamada (bare ground) would be close to
10% as opposed to 90% in a palm grove (oasis). This fact already leads
us to realize the importance of "microclimates."*
5. Luminosity
Because of the absence of nebulosity, the Sahara is one of the regions
of the earth where direct solar insolation is the greatest, during 300-360
days/year. In 1964, at Reggan, the monthly mean cloud cover varied from
4% (July and September) to 3 1 % (December). Such high luminosity
is a dehydrating factor, since it increases the temperature effects. It could
become a considerable source of energy. Each square meter of the Sahara
receives from the sun an average of 1 kW of heat and luminous power. An
area of 100 X 100 km receives approximately 25 X 10
12
kWh annually,
or the heat equivalent of roughly 2 X 10° metric tons of fuel, (derived
from Chouard, 1957). Let us remember that the surface of the Sahara
is 8 X 10
6
km
2
.
6. Aridity Concepts
Aridity is closely related to rainfall and temperature. Thus, "water" and
"temperature" counteract one another, particularly in regards to vegetation
(Ozenda, 1958).
Aridity is defined by numerous formulas which vary according to their
authors (de Martonne, 1926; Emberger, 1938; Bagnouls and Gaussen,
1953; Thornthwaite, 1948, 1958). We will only refer here to the definitions established by Capot-Rey (1951, 1953) and Dubief (1950, 1959,
* See also Volume I, pp. 6, 46, 364, and 399 for discussions dealing with microclimates—ED.
CLAUDE J. GRENOT
reach 4.80 meters in Béni-Abbès, while in California or in the Australian
desert, for example, it is generally less than 3 meters (Dubief, 1950).
Air hygrométrie degree, or relative humidity, remains very low. "Nowhere else on the earth has air been found as dry as in the Sahara region"
(Savornin, 1947). In the arid Sahara, delimited by the isohyet of 2 or
5 cm annually, the relative humidity varies between 4 and 20% (Table
III). But in the septentrional Sahara it reaches 20 to 30% during the summer, and goes up to 50% during the winter.
Vegetation, a relay station between the water contents of the ground
and of the atmosphere, plays an important role in contributing to humidity
of the air. For example, in the same area and at the same moment, the
relative humidity in a reg or hamada (bare ground) would be close to
10% as opposed to 90% in a palm grove (oasis). This fact already leads
us to realize the importance of "microclimates."*
5. Luminosity
Because of the absence of nebulosity, the Sahara is one of the regions
of the earth where direct solar insolation is the greatest, during 300-360
days/year. In 1964, at Reggan, the monthly mean cloud cover varied from
4% (July and September) to 3 1 % (December). Such high luminosity
is a dehydrating factor, since it increases the temperature effects. It could
become a considerable source of energy. Each square meter of the Sahara
receives from the sun an average of 1 kW of heat and luminous power. An
area of 100 X 100 km receives approximately 25 X 10
12
kWh annually,
or the heat equivalent of roughly 2 X 10° metric tons of fuel, (derived
from Chouard, 1957). Let us remember that the surface of the Sahara
is 8 X 10
6
km
2
.
6. Aridity Concepts
Aridity is closely related to rainfall and temperature. Thus, "water" and
"temperature" counteract one another, particularly in regards to vegetation
(Ozenda, 1958).
Aridity is defined by numerous formulas which vary according to their
authors (de Martonne, 1926; Emberger, 1938; Bagnouls and Gaussen,
1953; Thornthwaite, 1948, 1958). We will only refer here to the definitions established by Capot-Rey (1951, 1953) and Dubief (1950, 1959,
* See also Volume I, pp. 6, 46, 364, and 399 for discussions dealing with microclimates—ED.