TABLE 1.—_Seasonal distribution of rain.__Botanical features of the Algerian Sahara

+-------+-----+------------+-------+------+-----------------------+

| | | | (10). | | |

+-------+-----+------------+-------+------+-----------------------+

| |_mm._| _mm._ | _mm._ | _mm._| |

| | | | | | |

|Winter |469.9|119.3 (50.8)| 113.5| 37.4| The Tell is |

|Spring |300.0| 80.5 (95.0)| 88.2| 39.0| National; the High |

|Summer | 38.1| 66.5 (35.8)| 50.4| 3.7| (with Ain Sefra, |

|Autumn |287.2|139.5 (58.5)| 123.1| 19.6| parentheses); the |

+-------+-----+------------+-------+------+-----------------------+

The seasonal percentages of precipitation give a more graphic

conception of the rainfall conditions for the four physiographic

provinces. In the Tell this percentage in winter is 42, in spring 27,

in summer 4, and in autumn 27. On the High Plateau the percentages

are 30, 20, 16, and 34 for the four seasons respectively. In the

Saharan Atlas 30 per cent of the rain occurs in winter, 24 in spring,

13 in summer, and 33 in autumn. In the desert the percentages of

rainfall are 37, 39, 4, and 20[3] for the four seasons.

It is of interest also to note the number of days on which the rain

fell on an average each year over a period running from 7 to 12

years. Thus at two typical stations on the Tell rain was reported on

102 and 118 days; at two stations on the High Plateau it rained 65

and 83.8 days; at a station in the Saharan Atlas rain was reported

on 70 days; at desert stations, at Ouargla rain fell on an average

14.2, and at Laghouat 49 days each year. As a comparison, it may be

mentioned that for ten years at Wady Halfi, Egyptian Sudan, there

were only 22 days on which rain-drops were seen to fall. (Engler,

_loc. cit._)

The amount of precipitation varies greatly for the different desert

stations, usually becoming less as one goes south from the High

Plateau. As has already been mentioned, the average rainfall at

Laghouat, which lies at the southern base of the Saharan Atlas, is

198 mm., the average at Ghardaia is 114 mm., while that at El Golea

is 47 mm. The latter station is about 225 miles south of Laghouat, in

the midst of the Sahara. The amount of rainfall, however, is greatly

influenced by altitude, although lack of adequate precipitation data

for the desert makes impossible a detailed presentation of this phase

of the subject. As the amount of the yearly precipitation is less in

the extreme southern part of Algeria than it is nearer the Saharan

Atlas, where the altitude also is greater, it might be expected that

the number of rainy days would vary in a like manner. Such records as

are at hand, however, do not show this to be the case. For instance,

at Ouargla rain falls on an average 14.2 days, average of 7 years,

while the rainfall is 90.2 mm.; yet at El Golea, with a rainfall

of 47 mm., there are 23.4 rainy days each year.[4]

On the desert the rains are often of a torrential nature, as facts

presented above would indicate, and sometimes as much rain falls

within a few hours, or even a few minutes, as usually occurs in

an entire year. How much of the annual precipitation is of this

character and how much is of the non-torrential kind the usual

summaries leave entirely out of the account. It is well known that

the former type of storm is more destructive and less useful to

plants than the latter type. To illustrate the irregularity of

the rainfall in the northern Sahara the monthly precipitation at

Ouargla for several years is presented in table 2.