These conditions continue until noon, when a change may be expected, but may be deferred until 4 p.m., or rarely even 6 p.m. The wind suddenly ceases, the world becomes again visible, and the temperature drops from say 105° F. to 95° F. But soon there comes the reverse wind, almost equally strong, from the sea, and the humidity increases so much that the fall of temperature is not the relief that might be expected, being but the change from oven to steam-kettle. The natives tell me that this wind, so hot in the plains, among the mountains is cold, and is heated by its passage over the sun-roasted plains. Apparently the great heat here originates miniature local cyclones, cold air from the mountain tops, or drawn over the mountains, rushing down to fill the low pressure area on the plains, being heated there and rushing on a few miles out to sea, whence the easterly return wind originates. At Dongonab these “hurûr” winds are rarer than they are further south, where they are of almost daily occurrence during the summer, while at Halaib, 100 miles further north, the natives tell me they do not occur at all. Consequently we are sometimes visited by the return wind in the morning, caused by “hurûr” at a point further down the coast. Such a cyclone is illustrated by the frontispiece, which represents the combination of thunder clouds over the mountains while a “hurûr” rages over the plain and for several miles out to sea. But among the barrier reefs, though the wind is blowing directly towards them, all is glassy calm.
The rainfall is extremely scanty and local, though markedly better in the south, where the population is correspondingly greater and the fauna richer.
There are two seasons when rain may be hoped for, viz. the “kharîf” which centres round August, and which is referred to in the frontispiece, and the winter months, but if rain fell for an hour or two on three days it would be considered a liberal supply for the whole year in most places. At Dongonab there has been no rain (above a millimetre or two) since December, 1907, though one or two showers have fallen on Rawaya and Makawar[59]. There is of course much more rain on the hills than on the plains, but even so grass grows only in scattered areas to which the people migrate.
Tides. The Red Sea undergoes considerable variations of level at its extremities, up to seven feet at Suez, but in the middle the variations are small, only a few centimetres at Port Sudan. At Dongonab the difference between highest and lowest levels recorded is 80 cm., but the maximum change in any 24 hours is rarely over 30 cm. Records shew a distinct tide, but this may be interfered with by changes of level due to wind and changes of atmospheric pressure, and in any case one of the usual two tides of the 24 hours is practically suppressed, the water remaining near high tide level until it falls for next day’s tide. In the summer the average level is lower than in winter and the tidal effects are partially masked by the results of the peculiar climatic conditions. The water may remain low for days, so that all the coral which has grown above that level since the last occasion of extreme low water, which may have been one or even two years ago, dies off.
I suppose that every school-boy looking at an atlas, is struck by the peculiar shape of the Red Sea, and is led to ponder on the usefulness of this peculiar canal, the sole value of which is that it gives communication between Europe and the East, a value which needed but the trifling addition possible to human effort to make it the great highway of the world. Its own shores are desolate wastes, in itself it has no attraction for traffic, and even its shape seems to indicate that it is but a passage to other seas. (See map inside the cover.) For so narrow a sea, only a little over a hundred miles wide, the depth is great, two hundred to five hundred fathoms at the side and a thousand in the middle. These peculiarities are also well marked in the deep Gulf of Akaba which bounds Sinai on the east—the Gulf of Suez, on the west, being a shallower branch valley. Both these gulfs, like the Red Sea, are bounded on either side by high mountains, and those of the southern part of Sinai are particularly grand in the savage barrenness of their jagged peaks and vast precipices.
The Gulf of Akaba is directly in line with the Jordan Valley, a similar depression on a smaller scale, only partially occupied by water, the Dead Sea, while southwards we find another dry valley running through British East Africa and adjoining territories, a great trough bounded by plateaux, several thousand feet above its bottom. We can thus trace
Diagram 7. Formation of a rift valley
this trough-like valley from Palestine to some degrees south of the Equator as a stupendous crack in the earth’s surface, well named “The Great Rift Valley[60].” The Red Sea is its greatest section, its total depth here being, say, 5000 feet from the summit of the mountains[61] to sea level and 6000 feet to the sea bottom, 11,000 feet in all.
The formation of such a valley, by the dropping down of a series of strips of country below the level of the remainder, is illustrated by Diagram 7. To study the simplest possible case we draw a section through the ground and imagine it formed of three kinds of rock, of which two form horizontal sheets, AA and BB, over the third CC. These were originally unbroken, and in the positions shewn by the lines of dashes, but were broken by the dropping down of the central part to form the valley shewn here in section. The floor of the valley has the same structure as the original surface of the ground, the same three beds, A, B and C, occurring in the same positions, but at a lower level. They are found again in each of the steps on the valley’s sides, their regular reappearance in this way being conclusive proof of the earth movements postulated.