In the northern part, down to almost 19° N., the prevailing winds are north and north-west. The middle region, to 14°-16° N., has variable winds in an area of low barometric pressure, while in the southern Red sea south-east and east winds prevail. From June to August the north-west wind blows over the entire area ; in September it retreats again as far as 16° N., south of which the winds are for a time variable. In the Gulf of Suez the westerly, or "Egyptian," wind occurs frequently during winter, sometimes blowing with violence, and generally accom panied by fog and clouds of dust. Strong north-north-east winds prevail in the Gulf of Aqaba during the greater part of the year; they are weakest in April and May, sometimes giving place at that season to southerly breezes. The high temperature and great rela tive humidity in summer, make it a difficult region for active life. The mean annual temperature of the surface waters near the head is 77° ; it rises to 8o° in about 22° N., to 84° in 16° N., and drops again to 82° at the Strait of Bab el Mandeb. Daily varia tions of temperature are observable to a depth of over so fathoms. Temperature is, on the whole, higher near the Arabian than the Egyptian side, but it everywhere diminishes with increase of depth and latitude, down to 38o fathoms from the surface; below this depth a uniform constant temperature of 70.7° is observed throughout. In the Gulf of Suez temperature is relatively low, falling rapidly from south to north. The waters of the Gulf of Aqaba are warmer towards the Arabian than the Sinai coast ; a uniform temperature of 70.2° is observed at all depths below 2 70 fathoms.
The salinity of the waters is relatively great, the highest reading being 41 per mille (Gulf of Suez), and the lowest 36.5 near Perim island. The distribution is, speaking generally, the opposite to that of temperature ; salinity increases from the surface downwards, and from the south northwards, and it is greater towards the western than the eastern side. This statement holds good for the Gulf of Suez, in which the water is much salter than in the open sea; but in the Gulf of Aqaba the distribu tion is exceedingly uniform, nowhere differing much from an average of 4o per mille.
The movements of the waters are of great irregularity and com plexity, rendering navigation difficult and dangerous. Two features stand out with special distinctness; the exchange of water between the Red sea and the Indian ocean, and the tidal streams of the Gulf of Suez. From the observations of salinity it is inferred that a surface current flows inwards to the Red sea in the eastern channel of the Strait of Bab el Mandeb, while a current of very salt water flows outward to the Indian ocean, through the western channel, at a depth of so to ioo fathoms from the surface. In the Gulfs of Suez and Aqaba, almost the only part of the Red sea in which tidal phenomena are well developed, a sharply-defined tidal circulation is found. Elsewhere, the surface movements at least are controlled by the prevailing winds, which give rise in places to complex "transverse" currents, and near the coast are modified by the channels enclosed by the coral reefs. During the prevalence of the north and north-west winds the surface level of the northern part is depressed by as much as 2 feet.
The Red sea was important in Egyptian maritime commerce at least as early as the 2nd millennium B.c. (see Q0sEIR, EL) and it had associations with India early in the 1st millennium B.C. Under the Arabs the Red sea was an important highway of trade with connections to India, Persia and East Africa. This sea helped to keep Islam in touch with the thinkers of the East and with the glories of the ancient world during "the dark ages" in the West; with the revolutions in shipping and movement by sea that fol lowed "the age of discovery" the Red sea seemed to retreat into the background; but with the cutting of the Suez canal (1869) and the shortening of the route to India, Australia and the East, the Red sea not only recovered its former importance but became one of the greatest commercial highways in the world.