Red Sea. The Red Sea is an arm of the Indian Ocean, running north-north-west from the Gulf of Aden, with which it communicates by the Strait of Bab-el-Mandeb, 13½ miles across. Its length is about 1200 miles, and its width in the central portion is between 100 and 200 miles, the greatest breadth being about 205 miles; it narrows towards the southern entrance, while in the north it is divided by the peninsula of Sinai into two gulfs, the Gulf of Suez, 170 miles long by 30 miles wide, and the Gulf of 'Akaba, 100 miles in length.
The Arabian coasts of the Red Sea are usually narrow sandy plains backed by ranges of barren mountains; the African coasts towards the north are flat and sandy, but farther south high table-lands rise some distance inland, culminating still farther south in the lofty mountains of Abyssinia. A marked feature in the configuration of the Red Sea is found in the large existing and upraised coral-reefs running parallel to both the eastern and western shores, those to the east being more extensive and farther from the coast than those to the west; the most important are the Farisan Archipelago in the eastern reef, and the large island of Dahlak, lying off Annesley Bay, in the western reef. In addition to the islands of organic formation mention may be made of the volcanic group lying in 14° N. lat., the largest of which, Jebel Zugur, is 10 miles long, 7 miles wide, and 2074 feet in height; farther north, on the islet of Jebel Teir, is a volcano which was active until quite recently. A dangerous reef, the Dædalus, lies directly in the path of steamers in 24½° N. lat., and a lighthouse has been placed on it. The principal harbours on the Red Sea are Mocha, Hodeida, Lokeyyah, Jiddah, and Yenbo' on the Arabian coast, and Massowah, Khor Nowarat, and Suakim on the African coast.
In ancient times the Red Sea was used as a means of communication by the Phœnicians and other maritime peoples, until the discovery of the route round the Cape of Good Hope diverted the traffic into another channel, only to be revived, however, on a much more extensive scale with the construction of the Suez Canal.
The tides are very variable, depending largely on the direction and force of the winds, which also to a great extent determine the direction and velocity of the surface currents. The hot climate is due to the almost cloudless sky, and consequent want of rain, the altitude of the sun, and the absence of rivers. The mean temperature of the air generally ranges between 70° and 94° F. during the day, though readings of over 100° are often registered in the shade; but during the night the temperature may fall to the freezing-point, owing to radiation in the clear atmosphere. The prevailing wind on shore is north-north-west almost universally, but from October to May south-south-east winds prevail over the southern portions of the sea, a belt of calms and variable winds occurring in the central regions, while in the northern portions the usual north-north-west winds are met with. Evaporation is very great, and the air over the water is always very moist in the summer; hurricanes are unusual, but rain-squalls frequently occur with the southerly winds, and moderate gales and sandstorms, called 'dragons' in the popular language of the Arabs, are not uncommon.
The mean temperature of the surface water in the Red Sea varies at the northern end between about 65° and 79° F., in the central regions between 75° and 86°, and at the southern end between 78° and 89°; readings of over 100° have been recorded at the south end of the sea. The temperature of the water below the surface decreases down to a depth of about 200 fathoms, from whence down to the bottom a mean temperature of about 71° is found all the year round; this agrees with the temperature conditions prevailing in the enclosed seas of the East Indies, for instance, according to the observations made on board the Challenger, the depth at which the minimum temperature occurs (i.e. 200 fathoms in the Red Sea) indicating the depth of water over the barrier separating the Sea from the open ocean. In winter, in the northern part, the whole body of water from surface to bottom usually has a mean temperature of 71°.
The salinity of the water is almost constant at about 1.030 (ordinary ocean water is about 1.026), and this is due to the fact that no rivers flow into it, little rain falls, and the evaporation is excessive. It has been estimated that, were the Red Sea entirely enclosed, it would become a solid mass of salt in less than two thousand years, but this is prevented by an inflow of water through the Strait of Bab-el-Mandeb, and it is also known that a current of very salt water flows out underneath the incoming surface current.
The greatest depth in the Red Sea is about 1200 fathoms, and the mean depth of the whole area about 375 fathoms. From the point of greatest depth, which is near the centre, the bottom rises towards each end. Owing to the absence of rivers the deposits approach in character those formed in the open ocean, being largely composed of Foraminifera, Pteropods, and other pelagic shells. The marine fauna and flora are extensive, and have been described by Haeckel and other naturalists; it has been shown that a migration of the Red Sea and Mediterranean faunæ is taking place along the Suez Canal. The origin of the name—the Lat. Mare Rubrum and the Gr. Erythra Thalassa—is much disputed. The Hebrew name is Yam Sâph, which Gesenius explains as the 'sea of reeds.' The path by which the Israelites went out of Egypt was along the course of the valley called Wâdy Tumeilât, apparently an old arm of the Nile now silted up. The Lake of Ismailieh (Timsah) was then most probably the head of the Gulf of Suez, but the exact point of passage of this arm of the sea still remains obscure.