The Gulf of Mexico is a mediterranean gulf almost surrounded by the coasts of the United States and Mexico, and forming the northern division of the extension westward of the west Atlantic trench. Its southern boundary is defined by the partly submerged ridge which extends eastwards from the peninsula of Yucatan, and on which the island of Cuba is situated: to the east it communicates directly with the Atlantic by the Strait of Florida. On the western side of Yucatan a southerly embayment is formed by the Gulf of Campeachy. The United States coast closely follows the parallel of 30° N., while the parallel of 20° N. cuts across the Gulf of Campeachy: the greatest length-Vera Cruz to. Florida-is 1120 m., and greatest width-Galveston to Campeachy-680 m. The total area is approximately 716,000 sq. m.

The deepest part of the Gulf of Mexico, the so-called " Sigsbee " deep, lies below the line of 2000 fathoms, between 23° and 25-1/2° N., and 84-1/2° to 95° W. It is widest to the west, where the breadth is about 120 m., and narrows to 25 m. at its greatest depth (2119 fathoms) between 86° and 88° W., widening again to some 8o m. farther eastward. The continental shelf is for the ' most part narrow: its breadth is 6 m. at Cape Florida, 120 m. along the west coast of Florida, 120 m. at the south pass of the Mississippi, 130 m. near the boundary of Texas and Louisiana, and 15 m. off Vera Cruz. The shores are low, sandy and marshy, the coast-line being frequently doubled by lagoons. There are no islands except the Keys of Florida and Yucatan, and Cuba. The tides in the Gulf of Mexico are of comparatively small range (springs rarely exceed 4 ft. and neaps 2-1/2 ft.), but a remarkable feature is the exaggeration of the diurnal inequality to such an extent as almost to extinguish the semi-diurnal tide in the inner parts of the gulf, giving high and low water only once daily. The mean level of the water in the Gulf of Mexico was formerly given as about 40 in. above that of mean sea-level at New York, but later reports on precise levellings from New York to Biloxi through St Louis describe it vaguely as " somewhat higher." The current movement in the Gulf of Mexico consists of a rotational movement in the direction of the hands of a watch, the branch of the equatorial current which enters the Caribbean Sea passing into the Gulf by the Strait of Yucatan and issuing from it by the Strait of Florida as the Gulf Stream, which unites with the remainder of the northward moving water, forming the Antilles current.

From March to September the prevailing winds are the northeast trades; these undergo considerable modification on account of the configuration of the surrounding land, and the rains which accompany them are interrupted by spells of calm thick weather, and rarely by northerly winds known as Norte's det hueso colorado and Chocolateros. In the colder dry season, from October to April, the climatic situation is dominated by the relatively high temperature of the surface of the gulf, causing a cyclonic inflow of air which is associated with the strong northerly winds or northers prevailing on the western side, more particularly along the Mexican coast. The northers sometimes blow with terrific force and are at times accompanied by rain. The form and position of the Gulf of Mexico exercise a profound influence on the climate of the whole of the southern and south-eastern states of the Union, and indeed of the greater part of North America.