Hurricanes

Hurricanes, name applied to migratory tropical cyclones that originate over oceans in certain regions near the equator, and particularly to those arising in the West Indian region, including the Caribbean Sea and the Gulf of Mexico. Hurricane-type cyclones in the western Pacific are known as typhoons.

Most hurricanes originate within the doldrums, a narrow equatorial belt characterized by intermittent calms, light variable breezes, and frequent squalls, and lying between the northeast and southeast trade winds. As the doldrums of the Atlantic are situated largely to the north of the equator, hurricanes do not occur in the South Atlantic Ocean. The Pacific doldrums extend north and south of the equator; thus hurricanes occur in the South and North Pacific oceans.

Hurricanes consist of high-velocity winds blowing circularly around a low-pressure center, known as the eye of the storm. The low-pressure center develops when the warm, saturated air prevalent in the doldrums is underrun and forced upward by denser, cooler air. From the edge of the storm toward its center, the atmospheric pressure drops sharply and the wind velocity rises. The winds attain maximum force close to the point of lowest pressure (about 724 torr, or about 28.5 in. of mercury). The diameter of the area affected by winds of destructive force may exceed 240 km (150 mi). Gale winds prevail over a larger area, averaging 480 km (300 mi) in diameter. The strength of a hurricane is rated from 1 to 5. The mildest, Category 1, has winds of at least 120 km/h (74 mph). The strongest (and rarest), Category 5, has winds that exceed 250 km/h (155 mph). Within the eye of the storm, which averages 24 km (15 mi) in diameter, the winds stop and the clouds lift, but the seas remain very violent.

Hurricanes generally move in a path resembling the curve of a parabola. In the northern hemisphere the storms usually travel first in a northwesterly direction and in the higher latitudes turn toward the northeast. In the southern hemisphere the usual path of the hurricane is initially to the southwest and subsequently to the southeast. Hurricanes travel at varying rates. In the lower latitudes the rate ranges from 8 to 32 km/h (5 to 20 mph) and in the higher latitudes it may increase to as much as 80 km/h (50 mph). Those areas in which the hurricane winds blow in the same direction as the general movement of the storm are subjected to the maximum destructive violence of the hurricane.

Since 1943 U.S. military aircraft have been flying into hurricanes to measure wind velocities and directions, the location and size of the eye, the pressures within the storms, and their thermal structure. A coordinated system of tracking hurricanes was developed in the mid-1950s, and periodic improvements have been made over the years. Radar, sea-based recording devices, geosynchronous weather satellites (since 1966), and other devices now supply data to the National Hurricane Center in Florida, which follows each storm virtually from the beginning. Improved systems of prediction and communication have been able to help minimize loss of life in hurricanes, but property damage is still heavy, especially in coastal regions. The strongest hurricane to hit the western hemisphere in the 20th century, Gilbert, devastated Jamaica and parts of Mexico in 1988 with winds that gusted up to 350 km/h (218 mph). Destructive hurricanes in recent U.S. history include Hugo (1989), with $4.2 billion in damage and more than 50 deaths, and Andrew (1992), with $15.5 billion in damage, more than 50 dead, and thousands left homeless.

Florida Keys

Florida Keys
Florida Keys, chain of islands, islets, and reefs, southern Florida, between the Straits of Florida and Florida Bay, extending southwest and west from Virginia Key, south of Miami Beach to the Dry Tortugas, for 309 km (192 mi). The keys are chiefly limestone and coral formations. The larger islands of the group are Key West, Key Largo, Sugarloaf Key, and Boca Chica Key. The keys, distributed between Miami-Dade and Monroe counties for administrative purposes, were devastated in a 1935 hurricane. Several of the islands are popular vacation resorts, and fishing and tourism are the leading industries. A causeway extends from the mainland to Key West, the southernmost city of the conterminous United States.

Atlantic Ocean

Atlantic Ocean, the second largest of the earth’s four oceans and the most heavily traveled. Only the Pacific Ocean is larger, covering about twice the area of the Atlantic Ocean. The Atlantic is divided into two nominal sections: The part north of the equator is called the North Atlantic; the part south of the equator, the South Atlantic. The ocean’s name is derived from Atlas, one of the Titans of Greek mythology. II BOUNDARIES AND SIZE The Atlantic Ocean is essentially an S-shaped north-south channel, extending from the Arctic Ocean in the north to the Antarctic continent in the south and situated between the eastern coast of the American continents and the western coasts of Europe and Africa. The Atlantic Ocean proper has a surface area of 82 million sq km (32 million sq mi). Including its marginal seas—the Gulf of Mexico-Caribbean Sea, the Arctic Ocean, and the North, Baltic, Mediterranean, and Black seas—the total area is 106 million sq km (41 million sq mi). The boundary between the North Atlantic and the Arctic Ocean is arbitrarily designated as lying along a system of submarine ridges that extend between the land masses of Baffin Island, Greenland, and Scotland. More clearly defined is the boundary with the Mediterranean Sea at the Strait of Gibraltar and with the Caribbean Sea along the arc of the Antilles. The South Atlantic is arbitrarily separated from the Indian Ocean on the east by the 20° east meridian and from the Pacific on the west along the line of shallowest depth between Cape Horn and the Antarctic Peninsula. III GEOLOGIC FORMATION AND STRUCTURAL FEATURES The Atlantic began to form during the Jurassic period, about 150 million years ago, when a rift opened up in the supercontinent of Gondwanaland, resulting in the separation of South America and Africa. The separation continues today at the rate of several centimeters a year along the Mid-Atlantic Ridge. Part of the midoceanic ridge system that girdles the world, it is a submarine ridge extending north to south in a sinuous path midway between the continents. Roughly 1,500 km (930 mi) wide, the ridge has a more rugged topography than any mountain range on land, and is a frequent site of volcanic eruptions and earthquakes. The ridge ranges from 1 to 3 km (0.6 to 2 mi) above the ocean bottom. Along the American, Antarctic, African, and European coasts are the continental shelves—embankments of the debris washed from the continents. Submarine ridges and rises extend roughly east-west between the continental shelves and the Mid-Atlantic Ridge, dividing the eastern and western ocean floors into a series of basins, also known as abyssal plains. The three basins on the American side of the Mid-Atlantic Ridge are 5,000 m (16,000 ft) deep: the North American Basin, the Brazil Basin, and the Argentina Basin. The Eurafrican side is marked by several basins that are smaller but just as deep: the Iberia, Canaries, Cape Verde, Sierra Leone, Guinea, Angola, Cape, and Agulhas basins. The large Atlantic-Antarctic Basin lies between the southernmost extension of the Mid-Atlantic Ridge and the Antarctic continent. The Atlantic Ocean has an average depth of 3,600 m (11,810 ft). At its deepest point, in the Puerto Rico Trench, the bottom is 8,605 m (28,231 ft) below the surface. IV ISLANDS The largest islands of the Atlantic Ocean lie on the continental shelves. Newfoundland is the principal island on the North American shelf; the British Isles are the major island group of the Eurafrican shelf. Other continental islands include the Falkland Islands (Islas Malvinas), the only major group on the South American shelf, and the South Sandwich Islands on the Antarctic shelf. Oceanic islands, usually of volcanic origin, are less common in the Atlantic Ocean than in the Pacific. Among these are the island arc of the Antilles (including Puerto Rico, Hispaniola, Jamaica, and Cuba). In the eastern Atlantic, the Madeiras, Canaries, Cape Verde, and the São Tomé-Príncipe group are the peaks of submarine ridges. The Azores, Saint Paul’s Rocks, Ascension, and the Tristan da Cunha group are isolated peaks of the Mid-Atlantic Ridge system; the large island of Iceland is also the result of volcanic action at the Mid-Atlantic Ridge. Bermuda rises from the floor of the North American Basin, and Saint Helena from the Angola Basin. V CURRENTS The circulatory system of the surface waters of the Atlantic can be depicted as two large gyres, or circular current systems, one in the North Atlantic and one in the South Atlantic. These currents are primarily wind driven, but are also affected by the rotation of the earth. The currents of the North Atlantic, which include the North Equatorial Current, the Canaries Current, and the Gulf Stream, flow in a clockwise direction. The currents in the South Atlantic, among which are the Brazil, Benguela, and South Equatorial currents, travel in a counterclockwise direction. Each gyre extends from near the equator to about latitude 45°; closer to the poles are the less completely defined counterrotating gyres, one rotating counterclockwise in the Arctic regions of the North Atlantic and one rotating clockwise near Antarctica in the South Atlantic. See Ocean and Oceanography: Ocean Currents. The Atlantic receives the waters of many of the principal rivers of the world, among them the St. Lawrence, Mississippi, Orinoco, Amazon, Paraná, Congo, Niger, and Loire, and the rivers emptying into the North, Baltic, and Mediterranean seas. Nevertheless, primarily because of the high salinity of outflow from the Mediterranean, the Atlantic is slightly more saline than the Pacific or Indian oceans. VI TEMPERATURES The Atlantic Ocean may be described as a bed of water colder than 9° C (48° F)—the cold-water sphere—within which lies a bubble of water warmer than 9° C—the warm-water sphere. The warm-water sphere extends between latitude 50° north and latitude 50° south and has an average thickness of 600 m (2,000 ft). The most active circulation is found in the uppermost layer of warm water. Below this, circulation becomes increasingly sluggish as the temperature decreases. Surface temperatures range from 0° C (32° F), found year-round at the Arctic and Antarctic margins, to 27° C (81° F) in the broad belt at the equator. At depths below 2,000 m (6,600 ft), temperatures of 2° C (36° F) are prevalent; in bottom waters, below 4,000 m (13,200 ft), temperatures of -1° C (30° F) are common. VII MARINE RESOURCES The Atlantic Ocean contains some of the world’s most productive fisheries, located on the continental shelves and marine ridges off the British Isles, Iceland, Canada (especially the Grand Banks off Newfoundland), and the northeastern United States. Upwelling areas, in which the nutrient-rich waters of the ocean depths flow up to the surface, as in the vicinity of Walvis Bay off southwestern Africa, also have abundant sea life. Herring, anchovy, sardine, cod, flounder, and perch are the most important commercial species. Tuna is taken off northwestern Africa and northeastern South America in increasing numbers. The catch per unit area is much higher in the Atlantic than in the other oceans. A remarkable example of plant life is found in the Sargasso Sea, the oval section of the North Atlantic lying between the West Indies and the Azores and bounded on the west and north by the Gulf Stream. Here extensive patches of brown gulfweed (Sargassum) are found on the relatively still surface waters. Actively mined mineral resources in the Atlantic include titanium, zircon, and monazite (phosphates of the cerium metals), off the eastern coast of Florida, and tin and iron ore, off the equatorial coast of Africa. The continental shelves and slopes of the Atlantic are potentially very rich in fossil fuels. Large amounts of petroleum are already being extracted in the North Sea and in the Caribbean Sea-Gulf of Mexico region; lesser amounts are extracted off the coast of Africa in the Gulf of Guinea.