In fact, most Atlantic hurricanes can be traced to easterly waves that form over Western Africa.
Given favorable conditions, the tropical disturbance can become better organized, indicated by falling surface pressures in the area around the storm and the development of a cyclonic circulation (counter-clockwise in the Northern Hemisphere). Surface pressures fall as water vapor condenses and releases latent heat into areas within the tropical disturbance. (Latent heat is the heat energy released or absorbed during the phase change of a substance—in this case, water vapor.) In response to the atmospheric heating, the surrounding air becomes less dense and begins to rise. As the warm air rises, it expands and cools triggering more condensation, the release of more latent heat, and a further increase in buoyancy, thus allowing more air to rise. A chain reaction (or feedback mechanism) is now in progress, as the rising temperatures in the center of the storm cause surface pressures to lower even more. Lower surface pressures encourage a more rapid inflow of air at the surface, more thunderstorms, more heat, lower surface pressure, stronger winds, and so on. If the storm is far enough from the equator (generally at least 8° of latitude), the Coriolis force will induce the converging winds into a counterclockwise circulation about the storm's area of lowest surface pressure. Meanwhile, air pressures near the top of the storm, in response to the latent heat warming, begin to rise. In response to higher pressures aloft, air begins to flow outward (diverge) around the top of the center of the cyclone. Analogous to a chimney, this upper-level area of high pressure vents the tropical system, preventing the air converging at the surface from piling up around the center. If this were to occur, surface pressures would rise inside the storm and ultimately weaken, or even destroy it.
Once sustained wind speeds reach 37 km (23 miles) per hour, the tropical disturbance is called a tropical depression. As winds increase to 63 km (39 miles) per hour, the cyclone is called a tropical storm and receives a name, a tradition started with the use of World War II vintage code names such as Able, Baker, Charlie, etc. For a number of years beginning in 1953, female names were used exclusively, and then the alternation of male and female names began in the late 1970s. Finally, when wind speeds reach 119 km (74 miles) per hour, the storm is classified as a hurricane. Even when the conditions are ripe for hurricane formation at the surface, the storm may not form if the atmospheric conditions aloft (5-10 km above the surface) are not favorable.