25 5.5 Tropical Cyclones (Hurricanes)
Learning Objectives
- Describe how and why hurricanes form.
- Outline why hurricanes have the potential to be so dangerous.
- Explain why hurricanes mainly occur in the tropics.
Above the oceans just north and south of the equator, a weather phenomenon called a tropical cyclone can develop that can drastically alter the physical and cultural landscape if it reaches land. In the North Atlantic Ocean and the Caribbean Sea, this weather pattern is called a hurricane. In the North Pacific Ocean, the same type of weather pattern is called a typhoon. In the Indian Ocean region and in the South Pacific Ocean, it is called a tropical cyclone or just a cyclone. All these storms are considered tropical because they almost always develop between the Tropics of Cancer and Capricorn.
Hurricanes develop over water that is warmer than 80 ºF. As the air heats, it rises rapidly, drawing incoming air to replace the rising air and creating strong wind currents and storm conditions. The rapidly rising humid air then cools and condenses, resulting in heavy rains and a downdraft of cooler air. The rotation of the earth causes the storm to rotate in a cyclonic pattern. North of the equator, tropical storms rotate in a counterclockwise direction. South of the equator, tropical storms rotate in a clockwise direction.
Hurricanes start out as tropical depressions: storms with wind speeds between twenty-five and thirty-eight miles per hour. Cyclonic motion and warm temperatures feed the system. If a storm reaches sustained winds of thirty-nine to seventy-three miles per hour, it is upgraded to a tropical storm. Tropical depressions are numbered; tropical storms are named. When winds reach a sustained speed of seventy-four miles per hour, a storm is classified as a hurricane.
Hurricane Dynamics
Hot air rises. A water temperature of at least 80 ºF can sustain rising air in the development of a tropical depression. These storms continue to be driven by the release of the latent heat of condensation, which occurs when moist air is carried upward and its water vapor condenses. This heat is distributed within the storm to energize it. As the system gains strength, a full-scale hurricane can develop. Rising warm air creates a low-pressure area that draws in air from the surface. This action pushes water toward the center, creating what is called a storm surge. Storm surges can average five to twenty feet or more depending on the category of the hurricane. Cyclonic rotation is created by rotation of the earth in a process called the Coriolis effect. The Coriolis effect is less prominent along the equator, so tropical cyclones usually do not develop within five degrees north or south of the equator.
When a hurricane makes landfall (comes ashore), the storm surge causes extensive flooding. At the center of the cyclonic system is the hurricane’s eye, where there is a downdraft of sinking air but the wind is calm and there are no clouds. The eye c