Rise and Fall of a Hurricane

Rise and Fall of a Hurricane

People caught in a hurricane's fury often felt
the earth was in its death throes.

A hurricane is the offspring of the tropical seas, a whirling mass of energy formed when certain conditions of heat and pressure nourish and nudge the winds over a large area of ocean. The winds wrap themselves around an atmospheric low (the eye) with a significant degree of high energy. The hurricane is a heat engine generated and maintained by the energy of condensation of rain within its saturated spiral clouds. It is nature's way of releasing heat that builds up in the tropics.

The word hurricane is derived from the name Hunraken, an ancient Mayan storm god; among aboriginal Caribbean people a similar word, Hurakán, was the name of an evil spirit. In the North Pacific this same storm is called a typhoon (from the Cantonese t'ai fung, meaning "great wind"), and in the South Pacific and Indian Oceans it is a "tropical cyclone."

Witnesses who survive an encounter with a hurricane describe a most unforgettable experience. With the hurricane blowing in your face you can't breathe, nor can you see with rain and spray hitting your eyeballs at 100 miles per hour. You hear only the screaming of the wind that drowns out even thunder and roaring breakers. Speaking is impossible with your mouth blown out of shape, and if you try to walk or crawl you will be blown away. You can only hope to happen on to something solid around which to twist arms and legs.

Hurricanes form over warm ocean waters, 80 degrees Fahrenheit or higher, usually 5 degrees to 30 degrees north or south of the equator. The first and most important condition necessary for the formation of a hurricane is a vast stretch of open sea that has been heated decisively by the sun beaming down day after day from a cloudless sky. The warm sea creates an air funnel in which hot, moist air rises and condenses; heat is released, and the action increases. This funnel of warm air rises to perhaps 40,000 feet, producing vast cumulus clouds and violent thunderstorms. While more and more moisture is drawn from below into the funnel, these high air currents are distributed to the surrounding areas. The earth's rotation gives the system a twisting motion that gathers momentum, and a hurricane is born. It whirls counterclockwise in the Northern hemisphere, and clockwise in the Southern.

The hurricane is like a swirling doughnut with a hole in the middle, with winds over 75 miles per hour, averaging 130 mph, and reaching 200 mph or higher. As the storm system grows it may reach 400 to 500 miles in diameter. It is nature's most destructive storm because it covers such a wide area, traveling thousands of miles and raging for a relatively long time, usually about nine days. The San Ciriaco storm of 1899 churned the Atlantic waters for almost five weeks. The average traveling speed of an Atlantic hurricane is 12 mph, although it varies widely; a hurricane in September 1938 traveled through New England at about a mile per minute.

The energy involved in a hurricane is mind—boggling. The typical hurricane liberates heat from the condensation of moisture at a rate of 100 billion kilowatts per hour. This means that in a single day the average hurricane releases more energy than the electricity used by the entire United States in an entire year. Or, compared with nuclear power, the energy released by a hurricane in one day is equivalent to the explosion of 2,000 one—megaton hydrogen bombs. In addition to energy, the average hurricane squeezes out an estimated 200 billion tons of water a day as rain!

The direct action of the wind is not the most colossal wreaker of havoc in a hurricane. Most victims die by drowning, and most of the flooding comes not from the intense rains but from the storm surge. Winds and low pressure around the hurricane eye raise the level of the sea several feet. This produces a dome of water about 50 miles across that moves onto the coast, bringing the seas far inland. The National Weather Service estimates that storm surges cause 9 of every 10 hurricane fatalities.

The maelstrom of winds moves in a vast spiral around the storm center—the eye, the hole in the whirling doughnut. The eye is an oasis of comparative calm and, occasionally, sunshine. The temperature is appreciably higher than in the surrounding vortex. This is the low—pressure area around which the furious winds revolve. These winds become more violent—100 to 200 mph—at the approach to the calm center. Suddenly the eye arrives, and there is an almost instantaneous lull. Winds may drop to as low as 10 mph, rain stops abruptly, and patches of open sky are visible above. The calm continues while the eye passes; its average diameter is 14 miles, but it can range from 4 to 25 miles. This is only a brief time—out for battered victims of a hurricane; as soon as the eye passes, the destructive winds resume but from the opposite direction.

When the eye passes over a ship, flocks of birds land on the deck and rigging, sometimes completely covering all surfaces. The pilots of a weather plane observing Hurricane Carla in September 1961 reported the air so full of birds that they dared not fly through it. Any bird caught up in the violent winds is powerless and is carried willy—nilly wherever the winds flow. As the winds spiral toward the center, the birds eventually reach the calm eye, and there they must remain until the hurricane subsides. When the eye passes over a ship, the birds will land on it to rest from their life—and—death struggle with the elements. These birds sometimes travel great distances in the eye. Tropical birds have been found in New England, 2,000 miles from where they "boarded" the hurricane.

At sea the storm's energy is replenished by the constant drawing of warm, evaporated water into the funnel. But once the storm moves over the mainland the supply of evaporated water from the ocean is cut off, and its power eventually diminishes. Then the hurricane slowly dies, but not before it dumps enormous quantities of rain on the land below. Hurricane Diana (1955) caused little damage along the coast, but the rains brought floods to New England and the Mid—Atlantic, killing over 200 people and causing about $300 million additional damage.

The relentless violence a hurricane can inflict may inspire fear and dread, but its silver lining is the accompanying rainfall. Although the deluge may be too much, too fast, the average hurricane drops six inches of water over a given area. As parched land is revived, rivers resume their normal flow, and reservoirs are filled. Drought—stricken plants and animals are refreshed. Hurricanes, typhoons, or tropical cyclones are essential to successful agriculture in many countries. Toilers of the land depend on these storms for one—fourth of needed rainfall, and nature is more than willing to oblige.

From the book: 
Our Fascinating Earth