Introduction to Hurricanes
Major hurricanes are relatively rare events at any location. Coastal residents from Brownsville Tex., to Eastport, Me., have a good chance of living many years without experiencing one. But none of our coastal areas are immune. "Not here! We haven't had a hurricane in years," could be the most dangerous words you'll ever hear. It's best to be prepared. This could be the year.
Hurricanes are tropical cyclones in which winds reach constant speeds of 74 miles per hour or more, and blow in a large spiral around a relatively calm center he eye of the hurricane. Every year, these violent storms bring destruction to coastlines and islands in their erratic path.
Stated very simply, hurricanes are giant whirlwinds in which air moves in a large tightening spiral around a center of extreme low pressure, reaching maximum velocity in a circular band extending outward 20 or 30 miles from the rim of the eye. This circulation is counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. Near the center, hurricane winds may gust to more than 200 miles per hour. The entire storm dominates the ocean surface and lower atmosphere over tens of thousands of square miles.
The eye, like the spiral structure of the storm, is unique to hurricanes. Here, winds are light and skies are clear or partly cloudy. But this calm is deceptive, bordered as it is by maximum force winds and torrential rains. Many persons have been killed or injured when the calm eye lured them out of shelter, only to be caught in the maximum winds at the far side of the eye, where the wind blows from a direction opposite to that in the leading half of the storm.
Hurricane winds do much damage, but drowning is the greatest cause of hurricane deaths. As the storm approaches and moves across the coast line, it brings huge waves and storm tides which may reach 25 feet or more above normal. The rise may come rapidly, flooding coastal lowlands. Waves and currents erode beaches and barrier islands, undermine waterfront structures, and wash out highway and railroad beds. The torrential rains that accompany the hurricane produce sudden flooding as the storm moves inland. As its winds diminish, rainfall floods constitute the hurricane's greatest threat.
The hurricanes that strike the eastern United States are born in the tropical and subtropical North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Most occur in August, September, and October, but the six-month period from June 1 to November 30 is considered the Atlantic hurricane season.
The principal regions of tropical cyclone origin vary during the season. Most early (May and June) storms originate in the Gulf of Mexico and western Caribbean. In July and August, the areas of most frequent origin shift eastward, and by September are located over the larger area from the Bahamas southeastward to the Lesser Antilles, and thence eastward to south of the Cape Verde Islands, near the west coast of Africa. After mid-September, the principal areas of origin shift back to the western Caribbean and Gulf of Mexico.
On average, six Atlantic hurricanes occur per year. However, there are significant deviations from this average. In 1916 and 1950, 11 hurricanes were observed, and no hurricanes were observed in 1907 and 1914. During 1893, 1950, and 1961 seasons, four hurricanes were observed in progress at the same time.
Some hurricanes (usually weaker than their Atlantic counterparts) may strike Southern California and bring torrential rains to the southwest U.S.
Hurricanes begin as relatively small tropical cyclones which drift gradually to the west-north west (in the Northern Hemisphere), imbedded in the westward-blowing, tradewinds of the tropics. Under certain conditions these disturbances in crease in size, speed, and intensity until they become full-fledged hurricanes.
The storms move forward very slowly in the tropics, and may remain almost stationary for short periods of time. The initial forward speed is usually 15 miles per hour or less. Then, as the hurricane moves farther from the Equator, its forward speed tends to increase; at middle latitudes it may exceed 50 miles per hour in extreme cases.
The great storms are driven by the heat released by condensing water vapor, and by external mechanical forces. Once cut off from the warm ocean, the storm begins to die, starved for water and heat energy, and dragged apart by friction as it moves over the land.