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How Hurricanes Work

A hurricane's bands don't play marching music.
Circulating around the hurricane's eye are large, swirling bands of wind, clouds, and driving rain. They circulate in bands causing the rains to alternate between downpour and drizzle, often remaining dangerously heavy long after the eye has passed. The winds, on the other hand, are always strongest in those bands closest to the hurricane's eye (alos called the "eyewall"). They decrease in strength the farther they are from the center. Winds must be traveling at least 119 km per hour for the storm to be classified a hurricane, but they can reach up to 250 km an hour, or higher, depending on the strength of the storm.

Hurricane winds are so strong they have been known to lift a 180 kg. piece of lead and move it more than 490 meters! These same winds drag across the surface waters of the ocean, whipping them into a frenzy of huge waves and tidal surges that eventually reach island coastlines.
In this image of Hurricane Andrew, one can clearly make out the eye, eyewall, and swirling rainbands. Image courtesy of NOAA.

Sailors suffered in the doldrums.
Doldrums: for humans, a period of listlessness or despondency [probably akin to Old English "dol foolish" ] (1811).

For centuries the crews of sailing ships dreaded sailing into a particular region of ocean near the Equator and just off the western coast of Africa. The northeast and southwest trade winds meet there, causing air to slowly rise. The rising air produces only light winds and few ocean currents. Wind-powered ships would become becalmed for days or weeks on end. The ship's crews often became listless or despondent. Over time, this region took on the name of this mental state – the Doldrums.

Ironically, some of the Caribbean's most violent storms begin in the calm, warm waters of the Doldrums. This deceptively tranquil region, southeast of the Cape Verde Islands, has come to be known as the hurricane incubator. The waters in this region may heat up to as high as 26º C, or higher. Baby hurricanes grow out of heated ocean waters. In fact, hurricanes cannot form in northern climates nor in winter months when the ocean's waters cool.

Late in the season (which officially starts June 1 and ends November 1), hurricanes in the Western Hemisphere also form off the Yucatan Peninsula, in Central America, and swirl around the Gulf of Mexico.

Why is low atmospheric pressure such a big deal?
The lowest atmospheric pressure ever recorded at sea level was an astounding 870 millibars, 143 millibars below normal. This pressure was measured at the center of Typhoon Tip in the North Pacific in 1979. Why is that astounding? Who cares about air pressure measurements anyway?

To answer these questions, we must understand a couple of ideas. Each particular location on Earth has a measurable, atmospheric pressure. The pressure at that location is caused by the weight of the column of air above that location. Typically, meteorologists measure atmospheric pressure using a mercury barometer. The weight of the air above the Earth is equal to the weight of the column of mercury in the barometer. This column's height can be precisely measured. Standard, sea-level atmospheric pressure is 1013 millibars. This amount of air pressure supports a column of mercury 76 cm. high. Air pressure decreases as we rise above the Earth because there is less air above us the higher up we go. But atmospheric pressures can change at the surface of the Earth as well. They go up and down by small amounts and cause the "highs" and "lows" you hear about on weather reports.

Under normal conditions and over a normal period of time, the atmospheric pressure usually changes no more than 20 millibars. The mercury in the barometer usually rises or falls only a little bit in a period of 12 to 24 hours. But even small differences in atmospheric pressure cause the air to be pushed around. This air movement is called wind. In this way air pressure and wind are closely related. Wind moves out of areas of high air pressure and sweeps into areas of low air pressure. The greater the difference in air pressure from one place on Earth to another, the stronger the wind.

Use the illustration above to understand this phenomenon. The atmospheric pressure in the eyes (A) of hurricanes and typhoons is lower than in the surrounding atmosphere, so air spirals inward in the form of strong surface winds (B). If the center of low pressure is over warm ocean waters, the spiraling winds whip up waves and froth. This adds to the already high levels of water evaporation. The warm, wet air spirals in toward the center of the storm and then cools as it rises (C). The water vapor the air is carrying condenses and forms clouds and rain (grey areas in the illustration). As the condensation evaporates, it again heats the rising air, causing the air to rise upward even faster. The expanding air rises to altitudes of 10 to 15 km. Then, as the air cools, it flows outward over the top of the storm. This cool air flow outwards, lowers the weight of the air above the storm's center even more. The winds increase and draw in more wet air. As long as the hurricane is over warm water, this cycle continues and tends to get stronger. The pressure in the storm's center gets lower and lower, and the winds blow faster and faster.

The strongest winds in a hurricane are in the eyewall. Typhoon Tip's very low atmospheric pressure reading of 870 millibars created catastrophically high winds that swirled in bands around its eye. Typhoon Tip not only had the lowest atmospheric pressure on record, but also 320 km per hour winds—winds moving as fast as a racing car qualifying for the 2003 Indianapolis 500!

The birth of a storm is not a time for celebration.
Pushed by the trade winds, low pressure systems that form off Africa's coast begin to head west across the Atlantic. As they travel they might die out, or they might gain speed depending on weather conditions in the region. When winds reach 62 km per hour and stay at that speed for at least one minute, meteorologists call the phenomenon a system and call the system a tropical storm. Satellite photographs of the region will begin to record the formation of a central eye and spiraling bands of clouds. While meteorologists marvel at the beautiful symmetry of the formation, they also begin to estimate its growing dangers. Caribbean islanders become very nervous. To them, a hurricane in the Atlantic is like a giant game of chance. Where will it land? Whose home will be spared? Whose will be destroyed?

Order out of chaos
When a weather system's winds reach 119 km per hour, the storm is formally called a hurricane. Meteorologists give the hurricane a name. The name makes it easier for islanders to distinguish from other possible hurricanes in the region. It also warns everyone that a storm can be as unpredictable as a person. The scientists monitor the changes in the storm's strength, the direction in which it is traveling, and other weather formations in the area. They classify the storm using five major categories (see next page). To track its direction and possible landfall, scientists compare the storm's changing longitudinal and latitudinal position with the paths of previous storms.

Over the centuries hurricanes have followed certain paths, heading west and veering toward the north as they reach the Caribbean. But meteorologists must also look at other factors that influence a storm's direction — time of year, ocean temperature, other storms in the region. Any storm system may surprise even the most seasoned meteorologist by wobbling, looping, turning back, or even stalling in its tracks!

Review Questions

  1. Where are the strongest winds of a hurricane? Where are the weakest winds?
  2. What factors do meteorologists examine when predicting the path of a hurricane?
  3. Describe what happens to the atmospheric pressure in a hurricane.

 


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