Eye on Hurricanes

Hurricanes can wreak havoc. In September 2003, Hurricane Isabel killed at least 38 people as it smashed into the United States' East Coast. About 5 million people lost electricity, and public transportation shut down in Washington, D.C. Early loss estimates neared $1 billion.

In 1998 Hurricane Mitch killed over 11,000 people in Central America. When Hurricane Andrew hit Miami in 1992, losses topped $26 billion. Hurricanes trigger evacuations, interrupt electric and telephone services, and disrupt people's lives in many ways. The environment suffers, too, as hurricanes rip through coastal habitats.

The total energy released during an average hurricane's life is roughly 200 times humans' total capacity to generate electricity. What happens in these awesome weather systems?

How Hurricanes Form

Hurricanes are strong tropical cyclones that form over the Atlantic or eastern North Pacific Ocean. “Typhoon” is the title when such storms form in the western North Pacific. Heavy precipitation and strong winds spiral around an area of low barometric pressure—the eye—at the storm's center.

Hurricanes start as “tropical disturbances” over ocean waters at least 80 degrees F warm. Evaporating water molecules carry heat energy from the water, called latent heat, into the atmosphere. Warm air rises above cooler air.

Eventually, the water vapor cools. It condenses into tiny water droplets or forms ice crystals, which build into storm clouds, causing rain and releasing heat energy. That process warms the surrounding air, which rises more. The more humid the air, the more this process repeats.

Air pressure in the system's upward flow is less than in the surrounding air. Air from higher-pressure areas moves in. This creates wind. If vertical wind shear is low enough, the spiral winds of a cyclone build. North of the equator, Earth's rotation makes the spiral swirl counterclockwise around the eye.

The system is now a tropical disturbance. When maximum sustained winds reach 39 miles per hour, it becomes a tropical storm. When winds top 74 mph, it's a full-fledged hurricane. The system stretches 7.75 miles or more high and 312 miles or more across.

Inside the eye, light winds and fair weather prevail. The surrounding eyewall has the highest winds. A major Category 3 hurricane on the Saffir/Simpson scale has winds from 111 to 130 mph. The strongest Category 5 hurricanes like Mitch and Andrew blow at 155 mph or more. (Fortunately, Isabel's winds dropped about 50 mph from Category 5 levels before it hit land in North Carolina.)

Winds and heavy rains cause much of hurricanes' harm. Winds pushing the ocean ahead of a cyclone also cause storm surges—huge waves made even higher by the tides. Hurricanes hitting land often trigger tornadoes, too.

Tracking Tropical Cyclones

Timely warnings are key to saving lives. The National Hurricane Center issues updated forecasts every six hours during hurricane season from May to November. One issue is tracking: Where will a storm go?

“Think of it as a cork in a stream,” says James Franklin at the National Hurricane Center in Miami. A cork bobs around wherever moving water pushes it. “The hurricane is the cork, and the surrounding environment is the stream.” Thus, scientists study prevailing winds, pressure systems, and other weather features that steer the storm.

How intense will a cyclone become? How strong is it now? How long might it last? Clues to these questions are inside the storm itself. Satellites collect some data, but it's incomplete. To get needed detail, planes fly into the storms.

“If you've ever been to Disney World and ridden on the Tower of Terror ride at MGM, it's a little bit like that,” says Franklin, “except that the Tower of Terror ride is not as bumpy.” Dropwindsondes fall from the planes. Traveling down on parachutes, the cylinders' instruments measure barometric pressure, temperature, wind speed, and humidity. Other tools calculate the precise latitude, longitude, and altitude for each measurement. The dropwindsonde transmits the data back to the plane, which relays it to the National Hurricane Center.

Computers help to analyze all the information. Statistical models compare measurements to data on past storms' performances. Dynamic models perform physics calculations on measurements to predict possible developments.

Stormier Weather?

If tropical cyclones seem more forceful lately, it's no accident. Hurricane activity and intensity naturally vary in cycles lasting several decades. The 1940s through 1960s were rather intense. The 1970s and 1980s were calmer.

“We've come out of an inactive period, and we appear to be entering an active one,” says Franklin. More intense hurricane activity now is probably part of that natural cycle, he says.

What might happen if a global warming trend continued over time? Some studies suggest that the theoretical limit on hurricanes' destructive power might rise. In practice, however, many factors affect how strong hurricanes actually are. Plus, it's hard to know how warming patterns will develop.

Meanwhile, scientists are constantly improving tools for tracking and studying tropical cyclones. “It's just inherently exciting, as well as something that's helpful to people,” says Franklin. “You're providing a service to people by helping them prepare.”

Vocabulary

barometric pressure:
A measure of the pressure in the atmosphere.
dropwindsonde:
Cylindrical package of instruments that measures atmospheric conditions and precise location within a storm system, and transmits such data back to the plane from which it was dropped.
Saffir/Simpson scale:
A scale that ranks hurricanes from 1 to 5, based on their potential for destruction.
vertical wind shear:
Differences in speed and direction of moving air at different altitudes (height above sea level).

Back to Top

Activity

  1. Write a paragraph describing how a tropical disturbance forms. Use your own words.
    Answer: A tropical disturbance is a storm that develops over ocean water that is at least 80°F. The storm has circulating air caused by the evaporation of water molecules into the air. These water molecules release latent heat, warming the air, and causing it to rise above the cooler air. The water vapor cools, and the water or ice collects in the clouds. These clouds become storm clouds. When the clouds release their energy, more heat is generated, making the air around the cloud more humid. Eventually vertical wind shear is created as wind moves in from the high-pressure exterior of the storm to the low-pressure interior. The process keeps repeating. The winds in tropical disturbances can reach speeds up to 39 miles per hour.

Activity

  1. Write a paragraph describing how a tropical disturbance forms. Use your own words.
    Answer: A tropical disturbance is a storm that develops over ocean water that is at least 80°F. The storm has circulating air caused by the evaporation of water molecules into the air. These water molecules release latent heat, warming the air, and causing it to rise above the cooler air. The water vapor cools, and the water or ice collects in the clouds. These clouds become storm clouds. When the clouds release their energy, more heat is generated, making the air around the cloud more humid. Eventually vertical wind shear is created as wind moves in from the high-pressure exterior of the storm to the low-pressure interior. The process keeps repeating. The winds in tropical disturbances can reach speeds up to 39 miles per hour.