May 26, 1999

The Fury of Hurricanes

1. Hurricane Georges west of the Florida Keys on September 25, 1998.

2. Hurricane Mitch off the coast of Honduras on October 26, 1998.

3. Cloud structures surrounding the eye of Hurricane Bonnie, towering like giant skyscrapers 59,000 ft. into the sky, as observed on August 22, 1998.

Click on the images for larger views and more detailed captions.

Image Credits: Images 1 and 2 were taken in infrared wavelengths by the GOES-8 weather satellite of the National Oceanic and Atmospheric Administration (NOAA). Image 3 was taken by the world's first spaceborne rain radar aboard the Tropical Rainfall Measuring Mission (TRMM), a joint US-Japanese mission.

The 1998 Hurricane Season

Last year's Atlantic hurricane season was one of the most destructive and deadliest in more than 200 years. Besides hurricanes Georges and Mitch, eight other hurricanes developed over the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Some remained over water and never threatened the Western Hemisphere, but hurricanes Bonnie (August 19-30), Earl (August 31-September 3), Georges, and Mitch did. In addition, there were five tropical storms in the Atlantic.

In the Pacific Ocean, there were nine typhoons and 13 named storms. Howard was the strongest typhoon, with sustained winds of 150 miles per hour.

Hurricanes Are Enormous Heat Engines

What gives hurricanes their enormous violence, with wind speeds of up to 200 miles per hour, torrential rains that may amount to a foot or more in 24 hours, the ability to affect the weather over areas a thousand miles and more across, and the staying power of days to a week or more?

Put simply, hurricanes are engines of enormous size that are powered by heat from warm tropical ocean surfaces. The actual trigger mechanism is a low-pressure system in the tropics north or south of the equator toward which air from the surrounding regions flows (called a cyclone).

The airflow toward the low-pressure area follows an ever-tightening spiral pattern that in the Northern Hemisphere is counterclockwise (in the Southern Hemisphere it is clockwise). Along the way, the air picks up moisture (water molecules) and warmth from the ocean. To conserve its angular momentum, the spiraling motion becomes faster and faster as the air approaches the storm's center, until outward-pointing centrifugal forces prevent further convergence. This central region, which is about 10 to 50 miles across and visible in images 1 and 2 above as compact, round areas, is called the hurricane's eye.

Keeping the Winds of Hurricanes Flowing

As the spiraling and converging winds approach the hurricane's eye, they have nowhere to go but up. This updraft, which continues in a spiraling pattern, is promoted further by the buoyancy of the warm, humid air. The buoyancy is due to the fact that the warm, humid air is lighter than the cooler, drier air of the surroundings.

As the warm air, laden with moisture, spirals upward around the storm's eye, it cools. This, in turn, causes its moisture to condense into rain and, at higher elevations, to freeze into ice (hail). Condensation and freezing release heat, which ensures that the rising air cools more slowly than the surrounding air and continues to rise.

The updrafts of hurricanes typically reach heights of about 40,000 to 50,000 ft., but in exceptional cases they may approach 60,000 ft., as illustrated in image 3. Near the top of the updrafts, some of the risen air, now rid of most of its moisture by precipitation, spirals inward and is drawn down into the low-pressure, relatively calm region of the eye itself. But most of the risen air diverges outward for many hundreds of miles before sinking and intermingling with the prevailing winds of the surrounding areas (the outflow is called an anticyclone).

The air that spirals inward near the top of the updrafts and gets drawn downward into the eye warms as it sinks to lower elevations and dries the storm's central region. This destroys any updrafts in this region and is responsible for the relative calm in the central eye (images 1 and 2). The warm subsiding air also plays a role in reducing the system's central pressure, thereby intensifying the winds that, near the ocean surface, flow from the surroundings toward the eye. This increased airflow, in turn, raises the air's absorption of heat and moisture from the ocean surface, the buoyancy and strength of the updraft within the eye wall (the upward-spiraling winds just outside the eye), and the intensity of the storm. This intensification of the storm constitutes a "positive feedback loop."

The strongest winds and heaviest precipitation of a hurricane occur within about a hundred miles of the eye. However, a hurricane's impact can be felt outward to 1,000 miles and further in the form of increased winds, clouds, and rain.

Summary

It is the release of heat by condensation and freezing in a hurricane's upward-spiraling winds in the eye wall, along with the input of heat and moisture when the air flows across the warm ocean surface, that together constitute the "engine" of the hurricane. Intensification of the storm is brought about largely by the positive feedback loop described above. When a hurricane moves across land or to the northern latitudes of the ocean, where the input of heat is much less than over the warm tropical or subtropical ocean, its strength quickly wanes.

More Cool Stuff

We obtained the image of Hurricane Bonnie from the homepage the Tropical Rainfall Measuring Mission (TRMM):

http://trmm.gsfc.nasa.gov/

Summaries, images, and relevant links of the weather, including hurricanes, in 1998 can be found on these NOAA Web sites:

http://www.outlook.noaa.gov/98hurricanes/

http://www.ncdc.noaa.gov/ol/climate/research/1998/oct/oct98.html

An excellent source of weather forecasts, storm and hurricane warnings, satellite and radar images, and more is the NBC WeatherNet4 Web site in Washington, DC:

http://wxnet4.nbc4.com/home.html

For definitions of tropical storms and hurricanes by category, wind speed, and central pressure, go to a Web page of the Federal Emergency Management Agency (FEMA):

http://www.fema.gov/hu98/hur_cats.htm

To learn more about centrifugal and Coriolis forces, which are key to understanding hurricanes, go to our article "Centrifugal Force":

http://observe.ivv.nasa.gov/nasa/space/Centrifugal/centrifugal_entry.html

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the Earth and atmospheric sciences:

http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_land.html
http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_atmos.html