Tip Sheet: Hurricanes
BOULDER -- This year may bring a bumper crop of Atlantic hurricanes, thanks to the influence of La Nina and other factors. Below is a range of Web sites, answers to frequently asked questions, and a set of hurricane experts from the National Center for Atmospheric Research (NCAR) and from member institutions of the University Corporation for Atmospheric Research (UCAR). NCAR's primary sponsor is the National Science Foundation. NCAR is managed by UCAR, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
Roger Pielke, Jr., 303-497-8111, firstname.lastname@example.org
Robert Gall, 303-497-8160, email@example.com
Terry Hock, 303-497-8767, firstname.lastname@example.org
Christopher Landsea, 305-361-4357, email@example.com
Jay Baker, 850-893-8993, firstname.lastname@example.org
Related sites on the World Wide Web
Background information on hurricanes
What is a hurricane? Each year a number of tropical disturbances--centers of low pressure--move westward across the Atlantic, Pacific, and Indian Oceans. Some of these become depressions--more organized disturbances with sustained surface winds of up to 38 miles per hour (61 kilometers per hour). Should the winds become stronger, the system becomes a tropical storm and is given a name. If the winds reach 74 mph (119 km/hr), the storm is reclassified as a hurricane (other names, such as typhoon, are used outside of the Atlantic). Eventually, most of these systems either strike land and quickly weaken or recurve over the ocean, moving north and east as they become caught up in the midlatitude westerly winds and lose their tropical characteristics.
What's the difference between a hurricane and a tornado? Tornadoes are spawned by thunderstorms, while hurricanes are made up of many showers and thunderstorms (which can themselves spawn tornadoes upon landfall). Hurricanes gather energy from the warmth of the ocean. A hurricane's eye is typically 10 to 15 miles (16 to 24 km) wide, and winds around it are as strong as 150 to 200 mph (240 to 320 km/hr). The hurricane circulation can be hundreds of miles across. Even the largest tornadoes are only about a mile across, although their winds can reach 300 mph (480 km/hr). Tropical storms can survive for weeks, while most tornadoes exist for much less than an hour.
How many tropical systems occur on average each year? Globally, an average of around 85 tropical storms and 45 hurricanes/typhoons form per year. The tropical Atlantic and Gulf of Mexico--the sources of U.S. hurricanes--produce an average of nine named storms per year, with around six of those becoming hurricanes and two of those becoming intense hurricanes (ones with sustained surface winds exceeding 110 mph, or 177 km/hr). The period since 1995 has been unusually active. Even though 1997 saw few Atlantic storms, the 1995-1998 total was the largest on record for a four-year span in the Atlantic, with 53 named storms, including 32 hurricanes.
How are hurricanes predicted? Some climate factors are known to affect hurricane frequency for a given ocean or a given year. For instance, El Niño enhances upper-level winds that tend to suppress Atlantic hurricanes, while La Niña has the opposite effect. Each year, out of dozens of disturbances that cross the tropical Atlantic, only a handful encounter the right combination of light wind shear and warm ocean temperatures that allows for hurricane development. The behavior of tropical storms and hurricanes is predicted by high-resolution computer models at the NOAA Tropical Prediction Center, the federal agency that issues hurricane watches and warnings (see Web site above). Hurricane motion can be projected with some skill out to five days. Changes in storm intensity are still difficult to predict, even within a day or two, but some progress has been made with better computer models. New observing tools, such as GPS dropsondes (see Terry Hock, above) and satellite-based radars that can sense unusually deep layers of warm water, are also helping to improve forecasts by providing a better picture of current conditions.
What are the storm names for this year and upcoming years? Since 1953, Atlantic tropical storms have been named from lists originated by the National Hurricane Center and now maintained and updated by Region 4 of the World Meteorological Organization. The lists featured only women's names until 1979. Six lists are used in rotation. The names of the most severe hurricanes, such as 1998's Mitch, are retired permanently. Below are the lists for 1999-2004.
Atlantic basin hurricane names
UCAR NCAR UOP
The National Center for Atmospheric Research and UCAR Office of Programs are operated by UCAR under the sponsorship of the National Science Foundation and other agencies. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors. UCAR is an Equal Opportunity/Affirmative Action employer.
Prepared for the web by Jacque Marshall
Last revised: Fri Apr 7 15:38:50 MDT 2000
Last revised: Fri Aug 6 15:04:30 MDT 1999