WEATHER CORNER

BY JAN NULL
Special to the Mercury News

With the Atlantic hurricane season beginning June 1, it's a good time to look at this awesome and destructive natural force.

Q.  What's the difference between hurricanes, typhoons and cyclones?

A.  Just as in real estate, the difference is location, location, location. They are exactly the same meteorological phenomenon -- a tropical cyclone, or spiraling storm, with sustained wind speeds of at least 74 mph.

These storms are called hurricanes when they occur in the Atlantic, Caribbean or eastern Pacific ocean. The term comes from a Caribbean Indian storm god, Huracan.

In the Western Pacific, tropical cyclones are called typhoons from the Cantonese term tai fung, or great wind, and from Typhon, a gigantic monster in Greek mythology whose name means ``whirlwind.''

These same storms are called cyclones in the Indian Ocean. The word derives from the Greek term kyklos, or circle.

There are many local names for these monster storms as well.  One of my favorites is along the west coast of Mexico, where these storms sometimes are called cordonazo de San Francisco, or ``lash of St. Francis.'' This name originates from the strong southerly winds that can lash out with death and destruction along the coast near the time of the Oct. 4 Feast of St. Francis.

Q.  What are tropical cyclones?

A.  They are intense areas of low pressure in the tropics. Just as water flows downhill, warm, moist air is drawn rapidly inward from areas of higher pressure to the centers of these lows. Due to the Coriolis Force, they spiral counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This creates the strong winds that are the cyclone's trademark. The warm air, which derives much of its energy from the warm tropical ocean waters, rises high into the atmosphere and develops into huge bands of thunderstorms.

When a tropical cyclone's winds blow at less than 39 mph it is called a tropical depression. Winds of 39 to 73 mph qualify it as a tropical storm, and at 74 mph or greater it becomes a hurricane. Hurricanes are rated for damage potential using the Saffir-Simpson scale, which is based on wind speed. On this scale, Category 1 hurricanes have winds of 74 to 95 mph, Category 2 are 96 to 100 mph, Category 3 are 111 to 130 mph, Category 4 are 131 to 155 mph and Category 5 hurricanes have winds of more than 155 mph.

Category 4 and 5 storms are far more deadly and devastating than lesser categories, but they also are relatively rare. Since 1900 only 13 Category 4 storms and three Category 5s have made landfall in the United States.

Q.  What makes hurricanes so dangerous?

A. It's the flooding more than the winds. About four times more people are killed by hurricane-induced flooding than by the wind. Six to 12 inches of rain commonly fall as a hurricane moves inland.  Coastal areas also are susceptible to flooding from storm surges -- a combination of water piled up by the hurricane's winds and a rise in sea level because of the extremely low barometric pressure associated with tropical storms. The surge inundates coastal areas that are often densely populated and difficult to evacuate. It can range from five feet deep in a Category 1 storm to more than 18 feet with a Category 5.

Hurricanes also spawn tornadoes, spiraling storms that are much smaller but more intense as they move inland.

Q.  When are these storms most likely to occur?

A.  Like the names, this, too, depends on location. In the Atlantic and Caribbean, the official hurricane season begins June 1 and lasts until Nov. 30. Occasionally, a storm will occur out of season. Notably, Hurricane Arlene showed up early, intensifying to hurricane strength May 6, 1981, and Lili hung around almost until Christmas with hurricane-force winds as late as Dec. 22, 1994. The Atlantic season peaks in early September.

The season in the Northeastern Pacific runs from around May 15 until Nov. 30, with an early September peak in activity like the one in the Atlantic. In the northwest Pacific, the typhoon season is year-round with the highest probability of storms around Sept. 1 and the lowest about March 1. And the north Indian Ocean cyclone season is almost as long, extending from April through December with peaks in May and again in November.

In the Southern Hemisphere, the cyclone season in the southern Indian and southwest Pacific runs from late October through May with a peak about Feb. 1.

Q.  How are hurricanes named?

A.  Tropical storms in the Atlantic are named by the World Meteorological Organization. There are six lists used in rotation. Consequently, the list used this year will next be used in 2006. The lists are changed only if a storm is so infamous because of loss of life or property that its name is retired and replaced.

Q.  What will this year's hurricanes be named?

A. In the Atlantic the first named storm will be Alberto. It will be followed by Beryl, Chris, Debby, Ernesto, Florence, Gordon,
Helene, Isaac, Joyce, Keith, Leslie, Michael, Nadine, Oscar, Patty, Rafael, Sandy, Tony, Valerie and William.

In the Eastern Pacific the 2000 list is Aletta, Bud, Carlotta, Daniel, Emilia, Fabio, Gilma, Hector, Ileana, John, Kristy, Lane, Miriam, Norman, Olivia, Paul, Rosa, Sergio, Tara, Vicente, Willa, Xavier, Yolanda and Zeke.

You'll notice that none of the names starts with Q or U; there just aren't enough names beginning with those letters to go around. And for some reason, names starting with X, Y or Z are OK in the Pacific but not in the Atlantic.

Hurricanes were originally identified by their latitude and longitude. In 1953 the World Meteorological Organization began giving them women's names, and in 1979 the system was broadened to include men's names and to reflect Spanish and French influences.

Jan Null, founder of Golden Gate Weather Services and director of meteorology for Planetweather.com, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190; phone (510) 657-2246, e-mail weathercorner@ggweather.com or fax them to (510) 315-3015.