Published Tuesday, May 20, 2003 in the San Jose Mercury News

From the slow spin of rising warm air, tornadoes can reach dizzying speeds

Who says the West Coast and the center of the country don't have much in common? The recent record-breaking outbreak of tornadoes in the Plains and Midwest had its origins in the same jet stream pattern that brought California such a cool April and first week of May.

As the jet stream carved out one area of low pressure after another for the West Coast, it turned east. The resulting surface storms that formed in the Great Plains in response to this strong jet stream pushed warm, moist, unstable air northward.

During the day, thunderstorms formed in this air, and as these towering clouds built higher in the atmosphere, the turning of the wind with increasing height started the storms to rotate. The warm moist air moving northward from the Gulf of Mexico meeting a strong jet stream made the perfect recipe for the sustained, rotating thunderstorm (termed supercell) that then resulted in more than 400 tornadoes.

During the seven-day period May 4-10, the preliminary count showed 384 tornadoes in 19 states in a swath stretching from Colorado to the Atlantic seaboard and from the Gulf Coast states to as far north as Wisconsin.

A ranking system called the Fujita scale is used to rate tornadoes from F-0 to F-5, based on their estimated wind speed. An F-0 tornado has winds from 40 to 72 mph, F-1 from 72 to 112 mph, F-2 from 113 to 157 mph, F-3 from 158 to 206 mph, F-4 from 207 to 260 mph and F-5 from 261 to 318 mph.

Five of these recent storms ranked as F-4 on the Fujita scale. The previously most active tornado week since detailed records began in 1950 was May 12-18, 1995, when there were 171 tornadoes.

The last tornado outbreak of consequence was in May 1999, when 130 tornadoes hammered Kansas and Oklahoma over a three-day period, with 48 fatalities. The most devastating tornado event in recent history was the ``super outbreak'' of April 3 and 4, 1974, that produced 147 tornadoes in 16 hours across 13 states, with a death toll of 310 people.

Despite the breadth of this month's tornado rampage, the death toll of 42 is relatively low and most likely attributable to the improvement in monitoring and forecasting these severe weather events. A quarter-century ago there was seldom advance warning of a tornado; today the average lead time for a tornado is about 11 minutes.

The tornado season is generally from May to early June in the Southern Plains, while on the Gulf Coast it is earlier in the spring; in the Northern Plains and Upper Midwest, it is June or July. However, tornadoes have occurred in every state and during every month of the year.

Extensive current and historical information about tornadoes and thunderstorms can be found on the Web at  http://ggweather.com/tornado.htm.

Q. After seeing the recent devastation from the tornado in Tennessee, can you tell me if California has ever had a "real'' tornado touch down? Denise Olazar - Sonoma

A. Since 1950, there have been about 300 tornadoes in California. The strongest of these were two storms that ranked F-3 on the Fujita scale. Since 1950 there have been 22 F-2 tornadoes in the state. The most recent of these was the May 4, 1998, Sunnyvale-Los Altos storm that caused nearly $4 million in property damage.

Q. Has it been proved that there is such a thing as ``earthquake weather?'' Why does it seem as if earthquakes happen only on hot, still days? Does the weather contribute to earthquakes? Andrew Cassetta - Novato & Jody Kitchens - Rohnert Park

A. There is no scientific basis for the concept of ``earthquake weather.'' Tremors have occurred in all seasons and under all weather conditions. Earthquakes result from geologic processes that originate miles underground, but weather elements such as wind, rain, temperature and barometric pressure changes affect just the surface and immediate subsurface. The strong forces that cause earthquakes are focused at depths well out of the reach of the relatively weak forces exerted by weather.

The myth of warm days as ``earthquake weather'' may have originated with Aristotle. In 350 BC the Greek philosopher theorized in his ``Meteorologica'' that earthquakes resulted from warm winds that had become trapped in subterranean caverns. He thought small tremors resulted from the air pushing on the cavern roofs, and larger quakes were a result of those warm winds breaking the surface.

Q. What's the difference between climate and weather? Toni Vuong - Cambridge, Mass.

A. Weather describes the state of the atmosphere at a particular time or over a short period of time, on the order of minutes to weeks. Climate, on the other hand, is the synthesis of weather conditions over longer periods on the order of years to epochs. A common way to describe the climate of a location is to look at the average or ``normal'' conditions over a 30-year period. I have always liked the quote on the topic by science fiction author Robert A. Heinlein in ``Time Enough for Love'': ``Climate is what you expect; weather is what you get.''
 

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ggweather.com or fill out a form online at http://ggweather.com/questions.htm