WEATHER CORNER

BY JAN NULL
Special to the Mercury News

In a departure from the usual Weather Corner format, we're focusing today on a single topic: tornadoes. In light of the severe outbreak in Oklahoma and Kansas last week -- and the anniversary of the 1998 Sunnyvale tornado -- we'll look at this most powerful atmospheric force.

Q.  How common are tornado outbreaks like last week's?

A.  Fortunately, they are quite rare. It's been 25 years since the so-called Super Outbreak, which devastated parts of Ohio plus large areas of Indiana and Illinois in April 1974. Early estimates indicate there were about 60 tornadoes and 45 deaths last week; the 1974 outbreak spawned 148 tornadoes and killed 384.

Q.  What's the strongest tornado?

A.  Because of the difficulty in measuring the strongest winds in a tornado, their strength is estimated from how much damage they do.

The Fujita-Pearson Scale ranks storms from F0 (light damage, winds estimated up to 72 mph) to F6 (inconceivable damage, winds estimated at more than 318 mph). The strongest tornadoes ever recorded are rated at F5 (incredible damage, winds between 261 and 318 mph). Preliminary reports indicate that were several F5 tornadoes May 3 in Kansas and Oklahoma. Over the past 50 years there have only been 51 documented F5s in the United States.

Q.  How strong was the Sunnyvale tornado?

A.  The damage survey indicated a minimal F2 tornado, with winds in the  lower end of the 113 to 157 mph range.

Q.  How often do tornadoes occur in the United States?

A.  Nationwide there are an average of about 900 tornadoes per year. Of these, 74 percent are weak (F0 or F1), 25 percent are strong (F2 or F3), and only 1 percent reach the violent (F4 or F5) category. Despite the small percentage of F4 and F5 storms, they account for 70 percent of deaths. In California, the strongest recorded storms have been F2 tornadoes. This has happened 24 times out of the 262 tornado-producing storms in the state since 1950.

Q.  What is a ``typical'' tornado? How about some extremes?

A.  The typical tornado is about a mile long and 50 yards wide, lasts roughly five minutes and travels about 15 mph. The largest ones can be a mile wide and have been known to stay on the ground for more than 100 miles. They can persist for several hours and move as fast as 60 mph.

Q.  Do tornadoes occur elsewhere?

A.  Yes, in addition to all 50 states, they occur in many other parts of the world, but nowhere with the frequency or ferocity of the United States. Australia is second with about 200 per year, followed by Bangladesh and India.

Q.  Why is the Midwest so susceptible to severe tornadoes?

A.  Like many other meteorological events, their occurrence is a result of the confluence of several geographical influences. To the south is the Gulf of Mexico with its warm waters; to the north, Canada stretches toward the Arctic Circle. During spring (and to a lesser extent fall), southerly surface winds bring warm, humid air from the gulf into the Plains, while in the middle atmosphere, southwest winds bring hot, dry air from Mexico's highlands. In the upper atmosphere, westerly winds bring cool, moist air over the Rockies and into the Plains. This sets up warm, humid air near the ground, hot dry air above it and cool, moist air at higher levels -- the perfect recipe for the massive thunderstorms that can produce tornadoes.

Q.  Where can I find more information about tornadoes?

A.  On the Web at www.usatoday.com/weather/tornado/wtbasics.htm

Jan Null, founder of Golden Gate Weather Services, 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. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.