The last two tornado seasons have been the deadliest in a decade, with 206 deaths. Everyone from the insurance industry toAl Gore worry that global warming may be causing more tornado【S1】______activity. But there’s no baseline for comparison. That’s why we have【S2】______no accurate record of tornadoes before the 1950s, back when it was possible for these brief, freakish funnels of air to blow over the unpopulated areas without a notice.【S3】______ Tornado warnings have improved over the years; forecasterscan now issue warnings about 18 minutes after touchdown for 75【S4】______percent of twisters. As longer-term forecasts, the science isn’t there【S5】______yet, despite that you hear on the nightly news. " Television【S6】______forecasters cater with the public’s curiosity about extreme weather,"【S7】______says Michael H. Glantz, a professor at the University of Colorado at Boulder. Their goal, of course, is to be first with a forecast of trouble ahead. The number of deaths per tornado is greater in the South than in Tornado Alley(the Great Plains and part of the Midwest)—adisparity that can’t be explained for storm frequency or severity.【S8】______Experts say things like terrain, population density, and the numberof mobile homes play a role. And the biggest factor might be timing:【S9】______"In the Plains," explains Schaefer, "we have a many more clearly【S10】______defined period of tornado activity, generally from March to late June. In the Southeast, tornadoes can happen all year long, so there may be less vigilance. " Research also shows that the South may have a climatological propensity(倾向)for nighttime storms, which catch people unawares—asleep in bed. Last year’s two killer tornado outbreaks in North Carolina landed between 11: 30 p. m. and 3: 30 a. m.【S4】