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Keep your eyes on the sky ― it looks like 2006 will be a big year for tornadoes. As of the first week in April, there have already been 445 “twisters” in the United States. Last year, there were only 96 tornadoes by the beginning of April. According to the National Oceanic and Atmospheric Administration (NOAA), a government agency that studies the environment, this marks the most intense tornado activity in the first three months of any year since 1999. Previous years with a busy start have produced high numbers of tornadoes throughout the year. For those living in “Tornado Alley” from west Texas all the way up to North Dakota, springtime is tornado time. What exactly causes a “twister”? Tornadoes form when moist, warm air is caught under dry, cool air. When the warm air rises, it upsets the cool air, spinning (使快速旋转) the air into a funnel-like shape. The spinning air is officially categorized as a tornado once it touches the ground. These twisters can reach extremely high speeds, sometimes more than 250 miles per hour! Scientists try to measure the speed and damage of tornadoes using what is called the Fujita Scale, or F-scale. The scale ranges from F0, with winds under 73 mph, to F5, with winds between 261 and 318 mph. Violent tornadoes in the F4 and F5 categories sometimes last more than an hour, and cause incredible amounts of damage, often destroying everything for hundreds of miles. Fortunately, these account for less than 1 percent of all tornadoes. | 
