 Winter tornadoes that ripped across parts of the American South this week were unusually lethal but not particularly rare, a US government meteorologist said on Wednesday as the death toll mounted. Tornado season in the United States generally starts in March and continues through the summer months but winter tornadoes have become an almost annual occurrence, according to Harold Brooks of the National Oceanic and Atmospheric Administration. "While this is not a normal event, it's not an incredibly rare event," Brooks, based at the agency's National Severe Storms Laboratory in Norman, Oklahoma, said by telephone. Tornadoes that rolled through Arkansas, Alabama, Mississippi, Tennessee and Kentucky this week killed more than 50 people. Brooks said tornadoes in the southeastern United States occur in winter "roughly once a year," he said. The current tornado outbreak, which Brooks estimates includes some 30 to 40 tornadoes, is similar to a March 1, 2007, outbreak that killed 20 people in and around Enterprise, Alabama. There were previous deadly tornado outbreaks on March 12, 2006, in Missouri and Illinois and on January 1, 1999, in Arkansas and Tennessee, Brooks said. The difference between these other three outbreaks and the recent one is the death toll, he said. Tornadoes develop in warm, moist air ahead of east-moving cold fronts. There are 800 tornadoes reported in the United States in an average year, resulting in 80 deaths and over 1,500 injuries, according to the weather agency's Web site www.nssl.noaa.gov/edu/safety/tornadoguide.html. Big differences in temperature help fuel tornado development by whipping up strong winds aloft where masses of cold air and warm air meet. This year's cold northern temperatures and warm air in the US south created good conditions for tornado formation, Brooks said. Does climate change play any role in the frequency or intensity of tornadoes? Brooks said no, adding that the historical record of tornadoes is insufficient to let scientists figure out what impact, if any, climate change has. "Our current physical understanding of how tornadoes work (is that) some of the ingredients that are important to make a tornado will increase in a greenhouse-enhanced world, some of them will decrease and the balance is unknown," Brooks said.