Extreme tornado outbreaks have become more frequent: Study

​New York: The average number of tornadoes per outbreak, large-scale weather events that can last one to three days and span huge regions is rising in North America from about 10 to about 15 since the 1950s, says a new study.

Most death and destruction inflicted by tornadoes in North America occur during outbreaks, and the largest ever recorded happened in 2011. It spawned 363 tornadoes across the US and Canada, killing more than 350 people and causing damages worth $11 billion, the study pointed out.

"When it comes to tornadoes, almost everything terrible happens in outbreaks. If outbreaks contain more tornadoes on average, then the likelihood they'll cause damage somewhere increases," said lead study author Michael Tippett, climate and weather researcher at Columbia University in New York, US.

For this study, the authors calculated the mean number of tornadoes per outbreak for each year of the study period as well as the variance, or scatter, around this mean. They found that while the total number of tornadoes rated F/EF1 and higher each year hasn't increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.