Photo from Herb Stein, Center for Severe Weather Research

Oklahoma is bracing for another round of potentially violent weather, after multiple tornadoes ripped through the state on Monday, leaving at least six people dead. The odds a tornado in Oklahoma will cause at least one death are 1 in 39.85; the odds it will cause at least five deaths are 1 in 157.3.

Despite the toll of yesterday’s storm, it is very unlikely that a person will be killed by a tornado in a year—those odds are 1 in 4,513,000. It is more likely a person will die from a fall off a cliff—1 in 4,101,000—or will be diagnosed with leprosy—1 in 2,930,000.

The tornado with the highest death count was the 1925 Tri-State Tornado, which ravaged a path a mile wide and 219 miles across through Missouri, Illinois, and Indiana. The speed was 60 miles per hour—twice the forward speed of the average tornado. Although the Tri-State Tornado lacked the classic funnel cloud, and was therefore virtually invisible, the damage was catastrophic: nearly 2,000 people were injured, property losses totaled more than $16 million, and 689 people died.

At the end of the spring storm season last year, US fatalities from tornadoes stood at their lowest level in three years. At the end of June 2009, 21 deaths were on record, compared with an unusually high 121 deaths at the same time in 2008, and 74 deaths at the same time in 2007.

There is no fixed relationship between the number of lives lost and the number of tornadoes that occur every year. Despite so few deaths, the US tornado count at the end of June 2009 was only slightly below the three-year average: 850 versus 935.

Tornadoes are measured on the Fujita Scale, named after a professor who started his career cataloging the damage in post-1945 Hiroshima and Nagasaki, Japan. The 6-category Fujita Scale classifies a tornado by the damage it causes. The majority of tornadoes rank low on the scale and leave a minimal mark. At F0, a tornado might do nothing more than break some tree branches. The odds a tornado will be an F0 are 1 in 2.32. A tornado that measures F3 has the wallop to uproot those trees. At F5, a storm can easily uproot houses, even those with a solid foundation. The odds a tornado will be categorized as an F5—Tri-State level—are 1 in 634.5.