Tornadoes in the United States have been happening on fewer days since the 1970s, but more tornadoes have touched down (pictured) on those days.
The overall number of US tornadoes has not changed in recent decades. However, in analysing the national tornado database, Harold Brooks of the National Severe Storms Laboratory in Norman, Oklahoma, and his team found that the number of days with at least one tornado has fallen from 150 to 100 since the early 1970s. Over the past decade, 20% of US tornadoes occurred on just three days of the year.
Whether the change is linked to rising global temperatures is not clear, the authors say.