Warm, moist air is needed to create tornadoes.
Using this knowledge of atmospheric physics some scientists developed a theory that drought in the nation's mid-section may be killing tornadoes.
Evidence is found in the numbers.
The number of tornadoes in 2012 was well below average and 2013 is starting off with the same trend.
In the below graphic from Climate Central red indicates the average number of tornadoes over a 30 year period, while green shows the actual number of tornadoes per month in 2013.
Climate Central says" Pinpointing the reason for tornado droughts (and deluges) isn't always easy. Last year's drought — the actual drought that devastated much of the Great Plains — robbed the atmosphere of the water vapor that fuels severe thunderstorms. Since tornadoes come from thunderstorms, this key thunderstorm ingredient was missing."
This year drought conditions in the mid-section of the U.S. are less severe than last year, but Climate Central says the lingering drought could be combining with another weather factor to keep tornado numbers down this year.
"This year, while the drought is still going on, it's much less severe — yet we may be seeing another tornado drought in the making. This time, it may be the relatively cool spring air much of the nation has been experiencing. Heat is another key ingredient of thunderstorms." says Climate Central.
More research is needed to determine with certainty why there is a drop in tornado numbers.
Right now the above reasons are only theories on which to base research.
So far this year the numbers are 54% below average for the number of tornadoes in the first 4 months of the year.