Tornado damage in Henryville, Ind., after a tornado swept through the small community on March 2, 2012. There have been fewer tornadoes this year than most. (Michael Raphael/FEMA)
There were only 24 tornadoes throughout the United States last month, according to preliminary data from the National Oceanic and Atmospheric Administration, by far the fewest for a July since records began in the 1950s. This shatters the record low of 42 tornadoes set in July 1960.
Related on Our Amazing Planet: Storm Chasing Scientists
Why so few twisters? "The one-word answer is drought," said Bob Henson, a meteorologist and science writer for the National Center for Atmospheric Research in Boulder, Colo. Fewer rainstorms means fewer chances for tornadoes, which form only in thunderstorms. "If you don't have thunderstorms, you can't get a tornado," Henson told OurAmazingPlanet.
Record drought has gripped much of the country, with nearly two-thirds of the 48 contiguous states in some stage of drought. Part of the reason for the drought - and hence the lack of tornado-producing storms - is the presence of a high-pressure "heat dome" over much of the country.
The year started out differently, with unusually high tornado activity. In March, for example, there were 151 tornadoes, well above average, Henson said. These deadly storms made up the first billion-dollar disaster of the year.
Related on Our Amazing Planet: The Top 5 Deadliest Tornado Years in U.S. History
But tornadoes grew scarcer as drought started to take hold in mid- to late April. From May through July, there were 231 tornadoes, the fewest in this span since high-quality record-keeping began in the 1950s, according to the U.S. Storm Prediction Center in Norman, Okla.
In many years, a large percentage of tornadoes occur on a handful of days when conditions are right, such as strong upper-level winds and unstable air near the ground. This year was no different, with 153 twisters – a third of the year's current total – occurring on just three days: Feb. 29, March 2 and April 14. [Infographic: Tornado! How, When & Where Twisters Form]
A significantly greater number of tornadoes can be recorded now than in decades past due to the increased number of storm chasers and the ease of sharing photos or video of twisters, which makes this July's record all the more impressive. Researchers have created "inflation-adjusted" stats that reflect how many tornadoes likely occurred in years past.
Related on Our Amazing Planet: The Tornado Damage Scale In Images
After 2012, the most tornado-starved Julys were in 2002, 2006 and 2007, according to the adjusted data. The Julys of 2002, 2006 and 2012 also were among the nation's warmest months in the last century, Henson said. When a summer month is unusually hot, the polar jet stream generally has been pushed well to the north by domes of high pressure. That leaves less upper-level energy to fuel tornado-creating thunderstorms. Non-tornadic storms (which rely less on wind shear and more on heat and moisture) may still pop up, assuming drought hasn't taken hold, Henson said.
With the jet stream pushed to the north, Canada had more tornadoes than the United States last month, which is very unusual, Henson said.
Reach Douglas Main at firstname.lastname@example.org. Follow him on Twitter @Douglas_Main. Follow OurAmazingPlanet on Twitter @OAPlanet. We're also on Facebook and Google+.
Copyright 2012 OurAmazingPlanet, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
RELATED ON SKYE: Dramatic Photos of America's Drought