Powerful tornadoes like the one that leveled parts of an Oklahoma City suburb have varied widely in number in the past three years, for reasons that can’t be blamed on climate change, a federal researcher said.

“The climate has changed but not by a ridiculous amount,” said Harold Brooks, research meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma.

Science has gotten better since the 1950s at predicting where and when a storm will strike. More research will be needed to determine how a vast event such as global warming affects a relatively small-scale one like the tornado that hit Moore, Oklahoma, the National Oceanic & Atmospheric Administration said earlier this month.

A twister with winds of more than 200 miles (322 kilometers) per hour cut a 17-mile path through heart of Moore on May 20, according to the National Weather Service. The storm, which killed at least 24 people, was an EF5, the most powerful category of tornado, the agency said.

Tornado classification is based on damage, not actual wind measurement. The systems can last from several seconds to more than an hour, with most breaking up within 10 minutes, according to the U.S. Storm Prediction Center in Norman.

The area south of Oklahoma City, which includes Moore, was the site of a tornado in May 1999 that set a global record for wind speed at 302 mph. Oklahoma City tops the list of places most hit by twisters in the U.S.

Oklahoma City

Oklahoma City is hit more because it’s one of the largest municipalities by land area and sits in the heart of the most active U.S. tornado region, Brooks said. The city of 591,967 people covers 606 square miles, compared with Boston, which had a 2011 population of 625,087 in 48 square miles.

In the 1950s, roughly 550 tornadoes were reported in the U.S. each year, with 50 of them the equivalent of an EF0, the weakest level on the six-step Enhanced Fujita Scale,

There are now about 1,300 twisters a year, including 800 EF0s, Brooks said by telephone. Doppler radar and greater public awareness have contributed to an increase in the number of tornadoes reported, according to a fact sheet from the National Oceanic & Atmospheric Administration.

About 500 EF1 or stronger tornadoes have been reported in the U.S. each year since the 1950s, so “if we throw out the EF0s, there’s no long-term change,” Brooks said.

Strong Storms

What has changed is record variability in the number of tornadoes of EF1 or stronger, Brooks said. The most recorded in any 12-month period was 1,050 from June 2010 to May 2011, he said, while the fewest were the 217 from May 2012 to April 2013. To attribute any change to a warming planet would require a consistent rise or fall over several years, he said.

The climate in the U.S. is 1.5 degrees Fahrenheit warmer than it was in the 1970s, according to the National Climatic Data Center in Asheville, North Carolina.

Brooks said there have been other times when outbreaks of tornadoes have caused people to wonder if larger forces were at work.

“In 1953, people thought there were more tornadoes because of nuclear testing,” Brooks said.

In the 1970s, large numbers of tornadoes were thought to be caused by “global cooling,” said Tom Downs, a forensic meteorologist for Weather 2000 Inc. in New York.

From 1991 to 2010, an average of 1,253 tornadoes occurred in the U.S. per year, according to NOAA’s Climatic Data Center. Texas averaged 155, followed by Kansas with 96, Florida with 66 and Oklahoma with 62.

Alabama Leads

From 1950 to 2011, Alabama led the U.S. with the most EF5 or equivalent tornadoes originating there, with seven, while Oklahoma, Texas, Iowa and Kansas each had six, the Storm Prediction Center said.

Tornadoes tend to cause more deaths in Alabama than in Oklahoma because more people live in mobile homes, there is a greater rural population density, more of the storms happen at night and trees make the twisters harder to see, Brooks said.

About 77 percent of all U.S. tornadoes are EF0 or EF1, the two weakest categories, and about 95 percent are below EF3 strength, according to the data center. Only 0.1 percent reach EF5 power.

The annual U.S. average for EF3 to EF5 tornadoes from 1991 to 2010 was 37.5 per year. Kansas has an average of 3.1 per year, followed by Arkansas with 2.9, Texas with 2.8 and Oklahoma and Tennessee with 2.7.

The area called Tornado Alley, which includes Oklahoma, is at the convergence of three major air masses, Downs said. There is warm, dry air coming in from Mexico, warm, humid air moving off the Gulf of Mexico and cooler air from Canada sweeping across the U.S. Great Plains.

“It is special on the planet because of the presence of the Gulf and the presence of the Rocky Mountains,” Brooks said. “It is a great laboratory to create tornadoes.”