EF4 tornado devastates Mississippi city: What to know about the EF scale

The violent and deadly twister that leveled Rolling Fork, Mississippi, this weekend clocked in as a rare EF4 tornado on the Enhanced Fujita scale.

At least 22 people were killed from this weekend's devastating tornadoes across the South, including the EF4 in Rolling Fork.

What is the EF scale?

The EF scale, an updated 2007 version of the original Fujita scale created in the 1970s, assigns a number from zero to five based on wind estimates and damage.

The strength in miles per hour of a three-second wind gust determines the EF rating, according to the National Weather Service.

The wind gusts are an estimate, not a measured figure, and are determined based on damage. Damage to weaker structures, like barns, sheds and mobile homes, but not sturdier buildings, like homes, schools and malls, would result in a lower rating.

EF0

An EF0 tornado is categorized as having three-second wind gusts of 65 to 85 mph.

EF1

An EF1 tornado is categorized as having three-second wind gusts of 86 to 110 mph.

Eighty percent of tornadoes are rated EF0 or EF1.

EF2

An EF2 tornado is categorized as having three-second wind gusts of 111 to 135 mph.

EF3

An EF3 tornado is categorized as having three-second wind gusts of 136 to 165 mph.

EF4

An EF4 tornado is categorized as having three-second wind gusts of 166 to 200 mph.

EF4 tornadoes make up only 1% of all twisters, according to the Storm Prediction Center.

EF5

An EF5 tornado is categorized as having three-second wind gusts of over 200 mph and is the strongest level on the scale.

What are the strongest tornadoes in US history?

There have been just nine EF5 tornadoes to hit the United States since the Enhanced Scale started in 2007, according to the National Weather Service. Four of those took place on April 27, 2011, in Alabama and Mississippi. Two others took place just weeks later in Joplin, Missouri, and Piedmont/El Reno, Oklahoma.

The most recent EF5 tornado took place in Moore, Oklahoma, on May 20, 2013.

All nine of the EF5 tornadoes in U.S. history have taken place in April or May.

What are some of the deadliest recent tornadoes in US history?

High death tolls in tornadoes are relatively rare in recent decades.

In 2011, a monstrous EF5 tornado decimated Joplin, Missouri, killing 161 people and demolishing entire neighborhoods.

The Joplin tornado is the only one in the top 25 deadliest storms since 1955, according to the National Weather Service.

In 2021, dozens were killed by an EF4 twister that ripped through Mayfield, Kentucky.

ABC News' Dan Amarante contributed to this report.