The tornado that slammed into the Oklahoma City suburb of Moore on Monday, killing more than 50 people with winds up to 320 kilometres per hour, was given a preliminary ranking of EF4, the second most powerful category on the Enhanced Fujita Scale.
The Enhanced Fujita scale is an updated version of an earlier chart to measure the ferocity of tornadoes published in 1971 by a University of Chicago professor.
In rating the intensity of tornadoes, meteorologists and other experts study damage on the ground, along with wind speeds and other data, to rate twisters between EF0 (the weakest) and EF5 (the most intense).
The scale is as follows:
* An EF5 tornado, capable of causing incredible damage, is characterised by three-second wind gusts between 320 and 376 kph and can level strong frame houses off their foundations, according to the National Oceanic and Atmospheric Administration (NOAA). Automobile-sized missiles can fly through the air in excess of 100 meters, and trees can be stripped of their bark.
On May 3, 1999, a tornado rated as an F5 under the original Fujita scale struck the town of Moore, killing 40 people and destroying thousands of homes. That event caused more than $US1 billion ($1.02 billion) in damage, or $US1.3 billion in today's dollars.
The devastating tornado that struck the town of Joplin, Missouri, on May 22, 2011, killing 161 people, also was an EF5 tornado.
* An EF4 tornado can cause devastating damage, and is capable of levelling well-constructed homes and blowing away those with weak foundations. Cars can be thrown and large missiles generated. Such a twister includes wind gusts of between 270 to 320 kph.
The tornado that struck Moore on Monday, ripping up at least two schools, has been preliminarily rated an EF4, according to a spokeswoman for the National Weather Service Storm Prediction Centre in Norman, Oklahoma.
* An EF3 tornado can cause severe damage, tearing roofs and walls off well-constructed homes, uprooting trees, and lifting cars of the ground and throwing them. It has wind gusts of between 138 to 268 kph.
Several EF3 tornadoes have been recorded in the United States this year, including one that touched down in Kemper County, Mississippi, in April, killing one person and injuring four.
* An EF2 tornado can cause considerable damage, tearing roofs off frame homes, demolishing mobile homes, overturning boxcars and snapping or uprooting large trees. Such a tornado is characterised by wind gusts between 177 to 220 kph.
A number of EF2 tornadoes have struck the United States so far during 2013, in some cases causing injuries or substantial damage.
* An EF1 tornado can cause moderate damage, with wind gusts of between 138 to 175 kph. Damage caused in such twisters includes surfaces peeled off roofs, mobile homes pushed off foundations or overturned and cars blown off roads.
* An EF0 tornado causes light damage with wind gusts of between 104 to 136 kph. The impact can include damage to chimneys, branches broken off trees and shallow-rooted trees being knocked over.
Source: NOAA's National Weather Service Storm Prediction Centre