How Tornadoes Work


Tornado Ratings

Tornadoes are among the most dangerous storms on Earth and, as meteorologists strive to protect vulnerable populations through early warning, it helps to classify storms by severity and potential damage. Tornadoes were origina­lly rated on the Fujita Scale, named for its inventor, University of Chicago meteorologist T. Theodore Fujita. The meteorologist created the scale in 1971 based on the wind speed and type of da­mage caused by a tornado. There were six levels on the original scale.­

F0

  • Wind Speed: 40-72 mph (64-116 kph)
  • Light damage: Tears branches from trees; rips shallow-rooted trees from the ground; can damage signposts, traffic signals and chimneys

F1

  • Wind Speed: 73 - 112 mph (117 - 180 kph)
  • Moderate damage: Roofing materials and vinyl siding can be displaced; mobile homes are highly vulnerable and can easily be knocked from the foundation or toppled; motorists can be sent careening off road and possibly flipped over

F2

  • Wind Speed: 113 - 157 mph (181 - 253 kph)
  • Considerable damage: Well-established trees are easily uprooted; mobile homes are decimated; entire roofs can be ripped off houses; train cars and trucking hauls are knocked over; small objects become dangerous missiles

F3

  • Wind Speed: 158 - 206 mph (254 - 332 kph)
  • Severe damage: Forests are destroyed as a majority of trees are ripped from the ground; entire trains are derailed and knocked over; walls and roofs are torn from houses

F4

  • Wind Speed: 207 - 260 mph (333 - 418 kph)
  • Devastating damage: Houses and other small structures can be razed entirely; automobiles are propelled through the air

F5

  • Wind Speed: 261 - 318 mph (419 - 512 kph)
  • Incredible damage: Cars become projectiles as they are hurled through the air; entire houses are completely destroyed after being ripped from the foundation and sent tumbling into the distance; steel-reinforced concrete structures can be seriously damaged [source: NOAA]

In February 2007, the Fujita Scale was replaced by the Enhanced Fujita Scale. The new "EF" scale is similar to its predecessor. It classifies tornadoes into six different categories (EF0 through EF5 instead of F0 through F5). Where the EF­ scale differs, however, is in the number of criteria used to assess a tornado's level of damage. First, there are damage indicators -- objects that can be damaged in the tornado. These are classified from 1 (small barns) to 28 (softwood trees). Each damage indicator can also experience varying degrees of damage (DODs). Each DOD corresponds to estimated wind speeds.­

­For example, a motel has 10 degrees of damage, ranging from broken windows (3) to the collapse of most of the roof (6) to complete destruction of the building (10). If a motel's windows are broken, but it doesn't sustain more extensive damage, the estimated lowest possible wind speed is 74 mph (119 kph), while the estimated highest possible speed is 107 mph (172 kph). Meteorologists average these speeds, meaning the expected wind speed is 89 mph (143 kph). An examination of the EF Scale reveals that 89 mph falls into the EF1 category, so the tornado is classified as an EF1. For more information about the EF scale, see the official NOAA Web site.

Explore the links on the next page to learn even more about tornadoes and other weather anomalies.