Tornadoes, sometimes called twisters, are high-speed columns of rotating air connecting a thunderstorm to the ground. These storms vary greatly in size and strength and are difficult for scientists to predict. The average tornado damage path is about one and a half to three kilometers (one to two miles) with a width of 45 meters (50 yards); however, some paths can stretch more than 160 kilometers (100 miles) and have widths greater than three kilometers (two miles). 

Tornado paths are so small and unpredictable local National Weather Service (NWS) forecast offices usually only have about 14 minutes to alert residents with a tornado warning before the storm reaches them. Because of this, the NWS issues tornado watches over a large area to warn residents a tornado could form in their vicinity hours before one can touch the ground. 

Tornadoes only form when a thunderstorm has a certain combination of winds. As winds at varying speeds and directions cause rising air to start spinning, warmer air continues to rise and cooler air begins to sink to the ground. Once there are enough rising and sinking gusts of wind, the air near the ground begins to rotate. The rotating air throughout the tornado eventually speeds up to spin around one axis and begins to move horizontally across the land. Most tornadoes originate from supercell thunderstorms where there are drastic differences in air temperatures and wind speeds, but not all supercell thunderstorms produce tornadoes. 

Tornadoes occur in many parts of the world, including Australia, Europe, Africa, South America, and Asia; however, about 75 percent of the world’s known tornadoes have formed in the United States. About 1,200 tornadoes hit the U.S. every year. Although tornado season refers to the time of year when the United States sees the most tornadoes, peak tornado season varies across different regions of the U.S. The southern Plains experience peak tornado season from May to early June, the Gulf coast from March to April, and the northern Plains and upper Midwest see the most tornadoes in either June or July. Even though there are times of the year when tornadoes are most prominent, they can occur at any time given the right weather conditions.

To assess the wind speeds of a tornado, the NWS implemented the Enhanced Fujita Scale (EF Scale), a set of wind estimates based on the intensity of damage from structures in the path of the storm. Because buildings have varying structural integrity, the EF scale incorporates 28 damage indicators, such as building type (e.g., barn, school, motel, or a shopping mall), structures (e.g., gas station canopy, mobile home, or transmission line tower), and trees (e.g., hardwood or softwood). These damage indicators are then given a damage rating between 1 and 8, where 1 = no damage and 8 = completely destroyed. From the values given for each damage indicator, the NWS derives an EF number between 0 and 5 that estimates the overall intensity of the tornado.

  • EF-0: Gale winds with speeds between 105 and 137 kmph (65-85 mph).

  • EF-1: Moderate winds with speeds between 138 and 177 kmph (86-110 mph).

  • EF-2: Significant winds with speeds between 178 and 217 kmph (111-135 mph).

  • EF-3: Severe winds with speeds between 218 and 266 kmph (136-165 mph).

  • EF-4: Devastating winds with speeds between 267 and 322 kmph (166-200 mph).

  • EF-5: Incredible winds with speeds over 322 kmph (200 kmph).


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Enhanced Fujita Scale
Noun

scale that measures the intensity of tornadoes, from EF-0 (weakest) to EF-5 (strongest).

Noun

branch of the National Oceanic and Atmospheric Association (NOAA) whose mission is to provide "weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy."

supercell
Noun

thunderstorm with a strong, rotating updraft (mesocyclone) in the middle.

thunderstorm
Noun

cloud that produces thunder and lightning, often accompanied by heavy rains.

tornado
Noun

a violently rotating column of air that forms at the bottom of a cloud and touches the ground.