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Environmental Clues

Tornadoes are often preceded by a dark, often greenish sky, large hail, and often a loud roar, similar to a freight train. In the southern states, however, tornadoes frequently occur without the presence of large hail.

Most strong and violent tornadoes occur with "supercell" thunderstorms. A supercell storm is characterized by intense rotation within the storm. Typically, this area of rotation occurs in the south side of the storm. Visually, a low-hanging cloud base with rotation is often seen. This area may or may not be visible since rain sometimes wraps around the area of circulation.

Some tornadoes appear as a visible funnel extending only partially to the ground. Look for signs of debris below the visible funnel.

Environmental Clues
Things to Watch Out For Rotating Wall Cloud

Rotating wall cloud with very dark sky in the background, near Lahoma, Oklahoma. Note the greenish hue.
(Greg Stumpf)

Tornado near Alma, Nebraska

Tornado near Alma, Nebraska, showing visible condensation only partially to the ground. But, how can you tell it is a tornado? Note the swirl of dust near the ground, and the visible funnel. (Gene Rhoden)

Tornado nearly obscured by rain

Tornado nearly obscured by heavy rain near McClean, Texas. The red circle is where the tornado is located.
(Mike Emlaw)

Supercell Thunderstorm

Photo of a supercell thunderstorm looking east. The presence of an overshooting top above the anvil suggests a very intense storm with an increased potential of being severe. (Howard Bluestein, University of Oklahoma)

Supercell Thunderstorm

Photo of a tornado-producing supercell thunderstorm, looking west. The tornado is located just south of the precipitation region. (Alan Moller, NWS)

 In addition to tornadoes, thunderstorms often produce other hazards. In many cases, these other hazards occur in conjunction with tornadoes.
  • Flash Floods/Floods
    • The number ONE thunderstorm-related killer -- averages 146 deaths annually.
  • Lightning
    • Kills 75 to 100 people each year
  • Damaging Straight-Line Winds
    • Can reach speeds of 140 mph
  • Large Hail
    • Often the size of golf balls and occasionally can reach grapefruit size. Hail causes several hundred million dollars in damage annually to property and crops.
Other Thunderstorm Hazards
Flash Flood near Shadeyside, Ohio

The result of a flash flood at Shadyside, Ohio, June 14, 1990. (NOAA/NWS)

Large Hail Near Altus, Oklahoma

Large hail near Altus, Oklahoma. Some of the hailstones are larger than golfballs. (NOAA/NWS)

Lightning Flash

A stunning flash of lightning. (Bill Bunting, NWS)


'Tornadoes: Nature's Most Violent Storms' was developed by the National Oceanic and Atmospheric Administration (NOAA)/National Weather Service (NWS) in cooperation with the American Red Cross and the Federal Emergency Management Agency (FEMA). It is designed to assist in heightening public awareness and understanding the dangers associated with tornadoes. The package provides an introduction to tornadoes and their impact, discusses tornado development and occurrence, and describes safety information for homes, schools, places of work, and outdoors. In addition, basic environmental clues are presented.

Individual use of the pictures within these presentation must have prior approval from the credited source.
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