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Weather Conditions Before Tornadoes

Tornadoes are among the most violent storms found in nature. These rotating, funnel-shaped clouds extend from a thunderstorm cloud to the ground with circling winds that can reach up to 300 miles per hour and leave behind a path of damage up to 50 miles long and one mile wide. Although tornadoes can cause massive devastation in mere seconds, several weather conditions tend to occur prior to the formation of a tornado.

  1. Thunderstorms or Hurricanes

    • Tornadoes generally occur near the trailing edge of a thunderstorm. They may also accompany tropical storms or hurricanes as they move onto land. Season also affects the likelihood that a tornado will accompany a storm. Tornadoes east of the Rockies are most frequently reported March through May -- this is peak tornado season in the southern states. In northern states peak tornado season is late spring through early summer.

    Dark or Greenish Sky

    • A dark greenish or greenish-yellow color sometimes precedes a tornado or thunderstorm. Exactly why this occurs is unknown. One theory is that the golden-red light from a low sun and the natural bluing effect of air combine to create a green sky in front of the dark backdrop of a storm, which offsets the green or yellow hue. Another theory is that the storm clouds themselves, filled with water or hail, provide the blue color which, when illuminated by a low, golden-red sun, creates the green color.

    Wall Cloud

    • Tornadoes tend to be located under a wall cloud. A wall cloud appears as a relatively precipitation-free cloud base that extends further below the neighboring base and is located underneath a rapidly growing convective cloud. The wall cloud, when viewed from the front (the direction in which it is traveling), sometimes resembles a vertical wall. Sometimes a wall cloud also has a tail cloud extending into an area of precipitation. Some wall clouds begin as scud clouds. These amorphous clouds form below the preexisting cloud base, then ascend to the upper cloud.

    Large Hail

    • Large hail often precipitates in front of a wall cloud, with small hail even further in front. A tornadic supercell has a strong updraft with rapidly rising hot air. At an extremely high altitude the updraft slows and spreads out, creating an anvil composed of ice crystals. Some of this precipitation may freeze to grapefruit-sized hail. Shifts or fluctuations in the updraft can cause the hail to fall.

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