Generated by severe thunderstorm activity, tornadoes are nature’s most violent storms. Scientists do not know exactly what causes them, but atmospheric instability and wind shear are important contributing factors.
Instability in the atmosphere is created when a warm, humid air mass meets a cool, dry air mass. Through the process of convection, this encounter allows warm air to rise easily and produce thunderstorms. The greater the instability, the stronger a thunderstorm can become.
Wind shear occurs when there is a localized change of wind speed and direction. When surface winds blow in one direction and upper level winds in another, the air in-between is set in a horizontal rolling motion. The updraft of the thunderstorm can then tilt the rotating air into a vertical position. When that vortex extends from the cumulonimbus cloud to the ground, a tornado is born.
Tornadoes can occur anywhere in any season, but are more typical in the spring and summer months in the United States. According to NOAA, approximately 1,200 tornadoes touch down in this country every year.