How Tornadoes Form

Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, between two and six miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation. 

A lower cloud base identifies an area of rotation known as a rotating wall cloud. This area is often nearly rain-free. Note rain in the background. Moments later a strong tornado develops in this area. Softball-size hail and damaging "straight-line" winds also occurred with this storm.

Tornadoes Types

  • Weak Tornadoes
    • 69% of all tornadoes
    • Less than 5% of tornado deaths
    • Lifetime can last between 1 to 10 minutes, or more 
    • Winds less than 110 mph
  • Strong Tornadoes
    • 29% of all tornadoes
    • Nearly 30% of all tornado deaths
    • May last 20 minutes or longer
    • Winds 110-205 mph
  • Violent Tornadoes
    • Only 2% of all tornadoes
    • 70% of all tornado deaths
    • Lifetime can exceed 1 hour
    • Winds greater than 205 mph