Ingredients

• A number of ingredients are required for the formation of a tornado. First, there must be atmospheric instability. This instability is produced by two additional ingredients: warm moist air and cold dry air. Finally, tornado development requires wind shear. This occurs when strong winds change direction with height. This creates a horizontal circulating column of air.
  
  

Formation

• Tornadoes occur when cold dry air along a cold front collides with warm moist air. This collision rapidly uplifts the warm air, producing a thunderstorm. Thunderstorms develop a strong updraft, which lifts the horizontally rotating wind shear into a vertical position. This intensifies the circulation within the updraft, called a mesocyclone. If this area of circulation intensifies enough, the circulation reaches the ground as a tornado.
  
  

Location

• Tornadoes form out of a wall cloud. This type of cloud represents a lowering of the thunderstorm's base directly under the circulating updraft. The storm's downdraft can feed into this updraft, intensifying the circulation. Because wall clouds form under the updraft rather than the downdraft portion of the thunderstorm, tornadoes usually do not occur in areas of precipitation.
  
  

Intensity

• F0 tornadoes have winds between 65 to 85 miles per hour, and produce light damage. F1 tornadoes have winds between 86 and 110 miles per hour, producing moderate damage. F2 tornadoes have winds between 111 and 135 miles per hour, producing considerable damage. F3 tornadoes have winds between 136 and 165 miles per hour, producing severe damage. F4 tornadoes have winds between 166 and 200 miles per hour, producing devastating damage. F5 tornadoes have winds in excess of 200 miles per hour, producing catastrophic damage.
  
  

Breakdown

• Approximately 69 percent of all tornadoes are classified as weak, with winds below 110 miles per hour; they typically last less than 10 minutes. Around 29 percent of tornadoes are strong, with winds between 110 to 205 miles per hour. These twisters typically last less than 20 minutes and are responsible for 25 to 30 percent of all deaths. Only around two percent of tornadoes are considered violent, with winds over 205 miles per hour. However, these rare twisters can last as long as an hour and are responsible for 70 percent of all tornado deaths.
  
  

Activity

• In the southern United States, the peak of tornado activity occurs between March and May; however, the peak of tornado activity in the north occurs during the summer months. While tornadoes can occur at any time, they are most likely to occur between 3 p.m. and 9 p.m., when convection from daytime heating is greatest. Tornadoes typically travel to the northeast at an average speed of 30 miles per hour.