Tornado Protection: Selecting Refuge Areas in Buildings 

Chapter 1: Tornado Profile

The National Weather Service defines a tornado as a violently rotating column of air 
pendant from a thunderstorm cloud that touches the ground. 
From a local perspective, a tornado is the most destructive of all atmospheric-
generated phenomena. In an average year, a little more than 800 tornadoes hit various 
parts of the United States, though the number has varied from 500 
Determining Tornado Risk 
Detailed guidance for determining the magnitude of the tornado risk in a specific area 
of the United States is presented in FEMA publication 361, Design and Construction 
Guidance for Community Shelters (for more information, see the section of this booklet 
titled Information Sources).


Tornado Characteristics

The time of day when tornadoes are most likely to occur is the mid-after-noon, 
between 3:00 p.m. and 6:00 p.m. (Figure 1-3). Occasionally, severe tornadoes have 
been recorded in the early morning or late evening. 
The direction of movement is predominantly from the southwest to the northeast. 
However, tornadoes have been known to move in any direction along with the parent 
thunderstorms. 
The length of path averages 5 miles, but some tornado paths have exceeded 100 miles. 
The width of path averages 300 to 400 yards, but may reach up to 1 mile. 
The travel speed (translational) averages 25 to 40 miles per hour (mph), Figure 1-3 
Tornado occurrence by time of day. but speeds from 5 to 60 mph have been recorded. 
Rotation is generally counterclockwise in the northern hemisphere (Figure 1-6). About 
10 percent of tornadoes have been known to rotate clockwise. 
Wind speed is the sum of rotational speed and translational speed. The rotational speed 
decreases as the distance from the center of a tornado increases. With a 
counterclockwise rotation, the wind speed on the right side of the tornado is higher 
because the translational speed adds to the rotational speed. 
Because of the unpredictability of tornado paths and the destruction of commonly used 
instruments, direct measurements of wind speeds have not been made in tornadoes. 
Rather, wind speeds are judged from the intensity of damage to buildings. Engineering 
assessment of damage puts the maximum wind speed at 200 mph in most destructive 
tornadoes, and the speed is not likely to exceed 250 mph near ground level. 
Typical tornado rotation.