Tornado Science

This year’s tornado season could be one of the most active on record. Nearly 1,200 tornadoes have hit the U.S. this year, according to preliminary numbers from the National Oceanic and Atmospheric Administration. Four of these storms have been rated at the highest tornado strength, an EF-5.

A tornado is a violently rotating column of air extending from a thunderstorm to the ground. Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height create an invisible horizontal spinning effect in the lower atmosphere. Thunderstorms develop in warm, moist air, preceding an eastward-moving cold front.

The rising air within the thunderstorm tilts the rotating air from horizontal to vertical. An area of rotation, two to six miles wide, extends through a great deal of the storm. Warm moist air shoots upward, meeting colder, dryer air, and makes a strong updraft within the thunderstorm. As warm moist air rises, it may meet varying wind directions at different altitudes.

If these varying winds are staggered in just the right manner, with sufficient speed, they will act on the upward rising air, spinning it like a top. The storm will begin to show visible rotation, often forming a wall cloud--a large, lowering, and rotating base cloud. Inside the storm, these spinning winds can begin the formation of a tornado. On the outside, the thunderstorm rotation might be visible as well. Only about 30% of these storms form tornadoes. Most information about the inside of the thunderstorm comes from data collected by Doppler radar.

Tornadoes were originally ranked on the Fujita (F) Scale, which was developed by Dr. Tetsuya Theodore Fujita to estimate tornado wind speeds based on the damage left behind by a tornado. It was used from 1971 to 2007. An Enhanced Fujita (EF) Scale, developed by a group of meteorologists and wind engineers, makes improvements to the original F scale. The EF scale has been approved by the National Weather Service (NWS).

The original F scale had limitations, such as a lack of damage indicators, no account for construction quality and variability, and no definitive correlation between damage and wind speed. These limitations may have led to some tornadoes being rated in an inconsistent manner and an overestimate of tornado wind speeds.

The EF Scale rates tornado categories from zero to five, but the ranges of wind speed in each category are now more accurate. It takes into account more variables than the original F Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are eight degrees of damage (DOD), ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F Scale did not take these details into account.

The following ratings are given based on wind speeds: EF-0, 65-85 mph; EF-1, 86-110 mph; EF-2, 110-135 mph; EF-3, 136-165 mph; EF-4, 166-200 mph; and EF-5, greater than 200 mph.