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
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.