About how many tornadoes are recorded in the United States annually?

A typical tornado season sees about 800 tornadoes in the United States. However, better observational techniques, including advanced Doppler radar and growing numbers of tornado chasers, has led to increases in tornado observations since around 1990, so this official average might go up.

What is the Fujita and Pearson Tornado Scale?

The Fujita and Pearson Tornado Scale—usually just referred to as the Fujita Scale—was introduced in 1971 by University of Chicago professor T. Theodore Fujita (1920-1998) and Allen Pearson (1925-), who was then the director of the National Severe Storms Forecast Center. The scale ranked tornadoes by their wind speed, path, length, and width. The ranking ranges from F0 (very weak) to F5 (incredibly destructive). This scale was replaced in 2007 by the Enhanced Fujita Scale.

Fujita and Pearson Tornado Scale

Scale

Speed (mph/kph)

Damages

F0

40-72/64-116

Light damage: damage to trees, billboards, and chimneys

F1

73-112/117-180

Moderate damage: mobile homes pushed off their

foundations and cars pushed off roads

F2

113-157/181-253

Considerable damage: roofs torn off, mobile homes

demolished, and large trees uprooted

F3

158-206/254-331

Severe damage: even well-constructed homes torn apart,

trees uprooted, and cars lifted off the ground

F4

207-260/332-418

Devastating damage: houses leveled, cars thrown, and objects become flying missiles

F5

261-318/419-512

Incredible damage: structures lifted off foundations and carried away; cars become missiles. Less than 2% of tornadoes are in this category

F6

319-380/513-611

No F6 has been recorded, but if such a twister occurred it would be absolutely devastating

What is the Enhanced Fujita Scale?

Proposed by the National Weather Service in February 2006, and first put into use on February 1, 2007, the Enhanced Fujita Scale (EFS) was created to better reflect actual damages recorded since the original Fujita Scale was developed. Meteorologists have recently concluded that structures could be damaged by tornadic winds that were slower than previously thought. The original scale, which was felt to be too general, did not take into careful enough account the different types of construction, and it was hard to evaluate tornadoes that struck in low-populated areas where not many structures were present. The new scale also offers more detailed descriptions of potential damages by using 28 Damage Indicators that describe building types, structures, and vegetation, accompanied by a Degrees of Damage scale. Otherwise, the EFS uses the same categories, ranking tornadoes from 0 up to 5.

Enhanced Fujita Scale

Scale

Wind Speed (mph/kph)

Damages

EF0

65-85/105-137

Tree branches break off, trees with shallow roots fall over; house siding and gutters damaged; some roof shingles peel off or other minor roof damage.

EF1

86-110/137-177

Mobile homes overturned; doors, windows, and glass broken; severe damage to roofs.

EF2

111-135/178-217

Large tree trunks split and big trees fall over; mobile homes destroyed, and homes on foundations are shifted; cars lifted off the ground; roofs torn off; some lighter objects thrown at the speed of missiles.

EF3

136-165/218-265

Trees broken and debarked; mobile homes completely destroyed and houses on foundations lose stories, and buildings with weaker foundations are lifted and blown distances; commercial buildings such as shopping malls are severely damaged; heavy cars are thrown and trains are tipped over.

EF4

166-200/266-322

Frame houses leveled; cars thrown long distances; larger objects become dangerous projectiles.

EF5

>200/>323

Homes are completely destroyed and even steel-reinforced buildings are severely damaged; objects the size of cars are thrown distances of 300 feet (90 meters) or more. Total devastation.

Are there any other tornado strength scales in use?

Yes. In Great Britain meteorologists use the TORRO (the TORnado and storm Research Organisation) Scale, which is divided into T0 through T10 measurements of strength. A T0 tornado has winds beginning at 40 miles (64 kilometers) per hour, and a T10 is a "Super Tornado" with wind speeds of 270 to 299 miles (435 to 480 kilometers) per hour.

 
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