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Severe thunderstorms have the potential to produce damaging winds, lightning, hail, and at their worst, tornadoes. Wind gusts and sudden downdrafts within severe thunderstorms can be strong enough to cause significant wind damage. East of the Mississippi, the greatest number of thunderstorms occurs during the month of July when surface heat and atmospheric moisture is at its peak. Most of these storms develop during the late-afternoon or evening hours, when the atmosphere is most destabilized and the heat is at its maximum.

Tornadoes are funnel-shaped, rotating columns of air extending from the base of a thunderstorm to the ground (prior to making contact with the ground it is known as a funnel cloud). No wind-measuring instrument has ever survived a direct hit by a tornado, but estimates developed from scientific observation have estimated top winds to be approximately 280 mph (N. Audubon). Their powerfully concentrated winds can uproot trees, damage buildings, and turn small harmless objects into deadly projectiles.

A tornado's path of destruction may be more than one mile wide and may extend for 50 or more miles in length. Approximately 87% of all tornadoes travel a path from the southwest to the northeast, but numerous variations have been observed. The speed of their movement along the ground can range from as slow as 5 mph to as high as 73 mph. The average tornado ground speed is 35 mph. After making its first contact with the ground, the average life of a tornado is 15 minutes. This means that the average tornado, with a ground speed of 35 mph, would travel almost 9 miles before dying out (N. Audubon).

Like hurricanes, researchers use a scale, known as the Fujita-Pearson Tornado Intensity Scale to rate their strength.

Scale

Category

Windspeed

Path Length (miles)

Path Width (yd/ml)

Expected Damage

F0

Weak

0-72

0-1

0-17 yd

Light

F1

Weak

73-112

1-3.1

18-55 yd

Moderate

F2

Strong

113-157

3.2-9.9

56-175 yd

Considerable

F3

Strong

158-206

10-31

176-556 yd

Severe

F4

Violent

207-260

32-99

0.34-0.9 mi

Devestating

F5

Violent

261-308

100-315

1-3.1 mi

Catastrophic

 

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