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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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