Tornadoes
Disaster Fact Sheet
Tornadoes are violently rotating columns of air with circulation that reaches the ground. Tornadoes usually
start as a funnel cloud and are accompanied by a loud, roaring noise. Tornadoes:
• Are the most destructive of all the atmospheric phenomena with wind speeds up to 318 mph.
• Often strike with little or no advanced warning.
• Are extremely dangerous. One supercell storm can produce a series of tornadoes along the same general line.
Tornado Awareness: Tornadoes can occur at any time of the year in the United States.
• In southern states, peak tornado occurrence is March through May, while peak months in northern states are
June through August, with secondary tornado maximum occurrences in September through November.
• Tornadoes occur at all hours of the day and night but are most likely to occur between 3 and 9 p.m.
• Typically tornadoes move from southwest to northeast, but they have been known to move in any direction.
• The average forward speed of a tornado is 30 mph, but this may vary from nearly stationary to 70 mph.
Tornado Danger Signs: Tornadoes may develop from thunderstorms when warm, moist air is in advance of
eastward-moving cold fronts. Watch for skies that turn suddenly dark and greenish (a phenomenon caused by
hail) and with increasing wind speeds, and keep a particular lookout for the formation of funnel clouds.
• Large Hail: Tornadoes are spawned from powerful thunderstorms that often produce large hail.
• Calm before the Storm: Often just prior to tornado touchdowns, the winds may die down and the air may
become very still; clear, sunlit skies are not uncommon behind a tornado.
• Clouds of Debris: Approaching debris clouds can mark a tornado’s location, even if a funnel is not visible.
• Funnel Clouds: A visible rotating extension of the cloud base is a sign that a tornado may develop.
Tornadoes are evident when one or more clouds turn a greenish color and a dark funnel descends.
• Roaring Noise: High winds can cause a roaring sound often compared with a passing freight train.
Tornado Scales: The Fujita Scale (F Scale) rates the strength of tornadoes with intensity estimates through
after-the-fact examination of the damage they produced. Since February 2007, the Enhanced Fujita Scale (EF
Scale) has used knowledge of the wind forces needed to damage or destroy various buildings and their
component parts to estimate wind speeds.
Rating Wind Speed Typical Damage
F0 < 73 mph Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted
trees pushed over; sign boards damaged.
F1 73-112 mph Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or
overturned; moving autos blown off roads.
F2 113-157 mph Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars
overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted
off ground.
F3 158-206
mph
Severe damage. Roofs and some walls torn off well-constructed houses; trains
overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F4 207-260
mph
Devastating damage. Well-constructed houses leveled; structures with weak foundations
blown away some distance; cars thrown and large missiles generated.
F5 261-318 Incredible damage. Strong frame houses leveled off foundations and swept away;
automobile-sized missiles fly through the air in excess of 100 meters (109 yards); trees
debarked; incredible phenomena will occur.
