A tornado is a violent rotating column of air extending from a thunderstorm
to the ground
Most tornadoes form from thunderstorms.
You need warm, moist air from one direction and cool, dry air from another
direction. When these two air masses meet, they create instability in the
Next, a change in wind direction and an increase in wind speed with
increasing height creates an invisible, horizontal spinning effect in the lower
Rising air tilts the rotating air from horizontal to vertical.
When talking about tornadoes in the US (where the majority of tornadoes
occur), the warm air comes from the Gulf of Mexico and the cool, dry air
comes from Canada.
1) The rotating cloud base lowers
2) This lowering cloud becomes a
funnel, which continues
descending while winds build
near the surface, kicking up dust
and other debris.
3) Finally, the visible funnel extends
to the ground, and the tornado
begins causing major damage.
4) This tornado, near Dimmitt,
Texas, was one of the best-
observed violent tornadoes in
Tornadoes can happen at any time of the year
and at any time of the day.
In the southern US, peak tornado season is from
March through May, when there are strong winds
and atmospheric instability.
Peak times for tornadoes in the northern states
are during the summer.
Tornadoes are least common in the winter
Tornadoes are most likely to occur between 3
p.m. and 9 p.m.
The Great Plains (the central US): has all the
necessary ingredients for tornadoes.
More than 500 tornadoes typically occur in this
“Tornado Valley” every year
Other areas of the world that have frequent
tornadoes include South Africa, parts of Argentina,
Paraguay, and southern Brazil, as well as portions
of Europe, Australia and New Zealand, and far
There are several different scales for rating the strength
The Fujita Scale rates tornadoes by damage caused
An F0 or EF0 tornado, the weakest category, damages
trees, but not substantial structures. An F5 or EF5
tornado, the strongest category, rips buildings off their
foundations and can deform large skyscrapers.
The similar Torro Scale ranges from a T0 for
extremely weak tornadoes to T11 for the most powerful
SCALE WIND SPEED POSSIBLE DAMAGE
F0 40-72 mph Light damage: Branches broken off trees; minor roof damage
F1 73-112 mph
Moderate damage: Trees snapped; mobile home pushed off
foundations; roofs damaged
F2 113-157 mph
Considerable damage: Mobile homes demolished; trees
uprooted; strong built homes unroofed
F3 158-206 mph
Severe damage: Trains overturned; cars lifted off the
ground; strong built homes have outside walls blown away
F4 207-260 mph
Devastating damage: Houses leveled leaving piles of debris;
cars thrown 300 yards or more in the air
F5 261-318 mph
Incredible damage: Strongly built homes completely blown
away; automobile-sized missiles generated
over 200 mph
Attempts to warn of tornadoes began in the United
States in the mid-20th century.
Before the 1950s, the only method of detecting a
tornado was by someone seeing it on the ground. Often,
news of a tornado would reach a local weather office
after the storm.
Today, we use weather radar to detect tornadoes.
In the United States and a few other countries, Doppler
weather radar stations are used. These devices measure
the velocity and radial direction of the winds in a storm,
and so can spot evidence of rotation in storms from more
than a hundred miles (160 km) away.