A tornado is a swirling column of wind manifesting by
a funnel cloud (tuba) which form along the base cumulonimbus
clouds and accompanied by rain and sometimes a thunderstorm
or hail. 99 % of
them spin anti-clockwise in the northern hemisphere, and
clockwise in the southern hemisphere. The tornadoes can
successive rise and fall and touch the ground from time
to time on its way.
Comparable to tornadoes, but more modest, there is also
the waterspout that form over bodies of water (lakes, oceans,
seas) under conditions of moderate instability. Their wind
speed is less than 74 mph (120 km/h), the pressure drop
is about 20 to 30 hPa, heir diameter is less than 30 m and
their shelf life between 5 and 20 minutes.
Tornadoes are characterized by :
- violent winds with above 400 km/h (248 mph) were observed.
But the wind speed inside a tornado is difficult to measure,
but estimated at 410 mph (660 km/h) ;
- a sharp drop in pressure that can reach or even exceed
100 hPa during a tornado. This corresponds to a drop of
a tenth of the surface pressure, thus becomes comparable
to those prevailing sea level in the center of hurricanes
as David (1979), Allen (1980), Hugo (1989) and Andrew (1992),
to mention only the most famous among those who affected
the Caribbean region in recent decades.
- a diameter of the tip of the funnel cloud 15 to 30
m. But the diameter of the tornado varies from 15 m to 1
- it moves from 31 to 62 mph (50 to 100 km/h) for a distance
comprised between 4 and 6 km, and during an average of 10
to 30 minutes, but some may last several hours.
of a tornado
Tornadoes form when warm air in the lower altitudes
meets cold air at high altitudes. Then a cumulonimbus
form to a very high altitude and creates a severe
thunderstorm. Most often the tornado appears at the
periphery of the storm, aspiring enough hot air to
generate the whirl that primer.
As the warm, moist winds are attracted by depression,
which generates a twisted mouvment within the thundercloud
movement, swirling faster and faster. In extreme violence,
the rotation is amplified by a powerful upward and swirling
air column in the heart of the storm giving rise to a mesocyclone
(cyclonic circulation at the scale of a storm) that is caused
by the reaction of hot and cold air streams in a given area
of the storm. The central column speed up its rotation and
reaches the ground, creating a tornado.
If the cumulonimbus
which is formed to a very high altitude is above the jet
stream, then the storm cloud begins to turn (a so-called
to see this larger figure
to see this larger figure
The figures show the evolution of
the formation of a tornado : warm, moist air rises gradually
condenses to form a cumulonimbus.
In rising, the air causes a spiral movement. Cumulonimbus
such an extent that the upper winds are systematically deflected
upward, the base and sides. Sometimes
the rotational movement causes behind it another turn, the
descending currents, this time the formation of
tornado occurs when, in a small area of hurricane
which is close to the ground, winds converge more and more.
DYNAMICS OF A TORNADO
Thunderstorms are like an air pump, which provides it
with humidity and instability
The rising air begins a swirling motion
Often hail and flashings are observed near the funnel
A video explaining the formation of
A tornado may last for several minutes or greater than
one hour. After reaching its maximum intensity, tuba shrinks
then bowed horizontal and deforms ending by die.
animation by A.Gel, WVU and G.Giras, PSC
here to see a video
animation by G. Foss, PSC
here to see this simulation video
Régnier helped me correct mistakes, please you to
Im not english speaker, some improprieties can appear
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