Tornadoes, also called twisters or whirlwinds, are rapidly rotating columns of air that are in contact with both the surface of the Earth and a cumulonimbus cloud. The diameter of tornadoes varies between a few meters and several kilometers. Even though most tornadoes exist for a short period of time, they can be extremely powerful and can cause a great deal of damage.
Areas at risk
warm air - Warm and humid air masses flow close to the surface from the South.
cold air - A jet stream at high altitude brings cold, dry air masses from the West.
Australia and Oceania
Tornadoes can form on every continentexcept Antarctica. Most tornadoes, however, occur in the Great Plains in the United States because of the geographical features of the region. Air masses with different properties clash in this region: cold, dry air coming from the west collides with the warm, humid air coming from the south, the Gulf of Mexico.
The majority of tornadoes form from rotating thunderstorms called supercells, which, in turn, may form from cumulonimbus clouds in the presence of powerful convective updrafts.
The difference in wind speed and direction at different altitudes, a phenomenon called wind shear, creates a horizontal vortex, parallel to the ground. When the convective updraft of the thunderstorm draws up a section of the horizontal vortex tube, a "loop" is formed and the updraft splits into a pair of counter-rotating vortices.
It is usually the vortex rotating cyclonically, that is, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, that eventually becomes stronger, while the other one dies. This cyclonically rotating updraft is called a mesocyclone. When this forms, the cumulonimbus reaches the supercell state.
updraft - Air on the outside of the tornado moves upwards.
downdraft - At the center, air moves towards the ground, replacing air drawn upwards.
Tornadoes may form when the mesocyclone receives a supply of warm and humid air from the surface. At the same time, a downdraft is present in the center of the mesocyclone.
As the vertical vortex stretches towards the ground, the spinning air is drawn inward towards the axis of rotation and therefore it will spin faster. If it becomes fast enough, a funnel emerges from the supercell. If the funnel reaches the ground, it is called a tornado.
The tornado sucks in dust and debris from the ground, making the lower part of the funnel darker.
In most cases, the intensity of the tornado, as well as its diameter, quickly decreases. Eventually, the tornado disappears completely.
EF0 - Potential damage: some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over.
EF1 - Potential damage: major roof damage; broken windows; some damage to doors; mobile homes overturned.
EF2 - Potential damage: complete roof structures torn off buildings; mobile homes demolished; large trees snapped or uprooted; small objects become dangerous missiles.
EF3 - Potential damage: entire stories blown off high buildings; trains overturned; most trees uprooted.
EF4 - Potential damage: well-constructed houses are destroyed; structures with weak foundations blown away; cars thrown up in the air.
EF5 - Potential damage: multi-story, reinforced concrete buildings collapse, cars become missiles as they are hurled through the air.
105-137 km/h (65-85 mph)
138-178 km/h (86-110 mph)
179-218 km/h (111-135 mph)
219-266 km/h (136-165 mph)
267-321 km/h (166-199 mph)
˃ 322 km/h (200 mph)
Most tornadoes last for only a few minutes. However, they can still cause significant damage. Three-quarters of all tornadoes do not reach a speed of 180 km/h (112 mph). At this speed, they only cause moderate damage. Nevertheless, tornadoes attaining several times larger speeds can cause massive destruction, such as destroying buildings.
The Enhanced Fujita Scale, a modified version of the Fujita Scale that was created by a Japanese-born meteorologist, is used to characterize tornadoes. It consists of six categories based on tornado intensity and the inflicted damage.