A tornado, also sometimes called a twister or cyclone, is basically a rapidly spinning column of air which has descended from a thunderstorm (cumulonimbus) cloud to connect with the ground. Before it connects, it is called a funnel cloud. Many funnel clouds never become tornadoes.
Every tornado requires three basic elements to come into existence:
* warm, humid surface air
* cold air aloft, usually (but not always) resulting from an approaching sharp cold front
* wind shear: the wind in the upper levels of the cloud must be at right angles to the wind at the surface
The collision of warm surface air and cold upper air will create atmospheric instability, resulting in sharp updrafts and downdrafts which will slowly help build a thunderstorm. However, it is the wind shear which folds the updrafts into a spinning horizontal column of air (mesocyclone), which is then forced slowly vertical and downward in a rear flank downdraft.
The classical tornado is the supercellular tornado. The supercell cumulonimbus cloud will quickly build a high updraft tower, which will often look sheared off at the top in an anvil shape. The front of the cloud may overshoot its base. Its bottom will be very dark, often of a dark yellow or greenish colour, although from a distance the high towers may look bright with distant sunlight. From the ground (and at a comfortable distance), the gradually building supercell thunderstorm can be seen to have a leading edge of rain, while at the back, under the updraft, will be a relatively or completely rain-free base. This base may have a distinctive dimpled, or 'mammary', appearance, or it may sag downward as a distinctive wall cloud which may or may not show visible rotation. Either cloud formation indicates that the storm is capable of producing a tornado. Not all wall clouds produce tornadoes, but roughly half of all tornadoes descend from a previous wall cloud.
As the mesocyclone descends, it may or may not become visible as a white condensation funnel (or black, if the sun is behind it). The invisible rear flank downdraft will descend along with it and will often reach the ground before the funnel itself; and is itself capable of causing significant wind damage a large distance away from the funnel.
The spinning column becomes a tornado the moment the mesocyclone makes contact with the earth. Usually at this point the tornado finally becomes visible due to the upflung dust cloud, regardless of whether or not it was previously visible. The funnel may be only a few metres wide or it may extend over two kilometres; it may be columnar, wedge-like, or ropy. The larger funnels will often contain smaller internal funnels which rotate around each other and which may have a higher wind speed than the main funnel area. Other tornado variants are multiple vortex tornadoes, or 'sisters', where two or three vortices of similar size rotate around a common centre, and satellite tornadoes, where a second funnel descends from the same cloud base as the primary funnel and may rotate around it. Some waterspouts are true tornadoes which form or have moved over water; but most waterspouts and landspouts are not true tornadoes, as they have no association with a mesocyclone.
The tornado will often have a loud, distinct sound that is variously described as a freight train or a buzzing of hornets. Lightning has also been reported inside tornado funnels. In addition to the direct action of the spinning winds, a strong suction effect inside the funnel will also result from the extremely low pressure created by the spinning winds.
A single tornado may only briefly touch the ground before dissipating, or it may travel for long distances, most commonly in a northeast direction. It may last for seconds or for more than an hour. Some tornadoes repeatedly lose and regain contact with the earth, and may even seem to dissipate altogether before unexpectedly reforming again. Eventually the rear flank downdraft will begin to cut off the tornado's source of warm surface air, at which point the tornado will usually become ropy and eventually dissipate.
Wind speeds will range from 62 kph (F0 on the Fujita scale) to 512 kph (F5). F6 tornadoes with wind speeds greater than 512 kph have been postulated, but none have yet been confirmed. A tornado's wind speed is determined after the fact, based on the pattern and intensity of damage it has caused. For this reason, it can sometimes be very difficult to determine the true wind speed of a plains tornado which has not made contact with any manmade structure. In the United States, the Fujita scale has been replaced with the enhanced Fujita scale (EF0-5), which sets no upper limit on the EF5 and is believed to more accurately estimate wind speed. Another scale sometimes used to measure tornado wind speed is the TORRO scale, an extension of the Beaufort scale for sea winds. It is not in common use.