A tornado is a violently rotating column of air which is in contact with both a cumulonimbus (or, in rare cases, cumulus) cloud base and the surface of the earth. comes in many shapes, but are typically in the form of a visible condensation funnel, with the narrow end touching the earth. Often, a cloud of debris encircles the lower portion of the funnel.
Most have winds of 110 mph (175 km/h) or less, are approximately 250 feet (75 meters) across, and travel a few miles (several kilometers) before dissipating. However, some tornadoes can have winds of more than 300 mph (480 km/h), be more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 kilometers).
They have been observed on every continent except Antarctica; a significant percentage of the world's tornadoes occur in the United States. This is mostly due to the unique geography of the country, which allows the conditions which breed strong, long-lived storms to occur many times a year. Other areas which commonly experience tornadoes include Australia, south-central Canada, northwestern Europe, south-central and eastern Asia, east-central South America, and Southern Africa..
A tornado is defined by the National Weather Service (NWS) as "a violently rotating column of air in contact with the ground and extending from a thunderstorm base." A tornado does not necessarily have to be visible; however, the low pressures caused by the fast wind speeds (see Bernoulli's principle) usually cause water vapor in the air to condense into a visible condensation funnel.
A funnel cloud is a low-hanging, rotating cloud, with no associated strong winds at the surface. Funnel clouds are not tornadoes, however, many tornadoes initially descend from the parent storm as a funnel cloud. It is often difficult to tell the difference between a funnel cloud and a tornado from a distance. Many tornadoes can produce strong winds at the surface while the visible funnel is still a good distance from the ground.
Stronger tornadoes are often observed to have multiple vortices, or many columns of violently spinning air rotating around a common center. However, a satellite tornado is a term for a weak tornado which forms very near a large, strong tornado, often lasting no more than a minute. The satellite tornado may appear to "orbit" the larger tornado (hence the name), giving the appearance of one, large multi-vortex tornado. However, a satellite tornado is a distinct funnel, and is much smaller than the main funnel.
A waterspout is a tornado over water. In general, most tornadoes over land are associated with a severe thunderstorm; however, the National Weather Service in the United States considers all waterspouts—including "fair weather" waterspouts—to be tornadoes. These less severe relatives of classic tornadoes are almost always weak (F0 or F1 on the Fujita Scale), and spawn from non-rotating thunderstorms, or even regular summer showers. Typically, waterspouts moving onto land cause little or no damage, and dissipate within minutes. However, strong waterspouts from supercells can cause significant damage if they impact land areas. In addition, strong tornadoes can move over lakes or over the ocean, becoming waterspouts, without losing intensity.
A landspout is an unofficial term for a tornado not associated with a mesocyclone. Known officially as a dust-tube tornado, it is usually weak, features a small condensation funnel which often does not appear to reach the ground, and is often marked by a tall tube of dust and/or debris reaching as far up as the parent cloud. Though usually weaker than ordinary tornadoes, they are tornadoes, and can cause serious damage.
A gustnado is a small, vertical swirl associated with a gust front or downburst. Because they are technically not associated with the cloud base, there is some debate as to whether or not gustnadoes are actually tornadoes.  These usually cause localized areas of heavier damage among areas of straight-line wind damage caused by the gust front.
A dust devil is also a vertical swirling column of air. These phenomena resemble tornadoes, but are rarely as strong as even the weakest tornadoes, and tend to form under clear skies. Dust devils are not considered tornadoes because they form during fair weather, and are not associated with thunderstorms. However, they can, on occasion, result in major damage and fatalities, especially in arid areas. 
A tropical cyclone is a storm system fueled by the heat released when moist air rises and the water vapor in it condenses. The term describes the storm's origin in the tropics and its cyclonic nature, which means that its circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Tropical cyclones are distinguished from other cyclonic windstorms such as nor'easters, European windstorms, and polar lows by the heat mechanism that fuels them, which makes them "warm core" storm systems.
Depending on their location and strength, there are various terms by which tropical cyclones are known, such as hurricane, typhoon, tropical storm, cyclonic storm and tropical depression.
Tropical cyclones can produce extremely strong winds, tornadoes, torrential rain, high waves, and storm surges. The heavy rains and storm surges can produce extensive flooding. Although their effects on human populations can be devastating, tropical cyclones also can have beneficial effects by relieving drought conditions. They carry heat away from the tropics, an important mechanism of the global atmospheric circulation that maintains equilibrium in the earth's troposphere.
Factors in Formation
The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood. Six factors appear to be generally necessary, although tropical cyclones may occasionally form without meeting all of these conditions:
1. Water temperatures of at least 26.5 °C (80°F) down to a depth of at least 50 m (150 feet). Waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
2. Rapid cooling with height. This allows the release of latent heat, which is the source of energy in a tropical cyclone.
3. High humidity, especially in the lower-to-mid troposphere. When there is a great deal of moisture in the atmosphere, conditions are more favourable for disturbances to develop.
4. Low wind shear. When wind shear is high, the convection in a cyclone or disturbance will be disrupted, preventing formation of the feedback loop.
5. Distance from the equator. This allows the Coriolis force to deflect winds blowing towards the low pressure center, causing a circulation. The minimum distance is about 500 km (310 miles) or 5 degrees from the equator.
6. A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure center.
Generally, tropical cyclones generally form from four different types of systems: monsoon troughs, tropical waves, non-tropical lows, and decaying frontal boundaries. Monsoon troughs, which are broad areas of converging winds from both hemispheres, are the main trigger of tropical cyclone formation worldwide. When they strengthen, either due to strengthening high pressure poleward of the trough or by increased flow passing through the equator from the opposite hemisphere, thunderstorm activity increases and tropical cyclogenesis can occur.
Another common mechanism for tropical cyclone formation are tropical waves, also called easterly waves, which are westward-moving areas of convergent winds. These generate most of the hurricanes in the Atlantic and northeast Pacific basins. Tropical waves often carry with them clusters of thunderstorms, which can then develop into tropical cyclones. A similar phenomenon to tropical waves are West African disturbance lines, which are squalls that form over Africa and move into the Atlantic, often as a part of the Intertropical Convergence Zone. Tropical cyclones also frequently form from upper tropospheric troughs, which are cold-core upper-level lows. A warm-core tropical cyclone may result when one of these works down to the lower levels of the atmosphere and produces deep convection. Off-season tropical cyclones most often form in this manner. Finally, decaying frontal boundaries may occasionally stall over warm waters and produce lines of active convection. If a low-level circulation forms under this convection, it may develop into a tropical cyclone.
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