Cloud
A tornado is a violently spinning column of air in contact with both a cumiliform cloud base and the surface. A tornado is typically shaped like a funnel with the narrow end on the ground. Tornadoes are known for being extremely destructive and are usually visible due to water vapor from low pressure condensation and debris from the ground. Tornadoes form in storms all over the world, and though they have been recorded in all fifty U.S. states, they form most famously in a broad area of the American Great Plains, Midwest, as well as South known colloquially as Tornado Alley. In pure number of incidences, the United States reports more tornadoes than any other country, however, the Netherlands is the most tornado-prone country relative to land area.
Terminology
The word "tornado" comes from the Spanish word for "turned", which in turn comes from the Latin word torqueo, meaning "to twist." Some common, related slang terms include: twister, whirlwind, wedge, funnel, willy-willy, or rope. However, willy-willy usually refers to a dust devil in Australia.
Cyclone is also another term for a tornado, although it must be noted that in parts of the world (notably Australia) a cyclone refers to what is more correctly known as a tropical cyclone (also known as a hurricane, or a typhoon), and meteorologists use the term cyclone to refer to a wide range of circular weather systems (using adjectives to disambiguate).
In general tornadoes are associated with a thunderstorm; however, National Weather Service in the United States considers all waterspouts—including "fair weather" waterspouts—to be tornadoes. Waterspouts commonly form from rapidly growing cumulus clouds that have not become thunderstorms. They grow by stretching an already existing vortex. USA Today The Weather Book helps in defining a "waterspout" as a tornado-like rotating column of air under a cumuliform cloud occurring over water;they are most common over tropical and subtropical waters and dissipate upon reaching shore/land. Some waterspouts, however, are thunderstorm-spawned, identical to other tornados except for their occurrence over water.
Larger vortexes not associated with a thunderstorm over land are sometimes called landspouts.
Dust devils are small vortices that form near the ground, which may or may not be considered tornadoes.
Characteristics
Tornadoes normally rotate in a cyclonic (counterclockwise) direction in the northern hemisphere, as the warm air in which thunderstorms usually form sweeps north and jet streams come from the west, creating a situation in which the storms rotate. In the northern hemisphere, this rotation is counterclockwise, and in the southern hemisphere, clockwise. The tornadoes usually rotate the same way. Sometimes opposite direction swirls develop under a thunderstorm. About 1 in 100 tornadoes in the northern hemisphere rotate in an anticyclonic direction.
The most intense tornadoes can develop multiple vortices, all pinwheeling around the same axis. The additive wind velocity of the vortices' on the outer side of the pinwheel greatly increases the destructive power of this type of tornado.
Tornado funnels can range in width from a few feet to as much as one mile across. The latter can be particularly dangerous in that it may be mistaken for an area of rain or lowering of clouds.
No two tornadoes look exactly alike, nor have any two tornadoes behaved in exactly the same way. There are true incidents of tornadoes repeatedly hitting the same town several years in a row; however, forecasting the exact position a tornado will strike at a certain time is presently impossible.
Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. While tornadoes are invisible at night, some nocturnal tornadoes have been observed glowing diffusely due to lightning activity. Verified observations by Hall and others suggest a cellular structure inside tornadoes. A tornado must by definition have both ground and cloud contact. Thus, the exclamation "Tornado on the ground!" is redundant.
Not every thunderstorm, supercell, squall line, or hurricane will produce a tornado. Luckily, it takes exactly the right combination of atmospheric variables (wind, temperature, pressure, humidity, etc.) to spawn even a weak tornado. On the other hand, roughly 1,200 tornadoes a year are reported in the contiguous United States.
There are two general types of tornadoes, which are the ones that form in the outflow of air from a thunderstorm. These have a shallow, and very localized vortex and are generally weak storms that rarely exceed an F-2 on the fujita scale (about 157 mph). Waterspouts can be associated with this first kind of tornado.
The next type of tornado is the strongest and most dangerous. These are storms found where the surface air is flowing into the updraft area of a supercell. Tornadoes begin at the storms middle levels and grow up into the storm and down towards the ground. These types generally form the categories F-4 and F-5 tornadoes which range from 207 mph to above 261mph.
Intensity
In the United States (and predominately worldwide), the intensity of a tornado is measured on the Fujita-Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural and vegetative damage indicators compared to engineering data, as well as ground swirl patterns or photogrammetry / videogrammetry. Note that intensity does not refer in any way to the size, or width, of a tornado. The scale ranges from F0 for the weakest to F5 for the most powerful tornadoes. The Fujita scale is effectively a damage scale, wind speeds are estimates and have never been confirmed or fully tested, and there is no upper bound wind speed in the Enhanced Fujita Scale which has replaced the original Fujita scale.
The TORRO scale, developed in the United Kingdom by the Tornado and Storm Research Organisation (TORRO) and used primarily in the U.K., covers a broader range with tighter graduations, and TORRO says it is based solely on wind speed, though in practice damage analysis is used to infer wind speeds. It ranges from a T0 to T11 for the most powerful known tornado in the United States.
Of all tornadoes formed in the U.S., F0 and F1 tornadoes account for a large percentage of occurrences. On the other end of the scale, the massively destructive F5s account for approximately 0.1% of all
Frequency of occurrence
The United States experiences by far the most tornadoes of any country, and has also suffered the most intense ones. Tornadoes are common in most states in spring and summer, especially those east of the Rocky Mountains. There is a secondary peak in autumn, especially in the southeastern U.S. where tornadoes also occur more frequently during winter there than in other areas. The El Nino phenomenom is known to suppress the frequency of tornadoes.
Tornadoes can occur in the West as well, although they are usually very small and relatively weak. Recently tornadoes have struck the Pacific coast town of Lincoln City, Oregon, in 1996 and downtown Salt Lake City, Utah, in 1999 (see Salt Lake City Tornado). The California Central Valley is an area of some frequency for tornados, albeit of weak intensity. More tornadoes occur in Texas than in any other US state. The state which has the highest number of tornadoes per unit area is Florida, although most of the tornadoes in Florida are weak tornadoes of F0 or F1 intensity. A number of Florida's tornadoes occur along the edge of Hurricanes. The state with the highest number of stronger tornadoes per unit area is Oklahoma. The neighbouring state of Kansas is another particularly notorious tornado state. It should be mentioned that states such as Oklahoma and Kansas have much lower population densities than Florida and that tornadoes may go unreported.
On average, the United States experiences 100,000 thunderstorms each year, resulting in more than 1,200 tornadoes and approximately 50 deaths per year. The deadliest U.S. tornado on record is the March 18, 1925, Tri-State Tornado that went across southeastern Missouri, southern Illinois and southern Indiana, killing 695 people. More than six tornadoes in one day is considered a tornado outbreak. The biggest tornado outbreak on record—with 148 tornadoes, including six F5 and 24 F4 tornadoes—occurred on April 3, 1974. It is dubbed the Super Outbreak. Another such significant storm system was the Palm Sunday Tornado Outbreak, which affected the United States Midwest on April 11, 1965. A series of continuous tornado outbreaks is known as a tornado outbreak sequence, with significant occurrences in May 1917, 1930, 1949, and 2003.
Canada also experiences numerous tornadoes, although fewer than the United States. In Canada, an average of 80 tornadoes occurs annually, killing 2, injuring 20 and causing tens of millions of dollars in damage. The last killer tornado in Canada struck Pine Lake, Alberta, on July 14, 2000, killing 11.
Tornadoes do occur throughout the world as well; the most tornado-prone region of the world (outside North America), as measured by number of reported tornadoes per unit area, is the Netherlands, followed by the United Kingdom (especially England). Bangladesh, India, Argentina, Italy, Australia, New Zealand, Germany, Estonia, and portions of Uruguay also have pockets of high tornadic activity. Occasional strong tornadoes occur in Russia, France, Spain, Japan, and portions of Paraguay and Brazil. Tornadoes have recently hit South Africa and parts of Pakistan in 2001 as well. Approximately 170 tornadoes are reported per year on land in Europe. Perhaps the most notorious tornado of recent years was that which struck Birmingham , England in July 2005 which destroyed a row of houses though - amazingly - without fatalities.
Cloud color tells much about what is going on inside a cloud.
Clouds form when water vapor rises, cools, and condenses out of the air as micro-droplets. These tiny particles of water are relatively dense, and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color. As a cloud matures, the droplets may combine to produce larger droplets, which may themselves combine to form droplets large enough to fall as rain. In this process of accumulation, the space between droplets becomes larger and larger, permitting light to penetrate much farther into the cloud. If the cloud is sufficiently large, and the droplets within are spaced far enough apart, it may be that very little light which enters the cloud is able to be reflected back out before it is absorbed. (Think of how much farther one can see in a heavy rain as opposed to how far one can see in a heavy fog.) This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.
Other colors occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-sized droplets in the cloud.
A more ominous color is the one seen frequently by severe weather observers. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud which shows green is a pretty sure sign of imminent heavy rain, hail, strong winds, and possibly tornados.
Yellowish clouds are rare, but may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of smoke.
Red, orange, and pink clouds occur almost entirely at sunrise/sunset, and are the result of the scattering of sunlight by the atmosphere itself. The clouds themselves are not that color, they are merely reflecting the long (and unscattered) rays of sunlight which are predominant at those hours. The effect is much the same as if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds. The evening before the Edmonton, Alberta tornado in 1987, Edmontonians observed such clouds - deep black on their dark side, and intense red on their sunward side. In this case, the adage "red sky at night, sailor's delight" was clearly incorrect.
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