VALCANOS

Most volcanos occur at the edge of tectonic plates, where magma can rise to the surface. A volcano’s shape depends mainly on the viscosity of the lava, the shape of the vent, the amount of ash, and the frequency and size of the eruptions. In fissure through a crack, forming lava plateaux or plains. In cone-shaped volcanoes, the more viscous the lava, the steeper the cone. Some cones fall in on themselves or are exploded outwards, forming calderas.

Cross-section through a stratovolcano (vertical scale is exaggerated):
1. Large magma chamber 2. Bedrock 3. Conduit (pipe) 4. Base 5. Sill 6. Dike 7. Layers of ash emitted by the volcano 8. Flank	9. Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud

Conical mount Funji in japan, at sunrise from
lake Kawaguchi (2005)

Mayon, near-perfect stratovolcano in the
Philipines

Pinatubo ash plume reaching a height of 19km, 3 days
before the climate eruption of 15 june  1991

VERTEBRA

Divisions of
spinal segments

There are three major types of vertebra in the human spine: limbar, thoracic, and cervical. Lumbar vertebrae support a major part of the body’s weight and so are cmparatively large and strong. Cervical vertebrae support the head and neck.

IN HUMANS

There are normally thirty-three (33) vertebrae in humans, including the five that are fused to form the sacrum (the others are separated by intervertebral discs) and the fourcoccygeal bones that form the tailbone. The upper three regions comprise the remaining 24, and are grouped under the names cervical (7 vertebrae), thoracic (12 vertebrae) andlumbar (5 vertebrae), according to the regions they occupy. This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another. The number of cervical vertebrae is, however, very rarely increased or diminished.

Oblique view of cervical vertebrae

With the exception of the first and second cervical, the true or movable vertebrae (the upper three regions) present certain common characteristics that are best studied by examining one from the middle of the thoracic region.

URANUS

With a diameter of about 51,000 km, Uranus is the third largest planet in the solar system. It is a cold gas giant, believed to consist of a mixture of gas and ice around a solid core. Uranus is tilted at 97.9° and so rolls on its side along its orbital path. It has 15 moons, 10 rings made up of boulders, and a broad ring of dust.

Uranus presented a nearly featureless disk to Voyager 

Discovery 

Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights.

Sir William Herschel observed the planet on March 13, 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset, England (now the Herschel Museum of Astronomy), but initially reported it (on April 26, 1781) as a "comet".Herschel "engaged in a series of observations on the parallax of the fixed stars", using a telescope of his own design.

He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet". On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place". When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:

"The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed."

Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".

While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell was the first to compute the orbit of the new object and its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".^] Bode concluded that its near-circular orbit was more like a planet than a comet.

The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System." In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so that the Royal Family could have a chance to look through his telescopes.

ORBIT AND ROTATION

Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that on Earth. Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace. With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.

The rotational period of the interior of Uranus is 17 hours, 14 minutes. As on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. At some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

AXIAL TILT

Uranus has an axial tilt of 97.77 degrees, so its axis of rotation is approximately parallel with the plane of the Solar System. This gives it seasonal changes completely unlike those of the other major planets. Other planets can be visualized to rotate like tilted spinning tops on the plane of the Solar System, while Uranus rotates more like a tilted rolling ball. Near the time of Uraniansolstices, one pole faces the Sun continuously while the other pole faces away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon as in the Earth's polar regions. At the other side of Uranus's orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness. Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7, 2007.

Northern hemisphere	Year	Southern hemisphere
Winter solstice	1902, 1986	Summer solstice
Vernal equinox	1923, 2007	Autumnal equinox
Summer solstice	1944, 2028	Winter solstice
Autumnal equinox	1965, 2049	Vernal equinox

One result of this axis orientation is that, on average during the year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism which causes this is unknown. The reason for Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing the skewed orientation. Uranus's south pole was pointed almost directly at the Sun at the time of Voyager 2's flyby in 1986. The labeling of this pole as "south" uses the definition currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite shall be the pole which points above the invariable plane of the Solar System, regardless of the direction the planet is spinning. A different convention is sometimes used, in which a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation. In terms of this latter coordinate system it was Uranus's north pole which was in sunlight in 1986.

Visibility

From 1995 to 2006, Uranus's apparent magnitude fluctuated between +5.6 and +5.9, placing it just within the limit of naked eye visibility at +6.5. Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter. At opposition, Uranus is visible to the naked eye in dark skies, and becomes an easy target even in urban conditions with binoculars. In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25 cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania andOberon, may be visible.

INTERNAL STRUCTURE

Uranus's mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times Earth's. A resulting density of 1.27 g/cm³ makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, as different figures emerge depending on the model chosen; it must be between 9.3 and 13.5 Earth masses. Hydrogen and heliumconstitute only a small part of the total, with between 0.5 and 1.5 Earth masses.The remainder of the non-ice mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.

                                                                                     Diagram of the interior of Uranus

          The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron-nickel) core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20% of Uranus's; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus's radius. Uranus's core densityis around 9 g/cm³, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K. The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants. There may be a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions move freely within the oxygen lattice.

While the model considered above is reasonably standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow science to determine which model is correct. The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers. For the sake of convenience, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar (100 kPa) is conditionally designated as a "surface". It has equatorial and polar radii of 25 559 ± 4 and 24 973 ± 20 km, respectively. This surface will be used throughout this article as a zero point for altitudes.

Internal heat

Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun. Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in itsatmosphere. In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m². The lowest temperature recorded in Uranus's tropopause is 49 K (–224 °C), making Uranus the coldest planet in the Solar System.

One of the hypotheses for this discrepancy suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.

PLANETARY RING

Uranus has a complicated planetary ring system, which was the second such system to be discovered in the Solar System after Saturn's. The rings are composed of extremely dark particles, which vary in size from micrometers to a fraction of a meter. Thirteen distinct rings are presently known, the brightest being the ε ring. All except two rings of Uranus are extremely narrow—they are usually a few kilometres wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as result of those impacts only few particles survived in a limited number of stable zones corresponding to present rings.

The uranian ring system

William Herschel described a possible ring around Uranus in 1789. This sighting is generally considered doubtful, as the rings are quite faint, and in the two following centuries none were noted by other observers. Still, Herschel made an accurate description of the epsilon ring's size, its angle relative to the Earth, its red color, and its apparent changes as Uranus traveled around the Sun. The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. When their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet. Later they detected four additional rings. The rings were directly imaged when Voyager 2 passed Uranus in 1986.Voyager 2 also discovered two additional faint rings bringing the total number to eleven.

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet of the previously known rings. These new rings are so far from the planet that they are called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13. In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other red. One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light. In contrast, the planet's inner rings appear grey.

MAGNETIC FIELD

Before the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.

Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation. In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius. This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high as 1.1 gauss (110 µT). The average field at the surface is 0.23 gauss (23 µT). In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator. The dipole moment of Uranus is 50 times that of Earth. Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants. One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.

The magnetic field of uranus as observed by voyager 2 in
1986. S and N are magnetic south and north.

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts. Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's. Uranus's magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.

Uranus's magnetosphere contains charged particles: protons and electrons with small amount of H₂⁺ ions. No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona. The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively. The density of low energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm⁻³. The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps. The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years. This may be the cause of the uniformly dark colouration of the moons and rings. Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles. Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere.

MOONS

Uranus has 27 knownnatural satellites.The names for these satellites are chosen from characters from the works of Shakespeareand Alexander Pope. The five main satellites areMiranda, Ariel, Umbriel,Titania and Oberon.The Uranian satellite system is the least massive among the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton alone. The largest of the satellites, Titania, has a radius of only 788.9 km, or less than half that of the Moon, but slightly more than Rhea, the second largest moon of Saturn, making Titania the eighth largest moon in the Solar System. The moons have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light). The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.

The uranus system(NACO/VLT image)

Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest. Miranda possesses fault canyons 20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features. Miranda's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a formerly present 3:1 orbital resonance with Umbriel. Extensional processes associated with upwelling diapirs are the likely origin of the moon's 'racetrack'-like coronae. Similarly, Ariel is believed to have once been held in a 4:1 resonance with Titania.

major moons of uranus in order of increasing distance (left to right), at
their proper relative sizes and albedos( collage of voyager 2 photographer) 

EXPLORATION

Crescent uranus as imaged by voyager 2 while
departing for neptune 

In 1986, NASA's Voyager 2 interplanetary probe encountered Uranus. This flyby remains the only investigation of the planet carried out from a short distance, and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers of the planet's cloudtops, before continuing its journey to Neptune. Voyager 2 studied the structure and chemical composition of Uranus's atmosphere, including the planet's unique weather, caused by its axial tilt of 97.77°. It made the first detailed investigations of its five largest moons, and discovered 10 new moons. It examined all nine of the system's known rings, discovering two new ones. It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrewmagnetotail caused by Uranus's sideways orientation.

A Uranus orbiter and probe has been recommended by NASA's decadal survey; the proposal envisages launch during 2020–2023 and a 13-year cruise to Uranus

TORQUE

Torque is a force that makes an object rotate. A nut turns because of the torque applied by a spanner. The size of the torque is found by multiplying the applied force by its distance from the axis, so that a longer spanner exerts greater torque. 

Torque, moment or moment of force (see the terminology below), is the tendency of a forceto rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.

Loosely speaking, torque is a measure of the turning force on an object such as a bolt or aflywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

The symbol for torque is typically τ, the Greek letter tau. When it is called moment, it is commonly denoted M.

The magnitude of torque depends on three quantities: the force applied, the length of the lever arm connecting the axis to the point of force application, and the angle between the force vector and the lever arm. In symbols:

where

τ is the torque vector and τ is the magnitude of the torque,

r is the displacement vector (a vector from the point from which torque is measured to the point where force is applied), and r is the length (or magnitude) of the lever arm vector,

F is the force vector, and F is the magnitude of the force,

× denotes the cross product,

θ is the angle between the force vector and the lever arm vector.

The length of the lever arm is particularly important; choosing this length appropriately lies behind the operation of levers, pulleys, gears, and most other simple machines involving a mechanical advantage.

The SI unit for torque is the newton meter (N·m). 

Definition and Relation to Angular Momentum 

Torque is defined about a point not specifically about axis as mentioned in several books.

A force applied at a right angle to a lever multiplied by its distance from the lever's fulcrum (the length of the lever arm) is its torque. A force of three newtons applied two metres from the fulcrum, for example, exerts the same torque as a force of one newton applied six metres from the fulcrum. The direction of the torque can be determined by using the right hand grip rule: if the fingers of the right hand curl in the direction of rotation and the thumb points along the axis of rotation, then the thumb also points in the direction of the torque.

More generally, the torque on a particle (which has the position r in some reference frame) can be defined as the cross product:

where r is the particle's position vector relative to the fulcrum, and F is the force acting on the particle. The magnitude τ of the torque is given by

where r is the distance from the axis of rotation to the particle, F is the magnitude of the force applied, and θ is the angle between the position and force vectors. Alternatively,

where F_⊥ is the amount of force directed perpendicularly to the position of the particle. Any force directed parallel to the particle's position vector does not produce a torque.

It follows from the properties of the cross product that the torque vector is perpendicular to both the position and force vectors. It points along the axis of rotation, and its direction is determined by the right-hand rule.

The unbalanced torque on a body along axis of rotation determines the rate of change of the body's angular momentum,

where L is the angular momentum vector and t is time. If multiple torques are acting on the body, it is instead the net torque which determines the rate of change of the angular momentum:

For rotation about a fixed axis,

where I is the moment of inertia and ω is the angular velocity. It follows that

where α is the angular acceleration of the body, measured in rad·s⁻². This equation has the limitation that the torque equation is to be only written about instantaneous axis of rotation or center of mass for any type of motion - either motion is pure translation, pure rotation or mixed motion. I = Moment of inertia about point about which torque is written (either about instantaneous axis of rotation or center of mass only). If body is in translatory equilibrium then the torque equation is same about all points in the plane of motion.

Proof of the equivalence of definitions

The definition of angular momentum for a single particle is:

where "×" indicates the vector cross product, p is the particle's linear momentum, and r is the displacement vector from the origin (the origin is assumed to be a fixed location anywhere in space). The time-derivative of this is:

This result can easily be proven by splitting the vectors into components and applying the product rule. Now using the definition of linearmomentum p = mv (if mass is constant) and the definition of velocity 

The cross product of any vector with itself is zero, so the second term vanishes. Hence with the definition of force F = ma (Newton's 2nd law),

Then by definition, torque τ = r × F.

If multiple forces are applied, Newton's second law instead reads F_net = ma, and it follows that

The proof relies on the assumption that mass is constant; this is valid only in non-relativistic systems in which no mass is being ejected.

A particle is located at position r relative to its axis of rotation. When a force F is applied to the particle, only the perpendicular component F_⊥ produces a torque. This torqueτ = r × F has magnitude τ = |r| |F_⊥| = |r| |F| sinθ and is directed outward from the page.

UNITS

Torque has dimensions of force times distance. Official SI literature suggests using the unit newton metre (N·m) or the unit joule per radian.The unit newton metre is properly denoted N·m or N m. This avoids ambiguity with mN, millinewtons.

The joule, which is the SI unit for energy or work, is dimensionally equivalent to a newton metre, but it is not used for torque. Energy and torque are entirely different concepts, so the practice of using different unit names for them helps avoid mistakes and misunderstandings.The dimensional equivalence of these units, of course, is not simply a coincidence: A torque of 1 N·m applied through a full revolution will require an energy of exactly 2π joules. Mathematically,

where E is the energy, τ is magnitude of the torque, and θ is the angle moved (in radians). This equation motivates the alternate unit namejoules per radian.

In British unit, "pound-force-feet" (lbf x ft), "foot-pounds-force", "inch-pounds-force", "ounce-force-inches" (oz x in) are used, and other non-SI units of torque includes "metre-kilograms-force". For all these units, the word "force" is often left out, for example abbreviating "pound-force-foot" to simply "pound-foot" (in this case, it would be implicit that the "pound" is pound-force and not pound-mass).

Sometimes one may see torque given units that don't dimensionally make sense. For example: g x cm . In these units, g should be understood as the force given by the weight of 1 gram at the surface of the earth. The surface of the earth is understood to have an average acceleration of gravity (approx. 9.80665 m/sec²).

TORNADO

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of acumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider sense, to name any closed low pressure circulation. Tornadoes come in many shapes and sizes, but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds less than 110 miles per hour (177 km/h), are approximately 250 feet (80 m) across, and travel a few miles (several kilometers) before dissipating. The most extremetornadoes can attain wind speeds of more than 300 mph (480 km/h), stretch more than two miles (3 km) across, and stay on the ground for dozens of miles (more than 100 km).

A tornado near Seymour, Texas

                                                             An illustration of generation of infrasound in tornadoes by 
                                                             the Earth System Research Laboratory's Infrasound Program

Various types of tornadoes include the landspout, multiple vortex tornado, andwaterspout. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-supercellular tornadoes that develop over bodies of water. These spiraling columns of air frequently develop in tropical areas close to the equator, and are less common at high latitudes. Other tornado-like phenomena that exist in nature include thegustnado, dust devil, fire whirls, and steam devil.

Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the Tornado Alley region of the United States, although they can occur nearly anywhere in North America. They also occasionally occur in south-central and eastern Asia, thePhilippines, south east Asia, like Malaysia, northern and east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand. Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar by recognizing patterns in velocity and reflectivity data, such as hook echoes, as well as by the efforts of storm spotters.

There are several different scales for rating the strength of tornadoes. The Fujita scale rates tornadoes by damage caused, and has been replaced in some countries by the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and assign a rating.

DEFINITIONS

A tornado is "a violently rotating column of air, in contact with the ground, either pendant from acumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud". For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. Scientists have not yet created a complete definition of the word; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. Tornado refers to the vortex of wind, not the condensation cloud.

EXTREMS

 The most extreme tornado in recorded history was the Tri-State Tornado, which roared through parts of Missouri, Illinois, and Indiana on March 18, 1925. It was likely an F5, though tornadoes were not ranked on any scale in that era. It holds records for longest path length (219 miles, 352 km), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado (73 mph, 117 km/h) anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead). The tornado was also the second costliest tornado in history at the time, but in the years since has been surpassed by several others if population changes over time are not considered. When costs are normalized for wealth and inflation, it ranks third today.

The deadliest tornado in world history was the Daultipur-Salturia Tornado in Bangladesh on April 26, 1989, which killed approximately 1300 people. Bangladesh has had at least 19 tornadoes in its history kill more than 100 people, almost half of the total in the rest of the world.

The most extensive tornado outbreak on record was the Super Outbreak, which affected a large area of the central United States and extreme southern Ontario in Canada on April 3 and 4, 1974. This outbreak, which saw 148 tornadoes develop in 18 hours, included six of F5 intensity and twenty-four that peaked at F4 strength. Sixteen tornadoes were on the ground at the same time during its peak. More than 300 people, possibly as many as 330, were killed by tornadoes during this outbreak.

While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional anemometers would be destroyed by the intense winds, some tornadoes have been scanned by mobile Doppler radar units, which can provide a good estimate of the tornado's winds. The highest wind speed ever measured in a tornado, which is also the highest wind speed ever recorded on the planet, is 301 ± 20 mph (484 ± 32 km/h) in the F5 Bridge Creek-Moore, Oklahoma, tornado which killed 36 people. Though the reading was taken about 100 feet (30 m) above the ground, this is a testament to the power of the strongest tornadoes.

Storms that produce tornadoes can feature intense updrafts, sometimes exceeding 150 mph (240 km/h). Debris from a tornado can be lofted into the parent storm and carried a very long distance. A tornado which affected Great Bend, Kansas, in November 1915, was an extreme case, where a "rain of debris" occurred 80 miles (130 km) from the town, a sack of flour was found 110 miles (177 km) away, and a cancelled check from the Great Bend bank was found in a field outside of Palmyra, Nebraska, 305 miles (491 km) to the northeast. Waterspouts and tornadoes have been advanced as an explanation for instances of raining fish and other animals.

Safety

Though tornadoes can strike in an instant, there are precautions and preventative measures that people can take to increase the chances of surviving a tornado. Authorities such as the Storm Prediction Center advise having a pre-determined plan should a tornado warning be issued. When a warning is issued, going to a basement or an interior first-floor room of a sturdy building greatly increases chances of survival. In tornado-prone areas, many buildings have storm cellars on the property. These underground refuges have saved thousands of lives.

Some countries have meteorological agencies which distribute tornado forecasts and increase levels of alert of a possible tornado (such astornado watches and warnings in the United States and Canada). Weather radios provide an alarm when a severe weather advisory is issued for the local area, though these are mainly available only in the United States. Unless the tornado is far away and highly visible, meteorologists advise that drivers park their vehicles far to the side of the road (so as not to block emergency traffic), and find a sturdy shelter. If no sturdy shelter is nearby, getting low in a ditch is the next best option. Highway overpasses are one of the worst places to take shelter during tornadoes, as they are believed to create a Venturi effect, increasing the danger from the tornado by increasing the wind speed and funneling debris underneath the overpass.