SKYWARN~TORNADOS

From Wikipedia, the free encyclopedia

SKYWARN is a program of the United States' National Weather Service (NWS). Its mission is to collect reports of localized severe weather. These reports are used to aid forecasters in issuing and verifying severe weather watches and warnings and to improve the forecasting and warning processes and the tools used to collect meteorological data. It consists of a network of severe storm spotters that observe weather conditions and make reports of severe weather to their local NWS offices. These spotters are trained by personnel from the local NWS offices each spring in advance of the coming severe weather season.

Storm spotting

Where severe storms are possible, storm spotting groups such as SKYWARN in the United States coordinate amateur radio operators to keep track of severe thunderstorms and tornadoes. Reports from spotters and chasers are given to the National Weather Service so that they have the information to warn the general public. Spotters also give reports during winter storms, floods, hurricanes, and wildfires.

Other countries have similar programs, such as the Canadian spotting program Canwarn, the Tornado and Storm Research Organisation (TORRO) program in the United Kingdom, and Skywarn Europe for several European countries.

Amateur radio

SKYWARN has long been associated with amateur radio. Many NWS offices maintain an amateur radio station that is manned by amateur radio operators during times of severe weather. This allows licensed amateur radio spotters to transmit their severe weather reports directly to the NWS and receive up-to-date severe weather updates even if regular communications are disrupted or overloaded by the weather emergency.

Participation in SKYWARN does not require an amateur radio license; more than half of all SKYWARN spotters are not licensed amateur radio operators. The NWS encourages anyone with an interest in public service and access to some method of communication, such as amateur radio, telephone, the Internet, etc to join the SKYWARN program. Volunteers include police and fire personnel, dispatchers, EMS workers, public utility workers and other concerned private citizens. Individuals affiliated with hospitals, schools, churches, nursing homes or who have a responsibility for protecting others are also encouraged to become spotters. Those without radio communication capability make their reports via telephone or the Internet when those are still functioning.

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TORNADOS

A tornado is a violent, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. Tornadoes come in many 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.

Most tornadoes have wind speeds between 40 mph (64 km/h) and 110 mph (177 km/h), are approximately 250 feet (75 m) across, and travel a few miles (several kilometers) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).

Although tornadoes have been observed on every continent except Antarctica, most occur in the United States. They also commonly occur in southern Canada, south-central and eastern Asia, east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand.

A tornado near Anadarko, Oklahoma. The tornado itself is the thin tube reaching from the cloud to the ground. The lower part of this tornado is surrounded by a translucent dust cloud, kicked up by the tornado's strong winds at the surface.

The word tornado is an altered form of the Spanish word tronada, which means "thunderstorm." This in turn was taken from the Latin tonare, meaning "to thunder". It most likely reached its present form through a combination of the Spanish tronada and tornar ("to turn"); however, this may be a folk etymology. A tornado is also commonly referred to as a twister, and is also sometimes referred to by the old-fashioned colloquial term cyclone. The term "cyclone" is used as a synonym for "tornado" in the often-aired 1939 film, The Wizard of Oz. The term "twister" is also used in that film, along with being the title of the 1996 film Twister.

The Glossary of Meteorology defines a tornado as "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud... In practice, 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.

A tornado near Seymour, Texas

A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (see Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually causes water vapor in the air to become visible as a funnel cloud or condensation funnel.

There is some disagreement over the definition of "funnel cloud" and "condensation funnel". According to the Glossary of Meteorology, a funnel cloud is any rotating cloud pendant from a cumulus or cumulonimbus, and thus including most tornadoes under this definition. Among many meteorologists, a funnel cloud is strictly defined as a rotating cloud which is not associated with strong winds at the surface, and a "condensation funnel" is a broad term for any rotating cloud below a cumuliform cloud.

Tornadoes often begin as funnel clouds with no associated strong winds at the surface, however, not all of these evolve into a tornado. However, many tornadoes are preceded by a funnel cloud. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.

This tornado has no funnel cloud, however, the rotating dust cloud indicates that strong winds are occurring at the surface, and thus it is a true tornado. This was taken near Alva, Oklahoma in 1975.

Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same storm are referred to as a tornado family.

Occasionally, several tornadoes are spawned from the same large-scale storm system. If there is no break in activity, this is considered a tornado outbreak, although there are various definitions. A period of several successive days with tornado outbreaks in the same general area (spawned by multiple weather systems) is a tornado outbreak sequence, occasionally called an extended tornado outbreak.

A multiple vortex tornado is a type of tornado in which two or more columns of spinning air rotate around a common center. Multivortex structure can occur in almost any circulation, but is very often observed in intense tornadoes. These vortices often create small areas of heavier damage along the main tornado path.

A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957

A satellite tornado is a term for a weaker tornado which forms very near a large, strong tornado contained within the same mesocyclone. 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 defined by the National Weather Service simply as a tornado over water. However, researchers typically distinguish "fair weather" waterspouts from tornadic waterspouts.

Fair weather waterspouts are less severe but far more common, and are similar in dynamics to dust devils and landspouts. They form at the bases of cumulus congestus cloud towers in tropical and semitropical waters. They have relatively weak winds, smooth laminar walls, and typically travel very slowly, if at all. They occur most commonly in the Florida Keys and in the northern Adriatic Sea.
Tornadic waterspouts are more literally "tornadoes over water". They can form over water like mesocyclonic tornadoes, or be a land tornado which crosses onto water. Since they form from severe thunderstorms and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are considered far more dangerous.

A waterspout near the Florida Keys

A landspout (officially known as a dust-tube tornado) is a tornado not associated with a mesocyclone. The name stems from their characterization as essentially a "fair weather waterspout on land". Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel which often does not reach the ground. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, due to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they still produce strong winds and may cause serious damage.

A landspout near North Platte, Nebraska on May 22, 2004

A gustnado (gust front tornado) 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. They are formed when fast moving cold, dry outflow air from a thunderstorm is blown through a mass of stationary, warm, moist air near the outflow boundary, resulting in a "rolling" effect (often exemplified through a roll cloud). If low level wind shear is strong enough, the rotation can be turned horizontally (or diagonally) and make contact with the ground. The result is a gustnado. They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage. It is also worth noting that since they are absent of any Coriolis influence from a mesocyclone, they seem to be alternately cyclonic and anticyclonic without preference.

A dust devil resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are rarely as strong as even the weakest tornadoes. They form when a strong convective updraft is formed near the ground on a hot day. If there is enough low level wind shear, the column of hot, rising air can develop a small cyclonic motion that can be seen near the ground. They are not considered tornadoes because they form during fair weather and are not associated with any actual cloud. However, they can, on occasion, result in major damage, especially in arid areas.

Dust devil in Johnsonville, South Carolina

Tornado-like circulations occasionally occur near large, intense wildfires and are called fire whirls. They are not considered tornadoes except in the rare case where they connect to a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. However, they can produce significant damage.

A steam devil is a term describing a rotating updraft that involves steam or smoke. A steam devil is very rare, but they mainly form from smoke emitting from a power plant smokestack. Hot springs and deserts may also be suitable locations for a steam devil to form. There have also been reports of cold air steam devils as well.

Tornadoes also produce identifiable inaudible infrasonic signatures. Unlike audible signatures, tornadic signatures have been isolated; due to the long distance propagation of low-frequency sound, efforts are ongoing to develop tornado prediction and detection devices with additional value in understanding tornado morphology, dynamics, and creation. Tornadoes also produce a detectable seismic signature, and research continues on isolating it and understanding the process.

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

[http://www.esrl.noaa.gov/psd/programs/infrasound/] Infrasound Program.

FOR MORE ON TORNADOS CLICK ON >>> http://en.wikipedia.org/wiki/Tornado

WALL CLOUDS

A wall cloud, or pedestal cloud, is a cloud formation associated with thunderstorms. It is a marked lowering typically beneath the rain-free base (RFB) portion of a deep cumulus cloud (normally cumulonimbus but on rare occasion cumulus congestus), and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of the ambient cloud base. Most strong tornadoes form within wall clouds.

A rain-free base with a wall cloud lowering in the foreground and precipitation in the background. Taken in Miami, Texas.

Wall clouds are caused by the ascending and converging inflow air of the updraft ingesting moist, rain cooled air from the normally downwind downdraft. In supercells, this is the forward flank downdraft (FFD). Since temperature tends to decrease and dew point to (moisture content) increase as the updraft entrains this air, saturation occurs sooner as the air rises. Wall clouds may form as a descending of the cloud base or may form as rising scud consolidates and organizes.

Wall clouds can be anywhere from a fraction of a mile (0.25 km) wide to over five miles (8 km) across, and in the Northern Hemisphere typically form at the south or southwest end of a supercell. Wall clouds form in the inflow region, on the side of the storm coinciding with the direction of the steering winds (deep layer winds through the height of the storm). Rotating wall clouds are visual evidence of a mesocyclone.

Some wall clouds have a feature similar to an "eye". Attached to many wall clouds, especially in moist environments, is a tail cloud, a ragged band of cloud and cloud tags (fractus) extending from the wall cloud toward the precipitation. It can be thought of as an extension of the wall cloud in that not only is it connected to the wall cloud but also that condensation forms for a similar reason. Cloud elements may be seen to be moving into the wall cloud, as it is an inflow feature. Most movement is horizontal, but some rising motion is often apparent as well. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base; this feature is a collar cloud.

A wall cloud with tail cloud.

Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm, though small rotating wall clouds associated with mesovortices can occur within the leading edge on rare occasion. Wall clouds will tend to slope in, or toward the precipitation area, whereas shelf clouds as outflow clouds will jut outward from the storm. Wall clouds are inflow features with (often warm) air moving towards them whereas as shelf clouds are an outflow feature with cool air moving away from the storm, often as a gust front.

The wall cloud feature was first identified by Ted Fujita associated with tornadoes in tornadic storms. In the special case of a supercell thunderstorm but also occasionally with intense multicellular thunderstorms, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm that is most likely to produce tornadoes, and the vast majority of intense tornadoes.

Tornadogenesis is most likely when the wall cloud is persistent with rapid ascension and rotation. The wall cloud typically precedes tornadogenesis by ten to twenty minutes but may be as little as one minute or more than an hour. Often, the degree of ascension and rotation increase markedly shortly before tornadogenesis, and sometimes the wall cloud will descend and "bulk" or "tighten". Tornadic wall clouds tend to have strong, persistent, and warm inflow air. This should be sensible at the surface if one is in the inflow region; in the Northern Hemisphere, this is typically to the south and southeast of the wall cloud. Large tornadoes tend to come from larger wall clouds.

Although it is rotating wall clouds that contain most strong tornadoes, many rotating wall clouds do not produce tornadoes. Absent a low-level boundary, tornadoes very rarely occur without a rear flank downdraft (RFD), which usually manifests itself visually as a drying out of clouds, called a clear slot or notch. The RFD initiates the tornado, occludes around the mesocyclone, and when it wraps completely around, cuts off the inflow causing death of the low-level mesocyclone and tornadolysis. Therefore, in most cases, the RFD is responsible for both the birth and the death of a tornado.

Usually, but not always, the dry slot occlusion is visible (assuming one's line of sight is not blocked by precipitation) throughout the tornado life cycle. The wall cloud withers and will often be gone by the time the tornado lifts. If conditions are favorable, then, often even before the original tornado lifts, another wall cloud and occasionally a tornado may form downwind of the old wall cloud, typically to the east or the southeast in the Northern Hemisphere (vice versa in the Southern Hemisphere).

A tornadic wall cloud with RFD clear slot.

Shelf cloud

A shelf cloud is a low, horizontal wedge-shaped arcus cloud. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn.

Occasionally people seeing a shelf cloud may believe they have seen a wall cloud. This is a common mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally speaking, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm.

Panorama of a strong shelf cloud

Sign of danger

A sharp, strong gust front will cause the lowest part of the leading edge of an arcus to be ragged and lined with rising fractus clouds. In a severe case there will be vortices along the edge with twisting masses of scud that may reach to the ground or be accompanied by rising dust. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind squall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado.