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The EF Scale was designed so that a tornado rated on the Fujita scale would receive the same numerical rating, and was implemented starting in the United States in An EF0 tornado will probably damage trees but not substantial structures, whereas an EF5 tornado can rip buildings off their foundations leaving them bare and even deform large skyscrapers.
Doppler weather radar data, photogrammetry , and ground swirl patterns cycloidal marks may also be analyzed to determine intensity and award a rating. Tornadoes vary in intensity regardless of shape, size, and location, though strong tornadoes are typically larger than weak tornadoes. The association with track length and duration also varies, although longer track tornadoes tend to be stronger. This is apparently mostly due to the lesser number of tornadoes overall, as research shows that tornado intensity distributions are fairly similar worldwide. A few significant tornadoes occur annually in Europe, Asia, southern Africa, and southeastern South America, respectively.
The United States has the most tornadoes of any country, nearly four times more than estimated in all of Europe, excluding waterspouts. North America is a large continent that extends from the tropics north into arctic areas, and has no major east-west mountain range to block air flow between these two areas. In the middle latitudes , where most tornadoes of the world occur, the Rocky Mountains block moisture and buckle the atmospheric flow , forcing drier air at mid-levels of the troposphere due to downsloped winds, and causing the formation of a low pressure area downwind to the east of the mountains.
Increased westerly flow off the Rockies force the formation of a dry line when the flow aloft is strong,  while the Gulf of Mexico fuels abundant low-level moisture in the southerly flow to its east. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the year. A large portion of these tornadoes form in an area of the central United States known as Tornado Alley.
The United States averages about 1, tornadoes per year, followed by Canada, averaging 62 reported per year. Tornadoes kill an average of people per year in Bangladesh , the most in the world. Tornadoes are most common in spring and least common in winter, but tornadoes can occur any time of year that favorable conditions occur. Tornadoes can also be spawned as a result of eyewall mesovortices , which persist until landfall. Tornado occurrence is highly dependent on the time of day, because of solar heating.
The United Kingdom has the highest incidence of tornadoes, measured by unit area of land, than any other country in the world. The United Kingdom has at least 34 tornadoes per year and possibly as many as 50,  more than any other country in the world relative to its land area. Most tornadoes in the United Kingdom are weak, but they are occasionally destructive. For example, the Birmingham tornado of and the London tornado of both registered F2 on the Fujita scale and both caused significant damage and injury.
Associations with various climate and environmental trends exist. For example, an increase in the sea surface temperature of a source region e. Gulf of Mexico and Mediterranean Sea increases atmospheric moisture content. Increased moisture can fuel an increase in severe weather and tornado activity, particularly in the cool season. Ocean conditions could be used to forecast extreme spring storm events several months in advance. Climatic shifts may affect tornadoes via teleconnections in shifting the jet stream and the larger weather patterns.
The climate-tornado link is confounded by the forces affecting larger patterns and by the local, nuanced nature of tornadoes. Although it is reasonable to suspect that global warming may affect trends in tornado activity,  any such effect is not yet identifiable due to the complexity, local nature of the storms, and database quality issues.
Any effect would vary by region. Rigorous attempts to warn of tornadoes began in the United States in the midth century. Before the s, the only method of detecting a tornado was by someone seeing it on the ground. Often, news of a tornado would reach a local weather office after the storm. However, with the advent of weather radar, areas near a local office could get advance warning of severe weather. The first public tornado warnings were issued in and the first tornado watches and convective outlooks came about in In , it was confirmed that hook echoes were associated with tornadoes.
Today, most developed countries have a network of weather radars, which serves as the primary method of detecting hook signatures that are likely associated with tornadoes. In the United States and a few other countries, Doppler weather radar stations are used. When storms are distant from a radar, only areas high within the storm are observed and the important areas below are not sampled. Some meteorological situations leading to tornadogenesis are not readily detectable by radar and tornado development may occasionally take place more quickly than radar can complete a scan and send the batch of data.
Doppler radar systems can detect mesocyclones within a supercell thunderstorm. This allows meteorologists to predict tornado formations throughout thunderstorms. In the mids, the U. National Weather Service NWS increased its efforts to train storm spotters so they could spot key features of storms that indicate severe hail, damaging winds, and tornadoes, as well as storm damage and flash flooding.
The program was called Skywarn , and the spotters were local sheriff's deputies, state troopers, firefighters, ambulance drivers, amateur radio operators , civil defense now emergency management spotters, storm chasers , and ordinary citizens.
Know the difference between a watch and a warning
When severe weather is anticipated, local weather service offices request these spotters to look out for severe weather and report any tornadoes immediately, so that the office can warn of the hazard. Spotters usually are trained by the NWS on behalf of their respective organizations, and report to them.
In Canada, a similar network of volunteer weather watchers, called Canwarn , helps spot severe weather, with more than 1, volunteers. Storm spotters are required because radar systems such as NEXRAD do not really detect tornadoes; merely signatures which hint at the presence of tornadoes. Storm spotters are trained to discern whether or not a storm seen from a distance is a supercell. They typically look to its rear, the main region of updraft and inflow. Under that updraft is a rain-free base, and the next step of tornadogenesis is the formation of a rotating wall cloud.
The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell. Evidence of a supercell is based on the storm's shape and structure, and cloud tower features such as a hard and vigorous updraft tower, a persistent, large overshooting top , a hard anvil especially when backsheared against strong upper level winds , and a corkscrew look or striations. Under the storm and closer to where most tornadoes are found, evidence of a supercell and the likelihood of a tornado includes inflow bands particularly when curved such as a "beaver tail", and other clues such as strength of inflow, warmth and moistness of inflow air, how outflow- or inflow-dominant a storm appears, and how far is the front flank precipitation core from the wall cloud.
Tornadogenesis is most likely at the interface of the updraft and rear flank downdraft , and requires a balance between the outflow and inflow.
Only wall clouds that rotate spawn tornadoes, and they usually precede the tornado between five and thirty minutes. Rotating wall clouds may be a visual manifestation of a low-level mesocyclone. Barring a low-level boundary, tornadogenesis is highly unlikely unless a rear flank downdraft occurs, which is usually visibly evidenced by evaporation of cloud adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly afterwards; first, a funnel cloud dips and in nearly all cases by the time it reaches halfway down, a surface swirl has already developed, signifying a tornado is on the ground before condensation connects the surface circulation to the storm.
Tornadoes may also develop without wall clouds, under flanking lines and on the leading edge. Spotters watch all areas of a storm, and the cloud base and surface. The most record-breaking tornado in recorded history was the Tri-State Tornado , which roared through parts of Missouri , Illinois , and Indiana on March 18, It was likely an F5, though tornadoes were not ranked on any scale in that era.
In addition, it is the deadliest single tornado in United States history dead. 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, , which killed approximately 1, people. The most extensive tornado outbreak on record was the Super Outbreak , which spawned confirmed tornadoes over the southeastern United States — of them within a single hour period. The previous record was the Super Outbreak which spawned tornadoes.
While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional anemometers would be destroyed by the intense winds and flying debris, some tornadoes have been scanned by mobile Doppler radar units , which can provide a good estimate of the tornado's winds.
Staying Safe in a Tornado
Debris from a tornado can be lofted into the parent storm and carried a very long distance. 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. 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 as tornado 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.
Tornado Facts and Information
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 the constricted space can be subject to increased wind speed and funneling of debris underneath the overpass. Folklore often identifies a green sky with tornadoes, and though the phenomenon may be associated with severe weather, there is no evidence linking it specifically with tornadoes.
While there is a large drop in atmospheric pressure inside a strong tornado, it is unlikely that the pressure drop would be enough to cause the house to explode. Opening windows may actually increase the severity of the tornado's damage. Another commonly held misconception is that highway overpasses provide adequate shelter from tornadoes.
Staying Safe in a Tornado
This belief is partly inspired by widely circulated video captured during the tornado outbreak near Andover, Kansas , where a news crew and several other people take shelter under an overpass on the Kansas Turnpike and safely ride out a tornado as it passes by. An old belief is that the southwest corner of a basement provides the most protection during a tornado.
vipauto93.ru/profiles/spiare-messaggi/dati-cellulare-iphone-8-tim.php The safest place is the side or corner of an underground room opposite the tornado's direction of approach usually the northeast corner , or the central-most room on the lowest floor. Taking shelter in a basement, under a staircase, or under a sturdy piece of furniture such as a workbench further increases chances of survival. There are areas which people believe to be protected from tornadoes, whether by being in a city, near a major river, hill, or mountain, or even protected by supernatural forces. As a general rule, no area is safe from tornadoes, though some areas are more susceptible than others.
Meteorology is a relatively young science and the study of tornadoes is newer still. Although researched for about years and intensively for around 60 years, there are still aspects of tornadoes which remain a mystery. However, the step from supercell , or other respective formative processes, to tornadogenesis and the prediction of tornadic vs. Also under study are the low-level mesocyclone and the stretching of low-level vorticity which tightens into a tornado,  in particular, what are the processes and what is the relationship of the environment and the convective storm.
Intense tornadoes have been observed forming simultaneously with a mesocyclone aloft rather than succeeding mesocyclogenesis and some intense tornadoes have occurred without a mid-level mesocyclone. In particular, the role of downdrafts , particularly the rear-flank downdraft , and the role of baroclinic boundaries, are intense areas of study. Reliably predicting tornado intensity and longevity remains a problem, as do details affecting characteristics of a tornado during its life cycle and tornadolysis. Other rich areas of research are tornadoes associated with mesovortices within linear thunderstorm structures and within tropical cyclones.
Scientists still do not know the exact mechanisms by which most tornadoes form, and occasional tornadoes still strike without a tornado warning being issued.