The 'Picher Twister'... Tornado... What, When & How?

Quick Links To Tornado & Thunderstorm Information & Safety

A Closeup View Of Tornados

Tornadoes: Nature's most violent wind

Tornadoes have struck every U.S. state, including Alaska and Hawaii. But most tornadoes form in a belt from Nebraska southward through central Texas known as Tornado Alley and in the Southeast.

Wind speeds in tornadoes can vary from 72 to almost 300 mph. Fortunately, only 2 percent of all tornadoes have winds greater than 200 mph.

A tornado is defined as an intense, rotating column of air extending from the base of a thunderstorm cloud to the ground.

Air moves very rapidly upward around a tornado center. This distinguishes tornadoes from microbursts, which often do tornado-like damage and are often mistaken for tornadoes. Microbursts, on the other hand, features air blasting downward from thunderstorms. The large hail that often precedes tornadoes forms as a result of the intense updraft feeding the thunderstorm.

The United States is the world capital for tornadoes as conditions favorable for tornado development most often occur over the Plains during spring and summer.

A typical tornado outbreak often features an intense upper-level disturbance moving across the Plains during spring. This disturbance provides the strong vertical wind shear that gives an updraft its twisting motion, turning a normal thunderstorm into a potentially tornado spawning supercell.

Although, the United States has the most tornadoes of any nation in the world, tornadoes do occur in other locations such as Australia and Europe.

Fujita wind damage scale rates tornadoes

Intense tornadoes can have more than one vortex

Understanding Tornadoes

Deadly winds from thunderstorms...
Often wind damage that's blamed on tornadoes is actually done by winds coming down from a shower or thunderstorm.

Such "microburst" winds can reach more than 150 mph. In a tornado, the air is rising as it swirls around the vortex.

In addition to damaging buildings and blowing down trees, microbursts blasting down to the ground are a major aviation hazard and have caused several crashes.

Today, pilots are trained in how to avoid microbursts and special Doppler radars to detect them are being installed at airports.

Any strong winds coming down from showers and thunderstorms are known as "downbursts." If damaging winds are concentrated in an area extending 2.5 miles or less, it's called a "microburst." If the winds cover a larger area, it's a "macroburst."

Microbursts are most common from the Rockies eastward in the USA because showers and thunderstorms are more common.

In the humid East, "wet" microbursts are most likely while "dry" microbursts occur more often in the West. (Related graphic: wet and dry microbursts)

Lines of thunderstorms that cause one downburst after another are called derechoes.

How Hail Forms

Rising air creates spring, summer ice

Thunderstorms sometimes drop balls of ice known as hail in addition to rain.

Hail forms when strong currents of rising air, known as updrafts, carry water droplets high enough in a thunderstorm for the water droplets to freeze.

Most hailstones are smaller in diameter than a dime, but stones weighing more than a pound have been recorded.

A strong updraft allows hailstones to grow large enough to reach the ground. In general, the stronger the updraft, the larger the hail.

Once hailstones grow large enough to begin falling despite the updraft that's been holding them up, they hurtle toward the ground as fast as 90 mph.

Small hailstones often melt before reaching the ground, but the larger ones reach the ground and can cause extensive damage to crops and vehicles caught in the storm.

While crops are the major victims, hail is also a hazard to vehicles and windows.

Deaths are rare in the United States. The last known U.S. hail fatality was an infant killed in Fort Collins, Colo., in August 1979.

Deaths and injuries are more common in other parts of the world, especially places where many people live in poorly constructed buildings.

In the USA, hailstorms are most common on the Plains, especially just east of the Rockies. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy.

While hailstones are ice, hail is mostly a spring and summer phenomena because the strong thunderstorms needed to produce hail are much more common during warm weather.

Ice that falls during the winter is almost always sleet � raindrops that freeze on the way down to the surface.

Conditions Favorable For Tornado Development

The Convection Cap

Caps Often Aid Tornado Development

A convective cap is a layer of hot, dry air in the middle layers of the atmosphere above the surface. Often, temperatures increase with height in this layer and relative humidities are extremely low.

As you can see in the graphic above, warm humid air in the lower layers of the atmosphere near the surface is heated by the sun, but is not allowed to rise and initiate clouds and precipitation because of the hot, dry air above it.

As the air near the surface continues to heat up, it builds up an enormous amount of energy much the same way boiling water in a pot with a heavy lid on it would.

If a triggering mechanism, such as a cold front or dryline, moves into the area, the convective cap may weaken enough to allow the heated, humid air near the surface to burst through the cap and initiate extremely violent convection.

Supercells, along with intense tornadoes, often form as a result of this violent convection.

The conditions depicted in the graphic above often preceded some of the worst tornado outbreaks in history. Another graphic explains how the large scale weather features form the convective cap.

Disturbances Ripple Along

Disturbances spark unexpected showers

Upper-level disturbances ride along the high-altitude winds far above the earth's surface. Some times they can pass by without anyone but meteorologists knowing about them because they don't affect the weather at the Earth's surface.

At other times, they can spawn unexpected showers, such as the one that ruin's your family's picnic on a day when good weather was forecast.

An upper-level disturbance is a pocket of rotating air that ripples along with the generally west-to-east, upper-altitude winds. Complicated movements of air at different levels of the atmosphere form the disturbances..

As a disturbance moves along toward the east, air is rising rising on its east side. This rising air helps air rise from the ground and the rising air cools. If the rising air has enough humidity, clouds and precipitation form.

The air on a disturbance's west side is sinking. Since sinking air warms and warmer air evaporates clouds, the sky can rapidly clear as a disturbance passes by.

In addition to bringing showers on a day that forecasters expected to be dry, upper air disturbances can also trigger thunderstorms, and help make thunderstorms stronger than they would be otherwise been.

The Basics Of Wind Shear

Wind Shear Can Make Plane Rides A Bit Bumpy
The major cause of the air turbulence that sometimes makes planes bounce up and down in flight is wind shear. (Related: Aircraft turbulence)

Meteorologists use the term "wind shear" in a much wider way than the kind that can cause turbulence, as shown in the graphic here.

The term wind shear refers to a change in wind speed or direction, or both, over a short distance.

As the gaphic above shows, such changes help create eddies, or swirls of air, that cause turbulence. Wind shear can be both vertical and horizontal.

On a larger scale, wind shear describes a wind pattern with winds blowing from the south at ground level, from the southwest a few thousand feet above the ground, and from the west at higher altitudes.

Such large-scale shear is one of the conditions needed for strong or widespread tornadoes. The changes in wind direction help give the atomsphere the twisting motion needed for tornadoes.

Finally, during the hurricane season, you'll hear forecasters talking about "wind shear" being likely to weaken a storm, or maybe the lack of wind shear giving a tropcial storm or hurricane a chance to grow. (Related graphic and text: Wind shear weakens hurricanes)

What Goes On In Thunderstorms

A Look Inside A Thunderstorm

Lightning is the key ingredient that defines a thunderstorms since lightning is needed to create thunder.

Thunderstorms come in all shapes and sizes with some cells only a few miles in diameter and some clusters of storms, known as mesoscale convective complexes, that span hundreds of miles.

A typical thunderstorm produces a brief period of heavy rain and lasts anywhere from 30 minutes to an hour.

However, not all thunderstorms produce rain that reaches the ground. These dry thunderstorms are most common across the western USA and often spawn wildfires. They often form high above the ground with a large layer of very dry air between the base of the cloud and the ground. As rain falls from the cloud into the dry air, the rain drops evaporate before they reach the ground.

Warm, humid conditions are very favorable for thunderstorm development. This helps create the strong updrafts that feed warm, moist air into thunderstorms.

If the air is very unstable, severe thunderstorms with damaging winds, large hail, and sometimes tornadoes erupt.

Cold fronts, drylines, or afternoon heating, which causes warm air to rise, can trigger thunderstorms

Supercells Are The Ultimate In Storms

The largest, strongest and longest-lasting thunderstorms have an appropriate name: "supercells."

They are capable of producing tornadoes, large hail, dangerous bursts of wind or flash floods as well as lightning, which is a danger in all thunderstorms. These storms are characterized by rotating winds rising into the storm called a "mesocyclone."

Mesocyclones help give supercells their destructive power and also help the storms hold together for hours as they create a path of destruction along the ground.

Supercells spawn most of the USA's killer tornadoes. They can also bring hail more than three quarters of an inch in diameter and non-tornado winds faster than 57 mph.

Some supercells produce little rain, others, downpours that can cause flash floods. Supercells are most common on the Great Plains, but also occur in other parts of the USA.

Mesocyclones within supercells can be detected using the National Weather Service's Doppler weather radar. These state-of-the-art detection units allow forecasters to peek inside a supercell and look for a Tornado Vortex Signature, or TVS.

Fujita Wind Damage Scale Rates Tornadoes

Enhanced Fujita scale ranks tornadoes by damage

Forecasters and researchers use a wind damage scale to classify tornadoes. The original Fujita scale was developed by T. Theodore Fujita and was put into use in 1973. The scale was enhanced in 2007, with rankings running from EF (Enhanced Fujita) - 0 through 5. The ratings are based on the amount and type of wind damage.

The ratings are:

EF-0. Light damage

Wind 65 to 85 mph. Causes some damage to siding and shingles

EF-1. Moderate damage Wind 86 to 110 mph. Considerable roof damage. Winds can uproot trees and overturn single-wide mobile homes. Flagpoles bend.

EF-2. Considerable damage Wind 111 to 135 mph. Most single-wide mobile homes destroyed. Permanent homes can shift off foundation. Flagpoles collapse. Softwood trees debarked.

EF-3. Severe damage Wind 136 to 165 mph. Hardwood trees debarked. All but small portions of houses destroyed.

EF-4. Devastating damage Wind 166 to 200 mph. Complete destruction of well-built residences, large sections of school buildings.

EF-5. Incredible damage Wind above 200 mph. Significant structural deformation of mid- and high-rise buildings.

It should be noted that the Enhanced F-scale is a set of wind estimates (not measurements) based on damage. It uses three-second estimated gusts estimated at the point of damage. These estimates vary with height and exposure. Forensic meteorologists use 28 damage indicators and up to 9 degrees of damage to assign estimated speeds to the wind gusts. You can read more about the Enhanced Fujita scale on this Storm Prediction Center website.

The Fujita scale is different from the Saffir-Simpson scale of hurricane strength in one very important way. Fujita scale numbers are assigned to tornadoes only after the fact; after the National Weather Service inspects the damage. (Related: The Saffir-Simpson scale).

Hurricane scale numbers are assigned soon after a storm becomes a hurricane. The rankings are almost always based on data gathered by hurricane hunter airplanes. This isn't possible with even the strongest tornadoes because meteorologists have no way of collecting data during a tornado, even if they had time (which they hardly ever world) to send an airplane to examine it. Crews that fly into hurricanes would never fly into tornadic thunderstorm with its concentrated violence.

Intense Tornadoes Can Have More Than One Vortex

The most violent tornadoes often have more than one vortex - spinning air - rotating around the tornado center.

As you can see in the graphic above, the vortices are like small tornadoes moving around the parent vortex.

These subsidiary vortices form and die out as the center of the tornado moves along. They are called suction vortexes and are responsible for the strongest winds at the surface.

Dust, debris and clouds make it very difficult to see the individual vortices when viewing the tornado from the ground. Multiple vortices are most common in F4 or F5 twisters.

Understanding Tornadoes

Tornadoes Are Earth's Most violent Storms

Tornadoes are the most violent storms on Earth. Winds spiraling into them usually exceed 100 mph and can reach speeds of 300 mph. In the USA, an average of 1,000 tornadoes spin up beneath thunderstorms each year, and these typically kill about 60 people.

Tornadoes and the threat of tornadoes are a key part of the USA's spring weather because spring brings favorable tornado conditions. But tornadoes can occur any time of the year, during the day and at night.

The National Weather Service defines a tornado as "a violently rotating column of air in contact with the ground and pendant from a thunderstorm." In other words, a thunderstorm is the first step in the creation of a tornado. Then, if other conditions are right, the thunderstorm might spin out one or more tornadoes.

The three key conditions required for thunderstorms to form are:

� Moisture in the lower to mid levels of the atmosphere.

� Unstable air. That is, air that will continue rising once it begins rising from near the ground.

� A lifting force. Something is needed to cause the air to begin rising. The most common lifting force is heating of air near the ground. As the air warms it becomes lighter and begins rising. Advancing masses of cool air, which force warm air upward, also trigger thunderstorms.

When all the conditions are present, humid air will rise high into the sky and cool and condense into towering clouds, forming thunderstorms. This air rising into a thunderstorm is called an updraft. Tornadoes form in within a thunderstorm's updraft.

The strongest tornadoes are often near the edge of the updraft, not far from where air is descending in a downdraft caused by the thunderstorms with falling rain or hail. This is why a burst of heavy rain or hail sometimes announces a tornado's arrival.

Tornadoes are commonly associated with the nation's heartland � in a 10-state area stretching from Texas to Nebraska that also includes Colorado, Iowa, Illinois, Indiana, Missouri and Arkansas, known as Tornado Alley.

But, they are not limited to this region. Tornadoes have occurred in all 50 U.S. states and are, in fact, more common in Florida than they are in Oklahoma.

Florida tornadoes are generally weak � for tornadoes � with winds around 100 mph.

Tornadoes that have hit Oklahoma, on the other hand, are some of the most violent on record. A tornado that struck Oklahoma City and its southern suburbs in 1999 had winds of nearly 320 just above the ground.

Tornadoes are ranked by the damage they do using the six-tiered Fujita Scale. F0 and F1 tornadoes on the scale are considered "weak" and cause minimal to moderate damage with winds from 40-112 mph. F2 and F3 tornadoes are considered strong, packing winds of 113-206 mph that can cause major to severe damage. Violent tornadoes are those classified F4 and F5 with winds exceeding 206 mph. Damage is extreme to catastrophic.

Most weak tornadoes last 10 minutes or less, traveling short distances. Violent tornadoes have been known to last for hours and a few have traveled more than 100 miles.

Resources: Thunderstorms

Thunderstorms are one of the most common and most noticeable weather products of our atmosphere. They form worldwide, spit out deadly lightning, band together to form hurricanes, and can spin up the world's fastest winds inside tornadoes. The links below take you to information about the nature, structure and detailed studies of thunderstorms.

Weather To Watch Severe Weather Watches & Warnings

Severe weather alerts map from The Weather Channel

Latest NWS Watches, Warnings, Advisories & By State

Latest NWS watches, warnings, Advisories & By Type

Today's Outlook, From The Hydrometeorological Prediction Center

What Watches & Warnings Mean

The National Weather Service uses the words "watches" and "warnings" to alert you to potentially dangerous weather. Knowing the difference between the two can be a life saver.

Weather Watches...

A watch means conditions are right for dangerous weather. In other words, a "watch" means watch out for what the weather could do, be ready to act.

For events that come and go quickly, such as severe thunderstorms, tornadoes or flash floods, a watch means that the odds are good for the dangerous weather, but it's not yet happening.

For longer-lived events, such as hurricanes or winter storms, a watch means that the storm isn't an immediate threat.

For either kind of event, a watch means you should keep up with the weather and be ready to act.

When a severe thunderstorm, tornado or flash flood watch is in effect, it means you should watch the sky for signs of dangerous weather. Sometimes a severe thunderstorm, a tornado or a flash flood happens so quickly that warnings can't be issued in time. Many areas don't have civil-defense sirens or other warning methods. People who live near streams that quickly reach flood levels should be ready to flee at the first signs of a flash flood.

Hurricane or winter storm watches mean it's time to prepare by stocking up on emergency supplies and making sure you know what to do if a warning is issued. For those who live near the ocean, a hurricane watch may mean it's time to prepare for evacuation.

Weather warnings

A warning means that the dangerous weather is threatening the area.

For severe thunderstorms, tornadoes and flash floods, a warning means the event is occurring. Since tornadoes are small - a half-mile wide tornado is considered huge - a tornado will miss many more buildings that it hits in the area warned.

Still, a tornado warning means be ready to take shelter immediately if there are any indications a tornado is approaching. Severe thunderstorms are larger, maybe 10 or 15 miles across.

A Hurricane Warning means either evacuate or move to safe shelter.

A winter storm warning means it's not safe to venture out. If traveling, head for the nearest shelter.

How Alerts Are Issued

Before watches and warnings are issued, the National Weather Service, private forecasters, newspapers, radio and television normally try to alert the public to potential weather dangers.

Often, forecasters begin issuing bulletins on hurricanes and winter storms three or four days before the storm hits.

But forecasters can't issue alerts for the danger of severe thunderstorms, tornadoes and flash floods that far ahead. Usually, the Storm Prediction Center sends out alerts the day before dangerous weather is likely. Most television weathercasters highlight these alerts on the evening news the day before threatening weather.

Weather Radio

A weather radio is one of the best ways to stay tuned-in to dangerous weather. These radios receive broadcasts from the National Weather Service. The broadcasts are from weather service offices.

Broadcasts include ordinary forecasts of several kinds, including for boating, farming, traveling and outdoor recreation as well as general forecasts for the area.

The stations immediately broadcast all watches and warnings. Some weather radios have a feature that turn on the radio automatically when a watch or warning is broadcast. Such "tone alert" weather radios are highly recommended for places where large numbers of people could be endangered by tornadoes or flash floods. These include schools, nursing homes, shopping center security offices, hospitals, and recreation areas such as swimming pools.

This National Weather Service page has information on weather radio, including a list of weather radio stations in each state.

Thunderstorm Safety

Thunderstorm Safety Guide
Thunderstorms can throw one of nature's most spectacular shows, but at the same time prove to be deadly. Dangers associated with thunderstorms can include lightning, hail, heavy rain, flooding, strong winds associated with downbursts, microbursts and tornadoes.

When certain atmospheric conditions coexist, a general thunderstorm can become severe. According to the National Weather Service, a thunderstorm is classified severe when winds reach or exceed 57.5 mph or produces hail three-quarters of an inch (size of a dime) in diameter or larger.

Out of the estimated 100,000 thunderstorms that occur in the U.S. each year, only 10 percent become severe, but account for most of the damage and loss of life.

Thunderstorms typically last less than a half an hour, but at times they form into long-lasting lines, known as squall lines, or grow to become powerful supercells that can last for hours and spawn killer tornadoes.

Each state in the USA is prone to thunderstorms and the first step in staying safe is to stay tuned to the latest forecasts. If thunderstorms are possible in your area, keep up with forecasts. Plan your day accordingly, which may include postponing or canceling your outdoor activities. Severe thunderstorms might also prompt the National Weather Service to issue watches and warnings for tornadoes and floods.

When forecasters at the nation's Storm Prediction Center in Norman, Okla., determine that severe thunderstorms are possible in your area, a severe thunderstorm watch will be issued.

What To Do During A Thunderstorm:

Indoors...

Stay off the phone

Stay away from windows

Use flashlights if the power goes out

Discontinue use of phones and electrical equipment. You may also want to unplug appliances and computers.

Avoid taking a shower or bath. If lightning strikes your house it may send a current of electricity across metal plumbing throughout the house.

While Driving...

Reduce your speed

Pull off to the shoulder of the road. Be sure you're away from tall objects, such as trees, which could fall due to wind or lightning and do not clog highway underpasses.

Turn on your emergency flashers

Remain in the car until the storm passes

Do not touch any metal objects in the car

Avoid driving on roads covered by water

Outdoors...

Stay low

If possible, find shelter in a building

Keep away from trees, tall objects, metal objects and water

Boaters and swimmer should get to land as a soon as possible

If you're in a group caught outside, spread out.

If you begin to feel your hair stand on end, this indicates lightning is about to strike. You should drop to your knees and bend forward placing your hands on your knees and crouch down. Do not lie flat on the ground, this will only make you a larger target.

Resources: Lightning Science & Safety

During every minute of every day, roughly 1,800 thunderstorms are creating lightning somewhere on Earth. Though the chances of being struck by lightning are estimated at 1 in 700,000, these huge electrical sparks are one of the leading causes of weather-related deaths in the USA each year with an average of 73 people killed; about 300 people usually are injured by lightning.

Your guide to tornado safety Tornadoes have affected every state in the USA � although some more than others � and, as history shows us, even metropolitan areas are not immune to twisters. The U.S. has more tornadoes each year � about 1,000 on average � than any other country in the world. A large percentage of these tornadoes occur in "tornado alley," where the ingredients for tornadoes come together most often. The links below will help you plan for a tornado and then react in safe ways when one threatens, if one hits, and after a tornado. Before the tornado During the tornado After the tornado Tornado alerts Forecasters watch the atmosphere for signs of tornado ingredients coming together. When tornadoes are a possibility, a tornado watch is issued for the area.If a tornado is spotted by radar or by a person, a warning is issued. But these watches and warnings are only beneficial to those in harm's way if they are heard. Tornadoes at night often kill because people aren't aware of the approaching danger. One solution is to own a weather radio that sends out a tone alert and a warning message. (Related: NOAA Weather radio) A tornado's strength is rated after it hits, unlike a hurricane whose intensity is rated before it strikes land. A team of meteorologists and engineers survey the damage path, measuring track length and width, and study the most severe damage to estimate tornado winds. These factors will determine its ranking on the Fujita tornado intensity scale. More information about tornadoes, tornado safety The basics of tornadoes How tornadoes are classified Overpass: puts people in harm's way Home shelters can save lives Resources for learning more about tornadoes

5:20 p.m. is the time on Saturday, May 10, 2008 that the storm sirens sounded in Picher
A full six minutes before the National Weather Service station at Tulsa issued a tornado warning for the area.
175 mph: The wind speed of the EF4 tornado that hit Picher and then went on to Newton County.
1 mile: The estimated width of the tornado that hit Picher at its widest point.
150 plus: The number of people injured in Picher.
300: The number of structures in Picher that were destroyed.

A Closeup View Of Tornados...