Atmosphere And Weather

How Tornadoes Form



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As another spring thunderstorm marches across the sky, it begins to worsen. The dark, menacing clouds start to spin. Suddenly the sound of the storm is drown out by what many compare to the sound of a freight train. It's a twister, more technically known as a tornado, dropped from the sky and cutting a path of destruction.

Tornadoes can occur at anytime of the year. The majority form in May however, with them more severe ones occuring in April. Whenever the conditions are right, a tornado can form.

When cold dry polar air meets warm moist tropical air a thunderstorm develops. It is during huge rotating thunderstorms called supercells that tornadoes can form. These severe storms often grow to over 4000 feet and last much longer than regular thunderstorms.

The wind coming into these supercell storms sometimes start to swirl and form a funnel. How the air begins to rotate is n ot completely understood. One way it appears to happen is when winds at two different altitudes blow at two different speeds creating a wind sheer. If a column of air gets caught in a supercell updraft the updraft tightens the spin and it speeds up creating a funnel cloud. As the air in the funnel cloud spins faster, it creates a very low pressure area, sucking more air into it. Rain and hail in the storm causes the funnel to touch down. When one of these violently rotating columns of air reaches from the storm to the ground, a tornado has formed.

Globally, the middle latitudes between 30 and 50 degrees North and South are most favorable for tornadoes because this is where cold polar air meets warm tropical air. The air in these latitudes often flow at different speeds andin different directions at different levels of the troposphere. These wind sheers make it more ideal for development of rotation within a storm cell.

Each year there are an average of 1,000 tornadoes of varying size in the United States, followed distantly by Canada with 100 per year. Although North America does tend to have the most twisters, it certainly doesn't corner the market. There are also frequent sightingsin Northern Europe, Western Asia, Bangladesh, Japan, Australia, New Zealand, China, South Africa and Argentina. Relative to it's land mass, the United Kingdom has more sightings than any other country, but most are relatively weak.

Tornadoes can take several forms:

-Supercell tornadoes from from supercell thunderstorms and are someof the most violent. They are most common in the central part of the United States.

-Waterspouts resemble tornadoes but form over warm tropical waters. The funnel itself is made of fresh water as a result of condensation. They are rarely more than 50 yards wide andbreak up when they reach land.

-Landspouts are the same as waterspouts except they form over land. They are weaker than a supercell tornado and are not associated with a wall cloud or a mesocyclone(area of rotation of storm size that may often be found in the south-western part of a supercell).

-Gustnoda's are weak and usually short lived. They form along gust fronts (leading edge of cool gusty surface winds produced by storm down drafts).

Damage caused by the tornado depends on the size and wind speed. The most violent twisters are capable of tremendous destructionwith windspeeds up to 300 mph. They are capable of destroying largebuildings, uproot trees and hurl vehicles hundreds of yards. Their path of damage can exceed one mile in width and be up to 50 miles long.

Scientists use the Fugita Scale to rate tornadoes. It is sometimes difficult to get a full understanding of the destruction these wonders of nature are capable of when all you hear is, "F2 tornado touched down".

Fugita Scale and example:

F0- Gale

- Winds less than 73 mph (116 kmh).

-Can cause damage to chimneys, sign boards, breaks branches off trees and over turn shallow rooted trees.

F1- Moderate

- Wind speed73-112 mph (117-180 kph).

-Peel surfaces off roofs, push mobile homes off foundations or overturn them, push cars off the road.

F2- Significant

- Wind speed 113-157 mph (181-253 kmh).

-Cause considerable damage, tear roofs off light framed houses, demolish mobile homes, overturn boxcars, uproot or snap large trees, lift cars off the ground, turn light objsects into missles.

F3- Severe

- Wind speeds 158-206 mpmh (254-332 kmh).

- Tear roofs and walls off well-constructed houses, uproot trees in a forest, overturn entire trains, throw cars.

F4- Devastating

- Wind speeds 207-260 mph (333-416kph).

- Level well-constructed houses, blow structures with weak foundations some distance, turns large objects into missles.

F5- Incredible

- Wind speeds 261-318mph (417-509 kph)

-Lift and blow strong houses, debark trees, cause car shaped objects to fly through the air, and cause incredible damage.

F6-Inconceivable

-Wind speeds over 318 mph (509 kmh)

- To date has never been recorded and very unlikely. It would be very difficult to measure because there would be no objects left to study.

Researchers continue to study all aspects of tornadoes. Parts of their formation is still a mystery. The more they are studied, the more chance of forming better early warning systems.

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