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Killer twister's 40 minutes on ground made it so destructive

THE Oklahoma tornado ranks among the most destructive in history because of the length of time it remained on the ground and the path it took.

For nearly 40 minutes it travelled 20 miles across largely urban areas, sweeping up debris, trees and homes in winds estimated to have reached 200mph.

It was classified as a top-of-the-scale EF-5 twister with winds of at least 200 mph (322 kph) The upgrade was based on the enormity of the damage.

Yet the area is no stranger to storms of this magnitude.

In 1999 the city was hit by the most intense tornado ever recorded, with winds that reached 318mph. It caused 46 deaths and $1.5bn (€1.2bn) worth of damage.

It sits in the heart of what is known as Tornado Alley, an area of the Great Plains that experiences more tornadoes per year than anywhere else in the world.

At the height of the season, between May and June, there are two to four each day.

Each spring, moist warm air sweeps north from the Gulf of Mexico to meet cold dry air from the Rocky Mountains, creating the conditions needed to trigger tornadoes.

The collision between the airstreams creates an updraft that can begin to rotate as the winds change direction.

It is thought that a shift southwards in the jet stream – a current of air high in the atmosphere – which brought unseasonably cold spring weather to Ireland also brought a slow start to the tornado season.

The jet stream recently returned to its normal position, allowing warm air north with devastating results.

Despite being among the most violent meteorological phenomena, tornadoes are still poorly understood and highly unpredictable.

Meteorologists will often have only a few minutes' warning before one forms.

Most will last for an average of 10 minutes and often pass harmlessly through rural areas, especially on the Great Plains where there are large expanses of open land.

Some geographical features make tornadoes more likely by causing winds to create a vortex, but there is nothing unusual about the area around Oklahoma.

Meteorologists have reported being able to see debris picked up by the latest storm in radar images as it headed into Moore, giving an indication of just how powerful it was. It has been rated as a four on the five-point Enhanced Fujita Scale, making it the second most powerful type.

The scale measures the damage to buildings and infrastructure caused by a tornado as the wind speed often has little bearing on the harm it causes.

There are some fears that tornadoes will become more common as global temperatures change, but scientists say it is impossible to attribute the Oklahoma tornado to any long-term climate change.

The National Oceanic and Atmospheric Administration's National Weather Service issued more warnings of severe weather this week, and has placed Oklahoma and central Texas on tornado watch. (© Daily Telegraph, London)

Irish Independent