Even as trucks haul away debris and the search continues for the missing residents of Moore in Oklahoma, the one certainty is that more storms will follow in this part of the US known as Tornado Alley.
“Tornadoes are quite common in the Great Plains in May, averaging about three to four per day, often in clusters, but not usually as strong as this one and not usually in urban areas,” said Andrew Barrett of the department of meteorology at the University of Reading.
The town of Moore outside Oklahoma City suffered a category four storm in the five category Enhanced Fujita Scale, according to the US National Weather Service. It blasted the town with winds of up to 320kph and generated an exceptionally wide funnel – as much as 3km across – as it scoured the Oklahoma countryside. Such powerful storms are rare, however, with only 2 per cent reaching such intensity, Dr Barrett said.
There is no weather phenomenon to match a tornado for the ferocity of its wind speeds, said Gerald Fleming Met Éireann’s head of forecasting. Category five storms can reach wind speeds of between 350kph and 500kph.
The US midwest is known as Tornado Alley because it lies at a place where moist warm air coming north from the Gulf of Mexico encounters cold dry air moving south from the Rocky Mountains and Canada, said Peter Lynch, professor of meteorology at University College Dublin.
This difference in temperature generates energy as moisture is released and wind speeds rise. It often triggers cumulonimbus clouds, the towering anvil-topped forms that deliver thunder storms, he said.
A circular movement can form as energy is released. “If you have convergence with air flowing towards a central point it starts to turn faster and you get violent circulation,” Prof Lynch said.
Funnel clouds
Mr Fleming said there are powerful vertical movements of air within these thunder clouds which can trigger small funnel clouds at the underside of the cloud mass. They are nothing more than funnel clouds unless they begin to lengthen as they approach the ground. They are called tornadoes only when they touch the ground.
We don’t suffer the damaging tornadoes seen in the US but we regularly see funnel clouds and sometimes tornadoes here. “We are appearing to get more tornadoes, but we believe [this may be because] people are on hand with cameras to capture them,” he said.
“They are not common here but we had one on October 17th last year over Dublin,” said Ruth Coughlan in Met Éireann. It occurred above Crumlin and moved across Inchicore and Kilmainham towards the Phoenix Park.
The tornado produced a funnel 40m across and touched the ground along a track 2.75km long, she said.
