The people of Moore, a suburb of Oklahoma City, can count themselves incredibly unlucky to have felt the full force of the tornado which struck on Monday afternoon.
Just one percent of tornadoes reach category 4 on the Enhanced Fujita Scale, but that is what happened in Moore. ‘Devastating damage’ is expected in such a twister and early reports from the city would certainly back this up.
What made this tornado particularly devastating, apart from its incredible strength, was its size. A typical EF4 tornado has a width of half to one kilometre. This system seems to have covered close to three kilometres and it tracked across a densely populated area.
Astonishingly, Moore has experienced an even more powerful tornado in the not-too-distant past. In the same month back in 1999, an EF5 tornado ripped through the area for around 85 minutes. During that period it killed 48 people and caused $1 billion worth of damage. That storm produced the highest wind speeds ever recorded on Earth; Doppler radar deduced a wind speed of 486kph.
Although tornadoes occur throughout the year in the US, May is the peak month.
During this month the contrast between colder air moving southwards from Canada and across the Plains, and the warm, moist air from the Gulf of Mexico, is at its greatest. Such conditions are perfect for breeding the supercell storm clouds from which tornadoes may develop.
Yet other factors are necessary to turn the chance of an isolated tornado into a significant outbreak, and those factors were in evidence on Monday afternoon.
Hot, dry desert air from the southwest US is a contributing factor, as is the jet stream. This latter is of major significance.
The jet stream marks the boundary between warm and cold air and consists of bands of wind, blowing at more than 200kph, at an altitude of around 10km. Its significance is that it provides ‘directional shear’, a change in wind direction with height, which causes the clouds to rotate, thereby encouraging tornadoes to spawn.
Oklahoma City lies within ‘Tornado Alley’, a broad region lying between the Rockies in the west and the Appalachians in the east. Within this zone, the greatest concentration of tornadoes generally occurs in the states of Texas, Oklahoma, Kansas, Missouri, Nebraska and South Dakota.
These definitions are open to considerable interpretation and lines of tornadoes are inclined to develop along the Ohio, Mississippi and Tennessee river valleys.
People are entitled to ask whether it is safe to live in a city which has seen two killer tornadoes in 25 years, but statistical analysis suggests that an EF4 or EF5 tornado would be expected to touch down within 40 kilometres of Moore only four times per century.
The residents of Oklahoma City, and Moore in particular, must surely feel that they are due a change in fortune.