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Rise of the Superstorm


Rise of the Superstorm
Every year around 1,200 tornadoes touch down in the USA. Most occur in a region nicknamed Tornado Alley, with Texas, Oklahoma and Kansas at its core.
The most destructive of 2013 was the Moore Tornado, which touched down at 2.56pm CDT on 20 May, near Newcastle, OK. It was on the ground for 40 minutes and drew a 27-kilometre (17-mile) path through the state, 2.1 kilometres (1.3 miles) across at its widest point. Wind speeds were in excess of 322 kilometres (200 miles) per hour, placing the tornado in the highest category on the Enhanced Fujita (EF) Scale: EF5. Tornadoes of this class cause near-total devastation, levelling multistorey buildings, tearing homes from their very foundations and lifting asphalt from the roads.
North America has unique geography, which provides a deadly spawning ground for storms and tornadoes. The Rocky Mountains extend from north to south along the west side of the continent. As wind travels over the Rockies, it becomes cold and loses moisture via rain and snow, producing cool, dry air at high altitudes.
When this air hits warm, humid air from the Gulf of Mexico water vapour condenses and forms storm clouds. This releases huge amounts of energy, causing atmospheric instability. On 20 May 2013, severe weather warnings were issued for Oklahoma; a polar jet stream came over the Rockies into the southern Great Plains, and simultaneously a low-pressure system moved over the Upper Midwest region.

Differences in wind speed at different altitudes – known as wind shear – caused the air to spin, circulating in a horizontal vortex, and in combination with moisture and atmospheric instability. At 2pm CDT, this led to the development of a thunderstorm containing persistent, rotating mesocyclones.
Mesocyclones powerful enough to generate tornadoes often result in hailstorms. Updraughts of warm air carry water droplets high into the atmosphere, where they freeze before being carried downwards by cold downdraughts. If they become caught in an updraught again they will refreeze, adding a new layer of ice. This process can repeat several times, generating hailstones that are the size of golf balls or even larger. Oklahoma was pelted with hail as the storm intensified. If there is sufficient updraught to tighten the central vortex of a mesocyclone it begins to twist, resulting in a powerful vertical column.
The inward and outward airflows cause a drop in pressure at the centre, and form what is known as a dynamic pipe. At the core of the vortex, the pressure is lowered, which sucks in more air, causing the column to lengthen and extend down towards the ground.
A tornado warning was issued in Oklahoma at 2.40pm, and the tornado that ravaged Moore touched down 16 minutes later. It started out as a weak EF0 twister, capable of only minor damage to roof shingles, trees and guttering, but within ten minutes it had intensified to EF4.
EF4 tornadoes have extremely destructive winds of up to 322 kilometres (200 miles) per hour and, on its path to the city of Moore, it severely damaged a bridge and killed nearly 100 horses at the Orr Family Farm.
Once in the city, the storm intensified to EF5 – the highest rating for a tornado – and reduced many buildings to rubble. It lost its peak strength and returned to EF4 classification, but the intensity of the storm caused a great deal of damage: 13,500 homes were destroyed, or damaged, affecting 33,000 people, 24 people were killed and hundreds more injured.
The tornado continued to weaken until it eventually dissipated at 3.35pm, about eight kilometres (five miles) east of Moore.

Rise of the Superstorm