Over 1,000 tornadoes hit the US every year. This image shows the tornado forming over Oklahoma City, May 10, 2010

Spring in the northern hemisphere officially starts on March 20. This is the Equinox, when the sun creeps across the equator on its journey northwards. With just a few days of winter left, we are in the tail end of the season, and we have started to see the weather change across the US.

The cold wintry air from Canada is now jostling with much warmer air coming up from the Gulf of Mexico. As these weather systems meet, there is an explosive result: towering thunderstorms which may turn severe.

A thunderstorm is classed as severe if it produces hail over 1.9cm in diameter, has winds of at least 93km per hour (kph), or produces a tornado. This latest storm system has certainly produced some severe weather through many parts of the US.

Torrential rains have brought flooding to parts of Ohio. Here the ground was already saturated and the heavy rain has swollen the river levels further. In some places the flood warnings are expected to remain in force until Wednesday, March 8.

Further south, Louisiana reported a number of tornadoes. One struck the town of Rayne, causing a trail of devastation 300 metres across and 8km long. As the winds screamed over 180kph it ripped apart trees and houses and hurled them into other buildings. One person is reported dead after a tree smashed into her house, eleven more are injured.

Wind speeds in a tornado are only estimates, very rarely are they actually measured values. The tornado will be classed from 1 to 5, based on the damage that it’s caused and this will be used to estimate the wind speed. As of February 2007, the scale used to classify tornadoes is called the Enhanced Fujita Scale.

The original Fujita Scale was modified after several papers argued that the wind speeds were over-estimated.The tornado which struck Rayne was classed as an EF2, which means that the three-second wind gusts were between 179 and 218kph. A storm of this strength will tear roofs off even well-constructed houses, completely destroy mobile homes and lift cars off ground.

The storm system that produced these tornadoes is now heading east, but there will be more in the coming weeks and months. The peak season for tornadoes is between April and July.

Source: Al Jazeera