SHEFFIELD STORM: What caused last night's freak weather?

Sheffield weather expert Professor Ian Rotherham.
Sheffield weather expert Professor Ian Rotherham.
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From Sheffield's hottest September day to a storm of biblical proportions in a matter of hours - how did it happen?

One minute, we were roasting in unseasonably warm September temperatures, the next watching in disbelief as the heavens opened and the city was struck by one of the worst storms it has seen in years.

But what was behind yesterday's mixed bag of weather?

Professor Ian Rotherham, a climate and environment expert at Sheffield Hallam University tells us.

"It is basically all down to the jet stream.

"The jet stream is a channel of air at high altitude, the movement of which powers everything in the lower atmosphere and which gives us our weather.

"It crosses the Atlantic Ocean - and moves up and down - and depending on where it is depends on the type of weather we get.

"It is currently pulling warm air up from the south - the reverse of when we had extreme snowfall a few years back, when cold air was sucked up.

"Sometimes, when the jet stream hangs, the weather stops the same. Its like when your computer is loading and sticks - there's nothing you can do."

"Then when it moves, the weather changes again.

"The high temperatures and humidity and the unseasonably warm temperatures, as well as the extreme thunderstorms and torrential downpours are all tied into that.

"Unfortunately, the "tornado" many said to have witnessed in Sheffield last night wasn't - it was just an extreme thunderstorm. Tornadoes are normally focused in a very thin strip, about 200-300m across.

"We do get tornadoes, but they tend to form in wide, flat open areas, rather than residential and urban areas where the conditions are right. About 30-40 years ago there was a tornado on the edge of the Peak District which ripped up trees.

"The storm last night was created by ice particles moving up and down in the upper atmosphere and creating a huge surge of energy which is then released in electrical activity, in other words the lightning bolts that we see.

"We are forecast to have similar hot weather for the rest of the week - so chances are we could get storms again."