THE cause of the torrential rain and freak hail storm over Sheffield at the weekend has been revealed - and it’s partly down to the city’s location.
A massive downpour - accompanied by huge hailstones up to three centimetres in diameter - fell at around 3pm on Saturday, flooding roads and creating major traffic problems.
Cars came to a standstill on the Sheffield Parkway, and Penistone Road, Hillsborough, Parson Cross and Walkley were also awash with rainwater.
Readings taken by radar equipment show up to an inch-and-a-half of rain fell between 3pm and 4pm.
But some parts of the city escaped the extreme weather - a phenomenon which forecaster Paul Knightley, from MeteoGroup, said was due to light winds on the day of the deluge.
“Trying to find a trigger for these sorts of events is not always easy,” Paul said.
“I think it’s a combination of a few things, such as how the ground around the city is. The moors south-west of the city aid the lifting of air a bit more.”
Paul explained that hot and cold winds would have met in the Peak District, with the warm sunshine increasing the heat and moisture in the air.
“These ingredients came together over Sheffield - the lift from the hills, the sunshine, and the cold air in the upper atmosphere made the conditions moister,” he said.
“They came together in sufficient quantities over Sheffield to create a thunderstorm.”
He continued: “On Saturday the winds were very light, and kept shifting up from the south and the east, so it repeatedly developed over the same place and sat in one spot. The storm was very slow-moving.
“In a nutshell, it was the humidity near the ground surface, the uplift from the moors, and converging winds.”
Paul said the large hailstones were formed in freezing clouds around two-and-a-half kilometres above ground, and remained unusually big as they had only a short distance to plummet from the sky.
“The hailstones didn’t have too much warm air to fall through on the way down, so there wasn’t a great deal of melting. Generally the more unstable the conditions the bigger the hailstones will be,” he added.
Paul said readings taken using radar systems can be used to calculate the amount of rainfall in a given time.
“Between the hours of 3pm and 4pm, 42mm fell, which is an inch and a half in an hour. It was an intense rainstorm.
“The city also lies in a bowl, so all the rainwater has trouble going down the drains.”
The best of the hot weather has left us for the moment, Paul said. Temperatures are expected to hit up to 14 degrees Celsius today. Warmer air is set to move in again from the east tomorrow, but Friday - the day of the Royal wedding - will see a brisk wind and top temperatures of 17 degrees.