What caused yesterday’s severe thunderstorms?
Yesterday’s thunderstorms were remarkable, bringing torrential downpours of rain, large hail and flash flooding – not to mention frequent lightning.
The Midlands and north east England were worst hit with some locations having around 30mm, more than an inch of rain, in the space of just one hour. Northern Ireland was also hit again following Wednesday evening’s thunderstorms.
Flash floods were widely reported in these areas with drainage systems unable to cope with such a large volume of rain falling in such a short space of time.
Water that did manage to drain away caused a sharp and dangerously quick rise in river levels. The River Rede at Otterburn in Northumberland rose by almost 3 metres in just three hours as shown by the graph below from the Environment Agency.
Lightning strikes were captured by many people on camera, including this video from Marc Burton which shows the Tyne bridge in Newcastle being struck.
What caused these thunderstorms?
The answer lies in the weather set up that was in place across the UK yesterday. Two crucial ingredients were present – a plume of humid, moist air and a cold front sweeping in.
Parcels of air in our atmosphere only tend to rise when they are warmer than the surrounding air, so when the moist, humid air was solely present, all of the air was more or less at a similar temperature.
However, when the cold front arrived, the injection of cooler air at the surface suddenly led to the humid, moist air being relatively much warmer and wanting to rise rapidly.
In a nutshell the interaction of these two ingredients lead to the sudden destabilisation of the atmosphere, causing air to rise rapidly upwards into the sky and form cumulonimbus clouds reaching around 30,000ft – the height at which commercial planes fly.
Effectively it was a large scale version of someone taking a lid off some cooking popcorn and, as we know, once the corn pops, things really start to get interesting without a lid on the saucepan!
Why did some places get hit badly whilst others close by saw little or nothing?
When thunderstorms occur, they tend to travel in the direction in which the wind is blowing at around 3km high in the atmosphere – something called the steering wind because, as the word suggests, it steers them in a particular direction.
However, in the atmosphere, the air can’t rise enough to form thunderstorms everywhere, so you often find that there are gaps between them where some places see little or no rain and sometimes just sunshine.
The video below from the Met Office shows how two thunderstorms developed yesterday. Both formed over Wales, with one heading across the Midlands towards Lincolnshire, and another heading across Cumbria and into Northumberland.
Notice how the clouds seem to explosively develop as the storms grow. The red, yellow and orange crosses indicate the lightning strikes that were recorded.
Are any more thunderstorms likely?
With low pressure staying over us for the next few days, some thunderstorms are likely across northern parts of the UK. However, they are not likely to be as severe as those experienced yesterday – largely because the supply of humid, moist air has been cut off.
Whilst showers will tend to become fewer and lighter as the weekend progresses, there’ll still be some scattered around. The sunshine will come out from time to time but it won’t be as warm with temperatures around 17-21C.