Flood Alert: This Fascinating Video Shows Why Heavy Rain Can Be So Bad After A Heatwave

(Photo: University of Reading)
(Photo: University of Reading)

(Photo: University of Reading)

Thunder, lightning and flooding have been forecast across parts of the UK as the heatwave gives way to heavy rainfall.

So far, 33 flood alerts are in force across Britain, according to England’s Environment Agency, the Scottish Environment Protection Agency and Natural Resources Wales.

Much of Scotland has been hit by flood warnings, while in London, there’s concerns the Thames will overflow and wreak havoc. Mayor of London Sadiq Khan has said Transport for London has plans to “ameliorate any consequence of flash flooding”. If following flooding of the Tube and other networks last year.

But why is heavy rainfall so risky after a heatwave? We keep hearing about a drought, so isn’t it a good thing?

There’s been a serious lack of rain recently, leading to hosepipe bans in some parts of the country. And usually after a few days of sun, rain eventually follows.

But while a little bit of rain is good news for the environment, this video from the University of Reading shows that a lot of rain could be a problem.

In an experiment, Dr Rob Thompson demonstrated how heavy rainfall after a drought can lead to flash floods.

In the video, he puts a cup of water on wet grass, normal summer grass and grass after a heatwave. The cup of water after a heatwave doesn’t absorb into the grass compared to wet grass and normal grass.

One Twitter user asked Dr Thompson why the dry soil hardly absorbs any water.

“It gets compacted through the lack of water,” Dr Thompson replied. “But there’s a big factor from the small soil particles becoming hydrophobic (like the coating on waterproof coats) – which combines with water’s very strong surface tension meaning water molecules can’t fall through the small gaps.”

(Photo: Ali Majdfar via Getty Images)
(Photo: Ali Majdfar via Getty Images)

(Photo: Ali Majdfar via Getty Images)

Another user commented: “It may feel counter-intuitive, but dry ground just doesn’t absorb water as quickly. So as climate change exacerbates heatwaves and droughts AND heavy rain, flooding becomes a more dangerous probability.”

To which Dr Thompson replied: “Add to that – warmer air holds more water, so heavy rain is heavier... that’s a pretty bad combination and makes flash flooding an increasing threat.”

This article originally appeared on HuffPost UK and has been updated.