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The risk of flash floods during prolonged heatwaves has been starkly illustrated by a simple experiment.

In a video lasting barely over a minute, Rob Thompson, who is part of the University of Reading’s meteorology department, places three upturned glasses of water on different grass surfaces to demonstrate what happens when it rains after a drought.

On the left, depicting wet but not waterlogged grass, the cupful of water soaks into the soil within seconds.

In the middle, representing conditions typical during a ‘normal’ British summer, the water takes a little longer to drain away.

But in the final clip on the right, filmed during the current heatwave which has led to near-drought conditions, the water does not leave the cup – unable to penetrate the hard, parched soil.

Dr Thompson said: ‘Britain desperately needs rain to break this drought.

‘But we should be careful what we wish for.

‘Experience around the world has shown what can happen when heavy rain follows a very dry and hot period that has baked the soil hard.

‘The water can’t soak in easily, most of it just runs straight off the surface, which can quickly turn into flash floods.

‘If you’re praying for rain, you should pray for two days of drizzle, as dreadful as that sounds.’

The Met Office retweeted the post and added: ‘Dry ground takes more time to soak up water following a #heatwave than if it were during a normal summer.

‘This experiment shows how heavy rainfall following an extended period of extreme heat could lead to flooding.’

Temperatures have continued to soar across the UK and a drought is expected to be declared for some parts of England.

The National Drought Group – made up of Government and agency officials, water companies and other groups such as the National Farmers’ Union – is set to meet on Friday to discuss the prolonged dry weather.

There are expectations drought could be declared for the most affected areas of England in the south and east, after the driest July on record for some areas and the driest first half of the year since 1976.

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