A university professor has used a simple experiment to show how a downpour after a heatwave might not be such a good thing.

Rob Thompson, who is part of the University of Reading’s meteorology department, posted a video to YouTube showing him use three cups of water on different grass surfaces to demonstrate what happens when it rains after a drought.

He has shared the video to Twitter several times, once using it to question a weather report which said "we are fairly hopeful there could be some heavy showers to come on Monday", suggesting this was perhaps poor wording.

In the first experiment, a glass of water is put on top of wet grass and rapidly soaks into the ground, with the second experiment highlighting that water soaks into grass during a normal summer at a slower pace.

In the final experiment, the water is placed on to what appears to be dry grass and does not seem to budge, highlighting the potential for flash floods to occur if heavy rain follows a heatwave.

Met Office: Heavy rainfall after extreme heat could lead to flooding

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."

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Drought warning for parts of UK

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 (August 12) 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.