As their name suggests, flash floods strike quickly without warning.
A few hours - or even minutes - of heavy rain can leave a trail of destruction behind it as an area quickly becomes submerged in water.
They are at their most devastating when torrential downpours hit low-lying areas after a prolonged dry spell.
When the earth is baked hard by the sun in hot weather, it makes it more difficult for the water to filter down through the soil.
More rain falls than can be absorbed by the ground and water begins to flow downhill, washing away anything in its path.
Huge volumes of water collect in becks and streams, eventually flowing into rivers causing them to swell out of control.
This is what happened in North Yorkshire where a month's worth of rain poured on to the moors in just three hours.
This caused the River Rye to burst its banks at such a speed that motorists were forced to abandon their cars.
In the badly hit areas, more rain fell in a few hours than had done in the whole of June so far.
Thunder clouds emptied 69.4mm (2.5in) of rain on the town of Hawmby not far from worst-affected Thirsk.
At nearby Topcliffe, 40.4mm (1.6in) of rain was collected by Met Office rain gauges in three hours.
Some 29.2mm (1.15in) of that was in a single hour between 1700 and 1800 BST on Sunday - three times as much as would fall in an average heavy shower.
This led to villages on the valley floor becoming submerged.
The torrential downpour was produced by thunderclouds linked to the high temperatures.
"We had been predicting that there were going to be thunderstorms in that area. The huge amount of water came out of the thunderstorm," said Met Office spokeswoman Sancha Tetlow.
It was made worse by the dry conditions seen in the county in the weeks ahead of the deluge.
"It's like rain falling on a concrete surface," an Environment Agency spokesman said.
Mud slides often accompany flash floods
This was completely different from the floods in Boscastle, Cornwall, last year where water fell on sodden ground near the confluence of three rivers.
But the principle was the same - the water simply could not be absorbed quickly enough.
The Environment Agency said these kinds of flash floods are quite rare, but are likely to occur more frequently as climate change tightens its grip.
"More of these sorts of things are going to happen in the future. We will see more extreme weather events," the spokesman said.
"Over the past few years there has been a lot of intense rainfall and localised flooding similar to this linked to climate change."