In the case of the dramatic flooding which has hit the country this week, deep low pressure has formed over the North Sea.
This is the result of warm air from the Atlantic meeting cold air from the Arctic which has been sweeping towards the UK.
As low pressure builds it reduces the weight of air on the surface causing the water to bulge, as the low pressure moves winds form which push this rising body of water along.
In this case it has been pushed towards the British and European coasts and into the English channel, a much narrower body of water.
Water has backed up before the channel leading to levels around the east coast of England to rise further.
Coincidentally this has happened around the time of high tides which is why huge surges of water have developed and reached high enough to breach defences.
The surges have been further fuelled by very strong winds which are battering parts of the North and the east coast, worsening the problem.
Experts say that although the surges are not a result of the winds, very strong gusts have made things worse.
Phil Rothwell, head of flood strategy at the Environment Agency, said: “Sea surges are not caused by the winds, but in this case they have made the situation much worse.
“Low pressure has built around the North Sea and pushed water downwards which has backed up at the English Channel.
“Water just builds and builds as the pressure moves and eventually rises to cause a surge, bringing the risk of water coming over the top of defences.”
He said regions most at risk are Norfolk, Great Yarmouth, Ipswitch, North Kent and the Thames Estuary.
There is another surge expected along the east coast tonight (Friday) after which things are expected to settle down.
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