A storm surge is a change in sea level that is caused by a storm.
They can lead to extensive flooding and are dangerous for people living in many coastal areas.
The main cause of a storm surge is high winds pushing the sea water towards the coast, causing it to pile up there.
There is also a smaller contribution from the low pressure at the centre of the storm "pulling" the water level up which is called the inverse barometer effect, and is similar to what happens when you drink through a straw.
The strong winds in the storm create large waves on top of the surge which can cause damage to sea defences, or spill over the top adding to the flood risk.
The height of a storm surge depends on the size and strength of the storm, the direction it approaches the coast, and the shape of the coastline and seabed.
In areas with large tides the timing of a storm surge is particularly important and just a couple of hours' difference may mean the difference between an area being flooded or staying safe.
In the night of January 31 1953 a storm in the North Sea caused a storm surge which happened at the same time as a high spring tide.
Although the storm and surge were forecast in advance, public warning systems were not very effective and many people were not prepared for the flooding.
More than 2500 people were killed around the North Sea coastline, including 307 in England and 19 in Scotland.
Flooding also caused lots of damage to people's homes and businesses and ruined large areas of farmland.
Afterward the UK Government invested much more in improved sea defences, such as the Thames Barrier, and effective warning systems.