Most thunderstorms are associated with towering clouds known as cumulonimbus. The greatest storm surge on record for the North Sea as a whole occurred on 31 January and 1 February 1953. The highest waves wash away protective dunes, batter sea walls and break over coastal defences causing flooding. The opposite happens when pressure is above average, the sea level will fall. A deep depression, with a central pressure of about 960 hPa, causes the sea level to rise half a metre above the level it would have been had pressure been about average (1013 hPa). When air pressure decreases by one hectopascal, sea level rises by one centimetre.
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Storm surges are like big walls of water being pushed in from the sea in a 'surge' and caused by strong winds and low air pressure from a 'storm'. Sometimes, though, even when there is a lot of rain over a long period of time, we don't see any flooding along rivers, this is usually when the rivers were running low because there hadn't been any rain for a long time. Spring tides are very high tides and come much further inland than usual high tides and cause flooding. If heavy rain happens when the tides are high or the rivers are already full of water, flooding becomes more likely. However, snow melting can cause flooding in the UK, particualrly when we have big snow events, like in 2010/. In the UK most people don't get affected by flooding from snow melting very often, but in mountainous areas around the world it can be very common, particularly in spring when it can starts to get warmer quite quickly and the snow rapidly turns to water, which will start running downhill, so this can be a big problem for towns and villages in mountain ranges. Often when we have rain for a long period of time we see flooding in some places, usually along river banks and flood plains where there is already a lot of water on the ground, as well as places where we see flash flooding. When you have heavy rain for a short period of time that causes flooding, we call this 'flash flooding', because it can happen in a flash! This short-lived but heavy rain only usually causes flash flooding when there are other factors, for example, when drains in the road are blocked so that the water can't drain away, or at the bottom of steep hills as there is nothing to catch the water as it runs down the hill.
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a combination of high tides and high river levels.a large amount of rain - either from really heavy rain or rain over a long period of time.