A meteorological tsunami in the Netherlands is a tidal wave

Occasionally there is a kind of meteorological tsunami in the Netherlands: a higher tidal wave than the average flood height. The high tidal wave or meteorological tsunami puts the beaches under water and then drags everything that goes with it into the sea. The meteorological tsunami or mini-tsunami is caused by the still cold sea where several thunderstorms are active. In advance there are tropical temperatures where the wind comes from the southwest. The clouds that precede a thunderstorm are ominous but magnificent to see them coming.

The difference between a tsunami and a meteorological tsunami

A tsunami is an unexpected tidal wave from the sea, the most common cause of which is an earthquake in the bottom of the sea. The movement of different terrestrial scales makes this possible in the Indian and Pacific Ocean, but in the Atlantic Ocean, of which the North Sea is a border sea, this never happens. Nevertheless, an unexpected high tidal wave occasionally occurs. About eight times a year, tidal waves occur more than 25 centimeters higher than normal, up to a height of two meters. This is called a meteo-tsunami or seiche and causes sudden air pressure changes during the storming thunderstorms from the sea. These meteo-tsunamis occur mainly in the spring, when the sea water is still cold.

Tidal wave, spring flood and storm flood

A tidal wave or tsunami starts like a wave on the open sea and shallow water makes the wave bigger and bigger, up to twice as big as the start of the wave. This wave can start as a result of an earthquake on the seabed (tsunami) or as a result of the weather (meteo-tsunami). Under the influence of the moon, sun and earth, the tide (the movement of ebb and flow) can become very high, so-called spring tide or spring tide. The water along the coast in the Netherlands, for example, is then pushed up high. When it storms during a spring flood and the water is extra high, we speak of a storm flood. The flood of 1953 is the cause of the flood disaster in Zeeland when several dikes broke. Jumping tides and storm tides, however, do not cause tsunamis.

The emergence of storm clouds

When tropical days are measured in the Netherlands, it is thirty degrees or more. This is often the case with a southwest wind, a heat front. In tropical and hot weather, various storm clouds that form before and along a cold front are often called a 'squall line' or a quasi-linear convective system (QLCS). This is a cold front with lightning discharges, hail and a lot of wind.
Due to the earlier heat, the air rises above the North Sea, because warm air has a smaller density than cold air when it contains enough moisture (convection). The varying layers of air create the different storm clouds and the air pressure becomes very variable. Because the warm air starts to rise, it forces the cold air flow down, which pushes the sea water down. This creates a wave that gets higher with an erratic and shallow seabed. This phenomenon is visible on radar images and this is called a bow echo or bow echo. Thunderstorms can be accompanied by ominous clouds, namely a shelf cloud or rolling cloud.
A shelf cloud (literally plank cloud) is a cloud that precedes the thunderstorms. You often see the thunderstorm coming behind the shelf cloud. The shelf cloud arises when during heavy thunderstorms the warm air in front of the cloud rises sharply while a cold air mass falls down on a higher cloud. This is accompanied by gusts of wind such as a whirlwind or a downwind.

Roller cloud

The rolling cloud is created when colder air that comes with the thunderstorm comes into contact with the much warmer air on the surface of the earth from some height. The cold air then rolls up the warm moist air, which condenses into a cloud and rolls forward. A rolling cloud differs from a shelf cloud because rolling clouds are completely separate from the base of the thunderstorm or other clouds and roll horizontally around their own axis. A rolling cloud is a rare phenomenon.
Under the shelf cloud

Falling air pressure and a storm nose

The air pressure is measured with a barometer with the unit being hectoPascal (hPa) or millibar. The air pressure on the earth's surface is usually between 940 and 1060 hPa and depends on low and high pressure areas. Wind blows around high and low pressure areas. Around a low-pressure area counterclockwise and at a high-pressure area clockwise. During a thunderstorm, the air pressure can fall below 900 hPa and the wind from a quantity of cold air falls, as it were. A drop in air pressure below 900 is called a thunderstorm in meteorology because it gives the representation on paper a peak due to the drop and rise, just like a nose.

The wind

In addition to falling air pressure, the wind also plays a major role in the creation of a tidal wave in the North Sea. Due to the fast-moving thunderstorms from the southwest, the wind is blowing hard. For the storm clouds, however, a northeast wind blows and in the storm clouds a southwest wind blows. After passing the storm front, the wind then blows again from the northeast.

The sea

A strong air pressure difference is created by the wind when approaching and passing thunderstorms. Because the sea is still cold in the spring, a cold layer of air above the sea and a warm layer of air at a higher altitude give the start of the tidal wave.

Shallow water

The North Sea has an average depth of 30 meters, which is not deep compared to other seas, with an increasing bottom towards the coast. The bottom is covered with sandbanks and boulders. Due to the occurrence of the tidal wave due to weather conditions on the open sea, the wave rolls towards the coast and this becomes increasingly higher due to the shallow water. The cause is the friction that the wave experiences from the shallow and erratic seabed and so the wave gets higher and higher. In this way, the seawater can reach a height of two meters in a short time and flood the beach like a meteo tsunami; a phenomenon that is also caused by a seaquake during the tsunami, but where the tidal wave is up to ten times higher than the original tidal wave.

Damage in a tsunami

That a tidal wave or tsunami can cause a lot of damage is clear from the tsunami in 2004 during Boxing Day. A 9.3-magnitude quake on the Richter scale in the Indian Ocean west of the Indonesian island of Sumatra preceded the tidal wave or tsunami. Three large tidal waves emerged that landed along the coastal areas from Sumatra, Thailand to Sri Lanka, India and Somalia. The result was around 290,000 deaths, entire cities were destroyed and there was a huge mess.

Damage during a meteorological tsunami

A sudden tidal wave during a meteorological tsunami can be as high as two meters. A tidal wave caused by the weather in the Dutch coastal area increases the water level on the beach. Beach beds are swept away or damaged by the sudden rise of the water, gangways are dragged along and wooden houses are moved with nasty consequences afterwards.

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