Storm Ciarán rages over Europe as a bomb cyclone: ​​what is this rare weather phenomenon? And will it happen more often in the future? | Storm Ciaran

In recent days, meteorologists have regularly spoken of a so-called ‘bomb cyclone’ or a ‘weather bomb’ when talking about storm Ciarán. But what exactly do they mean by that?

It was two famous meteorologists, Fred Sanders and John Gyakum, who first used the term in 1980. At the time, there was criticism from European scientists that the term “bomb” was too reminiscent of war, to which Sanders said: “Then why do you use the word front?”

To find out the meaning of the meteorological term, you can divide it into two parts: bomb and cyclone. Cyclone is a synonym for a low-pressure area and bomb refers to its ‘explosive’ development. This means that the pressure in the core of the cyclone, expressed in hectopascals, drops rapidly. The limit for speaking of bomb cyclogenesis is a drop of more than 24 hectopascals in less than 24 hours. The lower the pressure, the stronger the wind can blow in the storm. This is because the rapidly rising air near the storm’s core must be replaced by air around the storm. The wind will therefore be drawn to the core of the storm at a higher speed, with all the consequences that entails.

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There are worldwide about 70 bomb cyclones per year. About two-thirds of these form in the Northern Hemisphere, mainly off the east coast of the United States and near Japan. Bomb cyclones near mainland Europe are rather exceptional. Although recently, in 2022, there was bomb cyclone Eunice.

That’s because for explosive cyclogenesis you need strong temperature contrasts. So along the east coast you have the difference between the warm Gulf Stream and the cool air from over land. This also means that most storms and therefore bomb cyclones will occur during the winter half-year from November to March when it cools down in the north and the south is still warm.

In the case of Storm Ciarán, we must also look to the Americas for the cause. Very cold air from the eastern US was blown into the warm, moist Atlantic Ocean. An active jet stream then launched it towards western Europe. That is a river of air at an altitude of about 10 kilometers. The jet stream was also responsible for the rapid deepening of the low-pressure area. This, as it were, sucked air from the core of the low-pressure area, causing that enormous drop and allowing a bomb cyclone to develop.

In 2015, Canadian scientists investigated in a scientific study what impact climate change would have on the development of future bomb cyclones in the Northern Hemisphere.

Their research showed that the so-called weather bombs will move more towards the North Pole, following the jet stream. This concerns a displacement of approximately 2.2° towards the north. This means that in the future we will encounter fewer and weaker bomb cyclones below the latitude of 45° and more and stronger ones above. Belgium is unfortunately above it at 50°. The scientists saw that overall the number of bomb cyclones would decrease by an average of 17% in the Atlantic Ocean. However, the biggest decline is on the east coast of the US, while they just saw an increase around the UK and the North Sea.

In a other study Scientists also saw that climate change will cause the number of bomb cyclones with a ‘sting jet’ to increase by 60%. This is a weather phenomenon in which the wind can accelerate significantly towards the earth’s surface, which can lead to extreme wind speeds during the storm. This was also the case with storm Ciarán.

However, there is still little data available about this weather phenomenon and more studies will be needed in the future to get a better idea of ​​what we can expect in the future.

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