Weather warning: When will Storm Ciaran hit and what makes it a bomb cyclone?

Storm Ciaran is a “bomb cyclone” forecast to move across southern England and northern France from early on Thursday 2 November. It could set a record for the lowest air pressure recorded in the region in the past 200 years. As of Wednesday afternoon, the strongest wind gusts of more than 137 kilometres per hour were forecast to stay just south of the UK in the English Channel. There are also multiple flood warnings, mostly around the southern coast of England.

What is a bomb cyclone?

A cyclone is the general term for any storm involving winds rotating around a region of low pressure. A bomb cyclone is the term for a storm where the air pressure in the centre drops very quickly, resulting in rapid or “explosive” intensification as the falling pressure produces higher wind speeds. Specifically, they are storms where the pressure falls 24 millibars (mb) or more in 24 hours. The term bomb cyclone is only used for extratropical or mid-latitude storms, not for tropical cyclones or hurricanes.

What causes bomb cyclones?

Extratropical or mid-latitude storms form when a mass of warm air meets a mass of cold air. If conditions are right, that leads to the warm air rising and rotating, air pressure dropping and the formation of a storm. The stronger the temperature difference between the air masses, the more powerful the storm can become. In the case of Storm Ciaran, a particularly cold mass of air swept across the eastern US into the Atlantic, where there was warm, moist air. A fast jet stream then propelled this weather system across to northern Europe, where it began to intensify rapidly.

Will Storm Ciaran set a record for the lowest air pressure in the region?

According to Ed Hawkins at the University of Reading, UK, the lowest air pressure recorded at sea level in the region where the storm is due to hit is 952mb, as measured in both Reading and London on 25 February 1989, with the second lowest being 957mb. The pressure in Storm Ciaran is expected to drop to around 953mb. So it is likely to be the second lowest, but it could set a new record if the pressure drops just slightly more. “With careful measuring, you might even be able to notice water boiling at lower temperatures during the storm, compared to when we have high pressure,” Hawkins said on X/Twitter.

Is global warming making Storm Ciaran more intense than it would have been?

“This storm will likely drop more rain than it would have done had it occurred a century ago as the atmosphere is warmer and more humid now,” Hawkins posted on the social media site Bluesky. So in the strict sense of more rain falling, you could say that global warming is responsible. There will also be more coastal flooding because of the 17 centimetres of sea level rise so far due to global warming. However, according to the Met Office’s State of the UK Climate report, there is no evidence that the wind speeds of storms hitting the UK have increased due to climate change, or that there have been more storms. However, climate models are predicting that winter storms will become more frequent in the future.

How long will the storm last?

The strongest winds will affect south-west England on Thursday from 0300 to 1100 and south-east England from 0600 to 1700, according to the UK Met Office, with amber alerts, used to warn of likely disruption and potential risk to life and property, in these areas.  South Wales and southern England will have high winds from 2100 Wednesday until midnight Thursday, with a yellow warning in place. There is a yellow warning for rain in southern and western England and in Wales from 1800 Wednesday until midnight Thursday, and for north-east England and eastern Scotland from Thursday 0600 until 0600 Friday.