Discover more about bomb cyclones – such as the most recent Storm Éowyn – from their formation to where exactly they can strike
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In the past few days, Storm Éowyn hit England, Scotland and Ireland with high – and in some cases, record-breaking – winds, heavy rainfall and snow. Scientists have referred to the storm as a ‘bomb cyclone’, but what exactly does this weather term mean, and how do these cyclones come about?
What are bomb cyclones?
Bomb cyclones get their name from the meteorological term ‘bombogenesis’, which refers to when a mid-latitude (between the tropics and polar regions) cyclone rapidly intensifies over a 24-hour period. This is often caused by cold and warm air masses colliding (such as air over warm ocean waters), an event that leads to pressure to drop. The lower the pressure drops within a cyclone, the more intense it will be.
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Much like other storms, all bomb cyclones have low atmospheric pressure. This is because the rising columns of air that make up cyclones produce a vacuum effect in the atmosphere, lowering pressure as they strengthen.
But the difference is bomb cyclones are able to drop pressure drastically, at levels scientists dub ‘explosive’. It is the quick pace at which bomb cyclones strengthen that defines them – other storms don’t develop at such a fast rate.
As a general rule, for a cyclone to be classified as a ‘bomb’, atmospheric pressure must drop by at least 24 millibars within 24 hours, although this varies depending on latitude.
Importantly, the term ‘bomb’ doesn’t make reference to the strength of the storm, but rather the speed at which a storm system forms, although bomb cyclones often bring about intense precipitation and strong winds that can wreak havoc.
Because bomb cyclones occur so quickly, it can make predicting their intensity and path even complicated, potentially endangering more lives and infrastructure than normal storms which can be easier to track.
Where and when do they occur?
Around 70 bomb cyclones occur around the world each year, predominately in the Northern Hemisphere off the east coasts of Japan and the US. The majority of bomb cyclones occur in winter months – roughly between October to March – unlike hurricanes which often form during an earlier season that runs between June and November.
Bomb cyclones are most common in areas with steep temperature gradients between different air masses, such as the northwestern Atlantic and Pacific Oceans.
In the last few decades, these types of storms have struck the US with increasing frequency. Between 1980 and 2020, the number of bomb cyclones in the Atlantic basin increased by around 70 per cent, according to data from the National Weather Service.
The East Coast of the US is a known hotspot for these weather patterns, since during winter there is a high contrast between cool air and the warm Gulf Stream current. Bomb cyclones can also occur on the Pacific Northwest, but in this region often bring atmospheric rivers (thin bands of concentrated atmospheric moisture) along with them.
Specific examples include a bomb cyclone in November 2024 that hit the Pacific Northwest with wind speeds of up to 75mph, killing at least two people and leaving more than 700,000 homes without power. Hurricane Milton, in October 2024, was also another instance of these type of cyclones impacting the US.
Is climate change making them worse?
In short, it seems likely. Climate change helps to drive ocean temperatures up and create further instability in the jet stream, creating a perfect concoction of conditions for these cyclones to occur.
Arctic warming can disrupt the jet stream, meaning pockets of cold air become trapped over the US, further exacerbating conditions that lead to bombogenesis.
‘Warming oceans are the fuel for these storms,’ says senior scientist at the Woodwell Climate Research Center Dr Jennifer Francis. ‘The increased heat and moisture create an environment where storms can intensify much more rapidly than in the past.’
