The term “weather bomb” often makes headlines when a rapidly deepening area of low pressure brings the potential for damaging winds and disruptive weather.
But despite the dramatic name, a weather bomb is not a separate type of storm. Instead, it describes a period of exceptionally rapid intensification within a low-pressure system, a process known as explosive cyclogenesis.
What is a weather bomb?
A weather bomb is an unofficial term used when the central pressure of a low-pressure system falls by at least 24 millibars within 24 hours. Meteorologists refer to this rapid deepening as explosive cyclogenesis, which is the preferred term at the Met Office. As the pressure drops, the low-pressure system becomes more intense, often leading to very strong winds, rough seas and heavy rainfall.
While areas of low pressure are a familiar feature of the UK's weather, only a small number deepen quickly enough to meet the criteria to have officially undergone explosive cyclogenesis. These systems are most commonly found over the North Atlantic, particularly during autumn and winter when temperature contrasts across the ocean are strongest. Although not all of these systems directly affect the UK, those that do can bring significant impacts.
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The science behind explosive cyclogenesis
The key to explosive cyclogenesis lies high in the atmosphere. One of the most important ingredients is the jet stream, a fast-moving ribbon of air several miles above the Earth's surface. When the jet stream becomes particularly powerful and develops a favourable pattern, it can help remove air from above a developing low-pressure system.
As air is removed from the atmospheric column, the weight of air pressing down on the surface decreases, causing pressure at sea level to fall. This falling pressure then draws in more air from the surrounding area. The inflowing air begins to rotate around the low-pressure centre, and as the system tightens, wind speeds increase.
A useful comparison is that of an ice skater pulling their arms in while spinning. As the skater becomes more compact, they rotate faster. In a similar way, as the pressure gradient around a deepening low-pressure system becomes tighter, the circulation speeds up and stronger winds develop.
Why do weather bombs produce such strong winds?
The rapid drop in pressure creates tightly packed isobars, the lines shown on weather charts that join areas of equal atmospheric pressure. The closer together these isobars become, the stronger the pressure gradient. It is this pressure gradient that generates powerful winds.
When explosive cyclogenesis occurs, winds can strengthen dramatically in a relatively short period of time. Peak wind speeds often occur over just a few hours, but can be strong enough to bring down trees, damage buildings and disrupt transport networks.
Combined with heavy rain and large waves, these winds can create hazardous conditions across coastal and inland areas alike.
Storm Doris: A classic example
One notable example of explosive cyclogenesis was Storm Doris. As the storm developed over the Atlantic, its central pressure fell from around 1004 millibars to 974 millibars within 24 hours, a drop of 30 millibars. This exceeded the threshold required to classify the system as a weather bomb.
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The rapid deepening was linked to a powerful jet stream and a favourable atmospheric setup that allowed the storm to intensify quickly as it approached the UK. The resulting tightly packed isobars helped generate the strong and damaging winds associated with the storm.
Are weather bombs becoming more common?
Weather bombs have occurred for many decades and remain a natural feature of the atmosphere. They are most likely to develop when strong temperature contrasts and a vigorous jet stream come together. While the term itself often attracts attention, the underlying science is well understood and closely monitored by meteorologists.
Modern forecasting tools allow forecasters to track developing low-pressure systems days in advance, helping provide early warning of potentially disruptive conditions. When a rapidly deepening storm is identified, forecasts and warnings can be issued to help people prepare for strong winds, heavy rain and other associated hazards.
Keep up to date with weather warnings, and you can find the latest forecast on our website, on YouTube, by following us on X and Facebook, as well as on our mobile app which is available for iPhone from the App store and for Android from the Google Play store.
