Storm Barra: What is a ‘weather bomb’?

One phrase used by Met Éireann head of forecasting Evelyn Cusack has made the whole country sit up and taken notice of the impending Storm Barra.

She described what is approaching us as a “weather bomb” – an unofficial title used most often in the US which tends to have a lot of such meteorological nasties.

The official name used by meteorologists is almost as scary. The process whereby atmospheric pressure drops very quickly at sea is called “explosive cyclogenesis”.

It is used to describe a low pressure system where pressure falls by 24 hectopascals (hPa) or millibars in a 24-hour period.

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In the case of Storm Barra the fall between midday on Monday and midday on Tuesday is forecast to go from 1010mb to 957mb, more than twice the threshold to meet the criteria for explosive cyclogenesis.

The approaching storm has resulted in the highest level of wind warning, red, for the counties of Cork, Kerry and Clare with gusts expected in excess of 130km/h. The warning for Cork and Kerry will be in effect from 6am to 9pm on Tuesday. The alert in Co Clare will be valid from 4pm on Tuesday until 1am on Wednesday.

"This is a very rapid deepening of that system," explained Met Éireann forecaster Aoife Kealy.

“The low pressure weather system gets caught up in the jet stream, the movement of air high up in the atmosphere. That suction of air reduces the weight and causes the atmospheric pressure to fall at sea level.

“When the pressure falls at sea level like that, the system starts to suck in all the air around it which makes the air spin faster. It’s like an ice skater who, when they draw their arms in, spins faster and faster.”

That is why really strong winds can be found around a system which has deepened rapidly.

She continued: “Storm Barra is caught up in the jet stream so the jet stream is enhancing that deepening. It is causing that very sharp fall in pressure.

“As it is approaching us, it is deepening quite rapidly and so it is still sucking in all that air. That sharp convergence of air is what is giving us quite severe winds tomorrow and even into Wednesday.”

A weather bomb is a “rare and severe event” and has led to the red warning, she explained. “Not every system develops like that which is why we are seeing this system give very strong winds.”

The best known weather bomb to hit Ireland was the storm of the Fastnet rock in August 1979 where the pressure dropped by 40 millibars in only 24 hours. Some 24 yachts involved in the Fastnet race sank and 15 sailors died.