Expert comment: Storm Ciaran brings 'rare' low pressure

● Storm Ciaran delivers ‘exceptionally rare’ atmospheric pressure readings

● Reading spared hurricane-force winds but caught heavy rainfall

● 2023 could be Reading’s wettest year on record

Dr Stephen Burt, from the University of Reading’s Department of Meteorology, has been looking at weather data from the University’s Atmospheric Observatory after Storm Ciaran hit the UK.

He said: “Depressions (low-pressure areas) form as a result of air being lifted by complex dynamical forces. Rising air cools and water vapour condenses, forming clouds and releasing heat energy which drives the storm, typically resulting in a fall in surface atmospheric pressure, and the familiar rainy or stormy weather.

"The most intense storms tend to be associated with very low atmospheric pressure, and this was certainly the case during the passage of Storm Ciaran across southern England this morning. Here in Reading, the barometer fell to 956.0 millibars (corrected to mean sea level), shortly before 0700 UTC. A lower value has been recorded only once before within more than 200 years - and that was more than 30 years ago, on 25 February 1989. On that date, the lowest value was 952.1 millibars around 1730 UTC. Before that event, we have to go back to Christmas 1821, when the barometer fell to 953 millibars in Oxford late on Christmas Eve that year, and to 946 millibars at 5 a.m. in north London on Christmas Day.

“Such low barometer readings are uncommon even in more storm-prone northern Scotland, but in southern England, they are exceptionally rare. Daily barometer readings at 9 a.m. GMT have been made in the university since 1908. At today's 9 a.m. reading, the barometric pressure (corrected to mean sea level) stood at 958.6 millibars, having begun to rise from its extreme low level. Even so, this was almost 4 millibars lower than any other 9 a.m. reading in our 115-year record.

“The centre of the depression passed across Dorset and Hampshire, a little to our south, and fortunately, we were spared the near hurricane-force winds on Ciaran's southern flanks. Instead of the wind, we caught many hours of moderate to heavy rainfall, amounting to over 20 mm in the last 24 hours, falling on already saturated ground and inevitably resulting in some flooding. Our total rainfall for 2023 has already surpassed Reading's average annual of 655 mm - that threshold was reached on 21 October - and with a very wet start to November, this may yet turn out to be Reading's wettest year on record.”