What happened
Storm Ciarán was a deep area of low pressure that swept in from the Atlantic at the start of November 2023. It brought severe gales to the Channel Islands and the south coast, and heavy rain across much of England. Its centre passed over southern England on 2 November.
At this station the pressure fell from 1000.0 hPa on Tue 31 Oct, 17:30 to 955.3 hPa at Thu 2 Nov, 09:05, the lowest it has ever recorded. In the three hours to Thu 2 Nov, 01:35 alone it fell 10.7 hPa.
Why the pressure falls
Air pressure is the weight of the air above. In a low, air is being drawn up and away high in the atmosphere faster than it can flow in at ground level, so there’s less air overhead and the pressure falls. The faster it falls, the more vigorous the low. Forecasters call a fall of more than 6 hPa in three hours “falling very rapidly”: a sign of strong winds close by. More on air pressure.
Two fronts in four hours
The station found a likely warm front at Thu 2 Nov, 01:00 and a likely cold front at Thu 2 Nov, 04:30. A low carries a wedge of mild, moist air (its warm sector) between two fronts. As the warm front passes, the temperature and dew point rise; when the cold front follows, colder, drier air arrives behind it and the pressure starts to climb. Two fronts only hours apart mean the warm sector was narrow: the cold front was catching up the warm one, as it does when a low is at its deepest. More on fronts.
Why the wind stayed light here
The strongest gust here was only 16 mph, on Wed 1 Nov, 11:50. Wind blows where the pressure changes quickly from one place to the next. Near a low’s centre the pressure is nearly level, so the wind there is often light; Ciarán’s strongest winds were on its southern side, over the Channel. This station is also sheltered by buildings and trees, so it reads lower than an open site. More on wind.
The rain
24.4 mm of rain fell over the 4 days, most of it as the fronts passed. The pressure then rose 5.2 hPa in the three hours to Thu 2 Nov, 17:35 as the low moved away to the north-east. More on rain.
What to look for in the charts
- Pressure: the steep fall to the low and the climb out of it: the deeper and sharper the “V”, the more vigorous the storm.
- Temperature and dew point: the step at each front, and how close together the two lines are while it rains (the air is nearly saturated).
- Wind: how little it picked up near the centre.
- Rain: when it fell, against the fronts marked on the charts.