A 14 hPa fall in six hours and what the sky did first
2026-09-20 · 3 min read
The barometer on the kitchen sill read 1019 hPa at 06:00 on Thursday and 1005 hPa at noon, a fall of 14 hPa in six hours, which is the steepest I have logged since the station went up. The display flashed its little falling arrow all morning. What I want to record here is not the fall itself but the order in which the sky told me about it, because the clouds had the story nearly a day before the needle did.
Wednesday afternoon, pressure steady at 1021 and a clear blue sky, there were cirrus hooks high up in the west, the kind with the little upturned tails. By Wednesday evening those had thickened into a sheet of cirrostratus thin enough to see the sun through, with a faint halo around it at about 19:00 that I measured with my outstretched hand at roughly the usual 22 degrees. The pressure at that point had dropped exactly 1 hPa. The halo is the thing to notice. It means ice crystals in a high sheet, which means warm air riding up over cold ahead of a front.
By Thursday 06:00 the cirrostratus had lowered and thickened into altostratus, a grey ceiling with the sun a pale disc behind it, and the pressure had started to move properly. By 09:00 the disc was gone, the cloud was nimbostratus, dark and featureless, and the first rain came at 09:40. The rain gauge recorded 11.2 mm between then and 14:00. The wind, which had been from the south at 10 to 15 km/h all Wednesday, backed to the south-east overnight and then veered sharply to the south-west at 13:10, at which point the pressure stopped falling and the rain eased within half an hour. That veer is the front going through.
So the sequence was: hooked cirrus, then a halo, then grey, then rain, then a wind shift and the pressure bottoming out, over about 20 hours. The barometer told me the same story but only from Thursday morning. I have read this sequence in books many times and this was the first time I watched all of it from one garden with a notebook in my hand and got every stage. The temperature, for the record, went from 11.8 °C at dawn to a maximum of only 14.1 °C, and then rose to 16 °C after the front passed and the sun came out at 15:30. Warm sector, cold front, clearance. Textbook, in the garden.