Mammatus over the allotments and the storm that missed
2026-05-17 · 3 min read
At about 17:30 on Friday the underside of a huge anvil cloud spread over the town from the south-west, and hanging from it were the pouches that the books call mammatus: dozens of rounded lobes, each perhaps a hundred metres across, lit from the side by the low sun so that each one had a bright rim and a grey belly. I have wanted to see proper mammatus for two years and I stood on the back step for twenty minutes with my neck cricked, looking like a fool to anyone in the upstairs windows along the row.
The station readings in the hour before were the classic signs of a storm approaching. Temperature had peaked at 24.6 °C at 15:00 and was holding. Humidity had climbed from 48 to 61 per cent. Pressure had wobbled, falling 2 hPa and then rising 1, which I have learned is what the gauge does when a large convective cloud is pushing air around nearby. The wind had been light and variable all afternoon and at 17:20 it swung to come from the direction of the cloud and picked up to 25 km/h in gusts. The anvil was the top of a cumulonimbus that must have been thirty kilometres away, and the mammatus were on its leading edge, hanging over us.
Then nothing. The storm went past to the north. We had seven minutes of large, widely spaced drops, 0.8 mm in the gauge, and the mammatus thinned and dissolved as the anvil moved on. By 18:30 the sky to the north was black with the occasional flicker, there was a long low rumble about once a minute, and over the garden it was clearing to a pale evening with a few ragged cumulus. The temperature dropped 5 °C in the outflow and then recovered 2 of them. The neighbours put their washing back out.
Mammatus, as far as I understand it, are sinking pockets of cold, moist air from the anvil, falling into drier air beneath and evaporating as they go, so they are a sign of a storm that has already done most of its work. They do not predict severe weather overhead so much as announce that it is happening somewhere nearby. Which is what we got: the spectacle without the soaking. I made a drawing in the cloud notebook with the time and the direction and wrote the station numbers alongside. Of all the pages in that book, this is the one I am most pleased with, and I did not get wet making it.