Learning the ten cloud genera from the back step
2025-03-08 · 3 min read
The station gives me numbers, but numbers do not tell you what the sky looks like, so this winter I set out to learn the ten main cloud genera properly, from the back step, with a notebook and a laminated card from a library book. The rule I set was to identify every cloud I saw to genus before writing down the station readings, for ninety days. Today was day ninety.
The high ones were the hardest at first because they all look like wisps. Cirrus is the hooked or feathery one; cirrostratus is the thin sheet that puts a halo round the sun or moon; cirrocumulus is the rare one, a fine rippled sheet like fish scales, which I have seen on exactly four of the ninety mornings. Halos turned out to be the most useful clue: if there is a ring, it is cirrostratus, regardless of how patchy it looks.
The middle ones I confused for weeks. Altostratus is the grey sheet with the sun as a watery disc behind it; altocumulus is the layer of rounded patches, often in rows, with blue between. The test from the card that finally stuck was shadows: altostratus is too thin to cast proper shadows on the ground, and altocumulus patches are large enough to give the sky a lumpy texture when you look up through a gap in your fingers. Nimbostratus, the low dark rain sheet, I had been calling altostratus for a month until I noticed that it was raining.
Low cloud: stratus, the featureless grey blanket that sits on the hill behind the town; stratocumulus, the lumpy grey layer that is the commonest cloud in the logbook by a wide margin, 41 of the 90 days; cumulus, the white heaps on a fine afternoon; and cumulonimbus, which is cumulus that has grown a flat top and means business. I only logged cumulonimbus twice, both in late February, both with hail.
The tally for ninety mornings, for what it is worth: stratocumulus 41, stratus 14, cumulus 11, altocumulus 8, cirrus 6, altostratus 4, cirrocumulus 4, nimbostratus 3, cumulonimbus 2, and clear 7, which adds up to more than 90 because some mornings had two layers. The thing I did not expect is that I now notice the sky constantly, walking to the shop, from the bus, and I cannot stop naming it. The station readings have become the footnote; the sky is the entry.