Cloud atlas

Stratus

Stratus with a base that stays put and a light, steady wind means a stable, moist, well-mixed boundary layer. If you can see the deck getting thinner and brighter from the top down, expect burn-off; if it darkens and smudges at the base, expect drizzle.

Cumulus

Scattered humilis and mediocris over land in the afternoon means a stable, dry-ish boundary layer and no rain today. A single congestus tower among flat cumulus means one locally favoured spot is being fed harder than the rest.

Cumulonimbus

A single congestus tower that glaciates at the top, acquires a dark flat base and starts showing a rain shaft is 20-40 minutes from the mature stage. The gust front arriving before the rain is the reliable ground-level warning.

Stratocumulus

A closed stratocumulus deck with a steady wind means a stable marine or post-frontal airmass and a good chance of drizzle. Scattered open cells with plenty of blue mean a showery, unstable airmass with a shower risk.

Nimbostratus

A uniform dark layer with steady rain and poor visibility is nimbostratus and the rain will last for hours. If the sky above shows any tower or anvil, it is convective and the rain will be showery and short.

Altocumulus

Scattered, thin, stable altocumulus in the morning means a quiet day. Altocumulus thickening and lowering with cirrus streaming in ahead of it means a front is on the way. Castellanus turrets are the one variety that justifies changing an afternoon plan.

Altostratus

Altostratus thickening and lowering with the wind backing and pressure falling means steady rain within a few hours. Altostratus that stays thin, warm-toned and static all morning is often the leftovers of last night's storms and will break up.

Cirrostratus

A 22° halo with a thickening veil means rain in the next 12-24 hours. A halo in a veil that then thins and breaks up means the front is missing you.

Cirrus

Cirrus increasing in coverage and thickening into a milky cirrostratus veil is the single most reliable day-ahead rain signal in the mid-latitudes. Cirrus in a narrow band that stays put and disappears is not.

Cirrocumulus

Cirrocumulus that thickens through the morning and is followed by a milky veil means rain within 24 hours. Patchy cirrocumulus in a clear sky with steady pressure and no thickening means fair weather.

Arcus

A low wedge advancing ahead of a dark storm base means a strong gust within minutes, followed by heavy rain. A shelf cloud with a smooth, sharply defined underside means a particularly well-organised outflow.

Virga

Virga under a developing cumulus or altocumulus means the sub-cloud layer is dry and the shower will not reach you. Virga under a mature cumulonimbus anvil with a fast-growing shaft is a gust-front warning.

Altocumulus lenticularis

Sharp, stacked lenticulars persisting through the morning are a sign of a strong, stable, windy airmass aloft. If they appear with cirrus thickening behind them, a front is on the way and the wind will strengthen further.

Mamma (mammatus)

Mammatus on the departing flank of a storm means the worst is over. Mammatus with new towers building upwind means more is coming. Mammatus with rotation underneath is an immediate shelter signal.

Cavum

A fallstreak hole means the cloud layer overhead is composed of supercooled water and is therefore unstable to any disturbance. If the layer later thickens into a frontal sequence, the precipitation from it may begin as snow even if the surface is above freezing.

Cirrus homogenitus

Contrails that start persisting and spreading mean the upper troposphere is moistening. If that is followed by a milky veil and a 22° halo, rain is 12-24 hours out.