British thunderstorms rarely announce themselves. There is no siren, no warning bell — just a sky that looked pleasant at lunchtime and now has a hard, dark underside with a top that keeps climbing. What began as a few puffy cumulus over a field can become a full thunderstorm in the time it takes to walk the dog.

The cloud responsible is cumulonimbus, and it is one of the most readable objects in the sky. It brings thunder, lightning, hail and the kind of downpour that floods a lane in twenty minutes. Learn its shape and its habits and you can often give yourself half an hour of notice before the first rumble.

The shape tells you more than the size

A fair-weather cumulus cloud is wider than it is tall, with soft, bright edges and a flat bottom. It is the sky at rest. What matters is what happens next.

When a cumulus starts to grow upwards, it becomes cumulus congestus: a towering cauliflower with crisp, sharp outlines and a hard, level base, often several times taller than it is wide. This is the stage where the cloud is still deciding what to be — and where a small change in the air can stop it in its tracks or push it further.

The change to watch for is at the very top. While the summit stays sharp and cauliflower-like, the cloud is still made almost entirely of liquid droplets and is unlikely to produce lightning. When the top begins to soften, lose its edges and turn fibrous — like cotton wool pulled apart — the cloud has reached the glaciated stage, cumulonimbus calvus. From there it usually takes only minutes for an anvil to spread out across the top, often sheared downwind into a flat hammer shape. That is cumulonimbus capillatus, and it is a thunderstorm.

A rough way to judge the height of the base

If you have temperature and dew point from a forecast app, subtract one from the other and multiply the difference in degrees Celsius by about 400 feet. A spread of 2°C suggests a cloud base near 800 feet; a spread of 8°C points nearer 3,200 feet. The smaller the spread, the lower and more humid the air — and low, dark, ragged bases beneath a towering cloud are a classic storm look.

Five things to look for in a suspect cloud

• A hard, flat, dark base. If the underside looks almost black against a brighter sky, the cloud is deep enough to be blocking a lot of light.
• Scud. Ragged, dark fragments racing beneath the main base, sometimes at a noticeably different angle to the cloud above. They show a strong, moist inflow of air.
• A rain shaft. A grey curtain hanging from the base. If it reaches the ground, the cloud is already precipitating heavily.
• A fibrous top. An anvil, a softening summit, or a small dome pushing up above the anvil — an overshooting top, which signals a particularly strong updraft.
• Flickers inside the cloud. At dusk or dawn you may see the whole cloud light up from within before you hear anything at all.

The order in which the warning arrives

1. The cloud stops looking friendly. Crisp edges, hard base, steady vertical growth.
2. The base darkens, lowers and grows scud. Light fades earlier than you expected.
3. The first distant rumble. If you can hear it, the storm is already within striking distance.
4. The gust front. A sudden cool, blustery wind arrives, the temperature drops several degrees, and there may be a shelf cloud rolling out ahead of the rain.
5. Heavy rain and often hail, with the strongest gusts and most frequent lightning.
6. The anvil passes overhead, rain eases to drizzle or stops, and the sky behind may show pouch-like mammatus clouds.

A single storm cell usually passes in twenty to forty minutes, but storms can also train over the same spot for hours, with new cells forming repeatedly along the same line.

Hail, downpours and the green tinge

Hail needs a strong updraft to hold ice pellets aloft long enough to grow, so any hail — even pea-sized — tells you the storm is vigorous. A greenish tint in the base is sometimes mentioned as a hail sign; it can be a hint, but it is not reliable on its own, and can also come from sunlight passing through a very deep, water-laden cloud. What you can trust is plain intensity: if the rain is heavy enough that you cannot see a few hundred metres, expect localised flooding on roads and standing water in low fields.

One safety point worth remembering: lightning can strike from the anvil several miles ahead of the rain, so a bright sky overhead is no guarantee of safety when a storm is nearby.

Reading the air around you

The cloud is only half the story. Before a storm arrives, the wind often drops away and the air feels close and heavy. Then comes the gust front, arriving as a distinct cool push, sometimes minutes before any rain. You may also notice the smell of rain on dry ground — petrichor — reaching you ahead of the first drops, and a sharp, clean smell after a nearby strike.

Watch which way the low cloud is moving compared with the high cloud. When the two run at different angles, the atmosphere is sheared, and sheared storms tend to be better organised, longer-lived and more likely to produce hail and strong gusts.

When to stop watching and take cover

Every cloud-spotting guide needs a stop line. Thunder means you are already within range. As soon as you hear it, go indoors or into a hard-topped vehicle and stay there until thirty minutes after the last rumble. Counting the seconds between flash and bang and dividing by three gives a rough distance in kilometres, but do not stand outside doing arithmetic.

• Avoid open ground, hilltops, isolated trees and water.
• Put down metal tools, fishing rods and golf clubs, and close the umbrella.
• Inside, keep away from landline phones and unplug sensitive equipment if the storm is overhead.
• Bring pets in — they feel the pressure and the static long before you do.

You do not need radar or an app to read a cumulonimbus. Find a growing cumulus, watch whether the summit stays crisp or turns fibrous, and keep an eye on the colour of the base. Twenty minutes of attention is usually enough to tell you whether to peg out the washing or head for the car.

Photo: PublicDomainPictures / Pixabay