Understanding cloud formations for outdoor plans

Why looking up beats checking the app

Weather forecasts for the hills are notoriously patchy. A valley town can be basking in sunshine whilst the ridge four miles away sits under a grey lid of drizzle, and no app will tell you which side of that line your walk will fall on. Clouds, on the other hand, are local, immediate and free. Learn to recognise half a dozen shapes and you can read the next three or four hours with reasonable confidence — often enough to decide whether to push on to the summit or drop into the valley for a brew.

Cumulus: the fair-weather builders

Cumulus are the clouds most of us drew as children: brilliant white, flat-bottomed, with rounded cauliflower tops. On a settled spring or summer morning they appear over the hills a couple of hours after sunrise and fade again by late afternoon. Small, well-spaced cumulus with plenty of blue between them are a gift — that is fair-weather cloud, and it is a grand day to be out.

Watch them, though. If the individual clouds start to grow taller, crowd together and develop dark, flat bases, convection is getting going. When a cumulus tower climbs above its neighbours and its top turns soft and fibrous, showers are usually only an hour or two behind. The taller and more solid they look, the heavier the shower.

There is a useful rule of thumb for how high the base sits. Take the air temperature in Celsius, subtract the dew point (your weather app will show it), and multiply by 400 to get the cloud base in feet. At 15°C with a dew point of 11°C the spread is 4, so the base sits around 1,600 feet — about 500 metres. A high base means dry air and light showers; a low one means moist air and, often, a wet afternoon.

Stratus: the flat grey blanket

Stratus is the featureless grey sheet that settles over the tops and refuses to move. It brings drizzle, hill fog and visibility that can drop to a few dozen metres, which makes navigation by landmarks a non-starter. If you can see stratus clinging to the fells at 400 or 500 metres and your route tops 700 metres, assume you will spend the day inside the cloud. Be ready to navigate by map and compass, or pick a lower route instead.

The good news is that stratus is often a morning phenomenon. If it starts to break into lumpy patches and lift — becoming stratocumulus — the day is improving.

Cirrus: high, wispy and full of hints

Cirrus sits high above the weather, usually above 20,000 feet, and is made of ice crystals rather than water droplets. It looks like stretched-out feathers, hooks or the trails left by a comb — the "mares' tails" of folklore. On a fine day, a little cirrus means very little.

What matters is what follows it. When cirrus thickens into a milky veil and you notice a pale ring — a halo — around the sun or the moon, you are looking at the leading edge of a warm front. Rain usually arrives somewhere between six and twenty-four hours later, with the cloud base steadily lowering: cirrostratus, then altostratus, then the grey gloom of stratus and steady rain. See a halo at teatime and plan for a wet tomorrow.

Reading the cloud base for rain

The height of the cloud base is one of the most useful things you can watch through a day on the hills. A base that stays put or lowers gently is a fairly settled picture. A base that climbs steadily through the morning while the cumulus above it grows taller and darker is a warning sign: the air is becoming more unstable, updraughts are strengthening, and showers or thunderstorms often follow within a few hours.

• Rising base plus towering tops — showers likely by mid-afternoon. Shorten the route or keep an escape path in mind.
• Lowering, darkening base — rain is close. Get waterproofs on before you need them.
• Anvil-shaped tops — the cloud has hit the top of the unstable layer. Expect heavy rain, hail and possibly thunder. Get off high ground, away from exposed ridges and away from streams.
• Cloud sitting on the summits — treat it as fog and navigate accordingly, even if the valley below is clear.

Practical habits for a day out

You do not need to become a meteorologist. A few small habits will do most of the work.

• Look up at breakfast and again at lunch. Cloud is a moving picture, and the change between those two glances tells you more than either one alone.
• Note the wind direction and whether it is strengthening. Cloud moving noticeably faster than the surface wind suggests a changeable, blustery day ahead.
• Compare the cloud base to hills you know. If the base is at 600 metres and your summit is 900 metres, you will be in the cloud — that is a decision made before you set off.
• Carry a paper map and compass, and know how to use them. Hill fog is the most common way a pleasant walk turns into a very long one.
• Tell someone your route, and give yourself permission to turn back. The hills will still be there next weekend.

None of this replaces a proper forecast, but it does something a forecast cannot: it tells you what is happening above the exact patch of ground beneath your boots. Learn to read it and you will pick better days, pack better and enjoy the countryside far more for it.