Understanding Clouds over Hills and Valleys

Why the sky deserves your attention

In Britain, weather moves faster than most forecasts can describe. A morning that begins under clear blue over the Yorkshire Dales can be grey and drizzling by lunchtime, and the hills only make it more interesting. Air forced up over a ridge cools as it rises, and the moisture within it condenses into cloud — which is why the same wind that dries a valley can bury a summit in fog within an hour.

Learning to read clouds is not about replacing the forecast. It is about gaining a second opinion, one that is visible from the stile you are standing at, and specific to the valley you are in right now. A little practice turns the sky into a running commentary on the weather to come.

Cumulus: the fair-weather friend that can turn

Cumulus are the clouds most people draw as children: crisp, bright, flat-bottomed and sharp-edged, drifting across blue sky. When they stay small — the sort that look like sheep grazing along a ridge — you are in settled, stable air, and the day should stay kind.

Watch their tops, though. Cumulus are honest clouds; they tell you what the air is doing. If the tops stay low and rounded, the rising air is being held back. If they begin to climb into towers, sharpening at the edges and darkening underneath, the atmosphere has become unstable. By early afternoon over a sunny ridge, that growth is often the first sign of showers.

• Flat bases sitting high above the valley suggest a dry, settled afternoon.
• Tops rising steadily through the morning mean showers later, likely over the hills first.
• A dark, slate-coloured base spreading out under a tall cloud means rain within the hour.
• A flattened, anvil-shaped top means thunder, hail and sudden squally gusts — time to drop off the ridge.

Cumulus over hills build fastest where the slope faces the sun, so the western or southern flanks of a valley often cloud over first. If you are planning a high-level route, note which side is growing and which is still clear.

Stratus and the grey blanket days

Stratus is the opposite character entirely: a featureless grey sheet that sits low and flat, hiding the tops of the fells and drizzling gently on everything below. When it touches the ground on the hills, we simply call it hill fog, and it is the reason a compass and the ability to walk on a bearing matter so much in the Pennines, Snowdonia or the Scottish Borders.

Not every grey layer means a washout. Stratocumulus — lumpy, textured, often broken enough to show patches of blue — tends to bring damp, drizzly, quiet weather rather than heavy rain. It is common in autumn and winter when the air is stable and there is little energy for anything dramatic.

Read the direction of change. A grey layer that thickens, lowers and darkens through the day is a warning that a front is approaching. A layer that begins to break and lift by mid-morning often means the sun is winning, and a bright afternoon may follow. Valley fog on a still autumn morning is a good example: if it lifts by nine or ten o'clock, the day is usually set fair.

Lenticular clouds and the wind above the tops

Lenticular clouds are the most distinctive of all: smooth, rounded, almond or saucer-shaped, often stacked in layers like a pile of plates above a summit. They form when stable, fast-moving air is forced up over a ridge, creating standing waves downwind. Crucially, the cloud stays almost stationary while the air rushes through it.

For a hill walker, a lenticular parked over your intended summit is a serious message. It signals strong winds in the upper air, even when the valley below is barely stirring. Expect fierce gusts on exposed ridges, especially on the lee side, and sudden changes in wind direction around spurs and cols. On days like this, a low-level walk in the valley may be the wiser choice.

There is a bonus to these clouds, too. As air descends on the sheltered side of the hills it warms and dries, so the lee of a range — the eastern Pennines, or the country east of Snowdonia — can be noticeably brighter than the wind-battered tops themselves.

Quick reads before you set off

Combine the clouds with the wind and the feel of the air, and you can build a reliable picture in a minute or two at the car park or the first gate.

• Small, sharp-edged cumulus in blue sky, bases high — a settled spell; enjoy it.
• Cumulus growing by mid-morning, bases darkening — showers likely, so plan an escape route off high ground.
• Flat grey stratus down to the valley — drizzle and poor visibility on the tops; take the compass.
• Lenticular cloud over a ridge with a light valley breeze — expect strong gusts on the summits.
• Cloud base lowering and thickening through the day — rain within a few hours.
• Valley fog that lifts by mid-morning — usually the prelude to a fine, clear day.

Clouds are a conversation, not a single sentence. Watch how they change from hour to hour, note whether the wind is backing or veering, and you will find that the sky over your valley has been telling you the forecast all along.