Reading Weather Clouds Before a Beacon Ascent

Why the beacon gets different weather from the car park

Almost every hilltop beacon in Britain stands a few hundred metres above the lane where you park. That gap is enough to turn a blue-sky morning into a wet, wind-scoured summit by lunchtime. The good news is that most of the warning signs are visible before you leave the trailhead, if you know what you are looking at. Cloud shape, wind direction and a falling barometer will tell you more about the next three hours on the ridge than any forecast issued at dawn.

Cloud shapes worth knowing

Look at what the cloud is doing, not simply how much of it there is. A grey sky that stays grey is often kinder than a bright one that starts to boil.

• Small, flat-based cumulus dotted across blue sky are the fair-weather sort. But watch the clock. If they begin building upwards during the morning into tall cauliflower towers, showers are likely by early afternoon, and the summits will catch them first.
• An anvil is the giveaway. A flat, fibrous top spreading outwards from a towering cloud means cumulonimbus: thunder, hail and violent squalls on exposed ground. Postpone the ascent rather than race it.
• Lenticular clouds, smooth lens or saucer shapes stacked above a ridge, show a standing wave in strong wind aloft. The summit can be eerily calm one minute and battered by a ferocious gust the next. They are common over the Pennines, Snowdonia, the Lakes and the Scottish hills.
• A mackerel sky of rippled altocumulus often arrives twelve to twenty-four hours ahead of a warm front. If it thickens into a grey sheet, then drizzle, you have your answer: rain is on the way.
• Cirrus with hooked tails, thickening and lowering, points to an approaching front. The more fibrous and widespread it becomes, the closer the change.
• Sea fret or haar deserves special respect. On east-coast headlands and around Cornish and Welsh promontories, a light onshore breeze in spring can push fog inland by early afternoon and swallow a coastal beacon that was crystal clear at ten in the morning.

Reading the wind, not just the forecast

Wind direction tells you which way the weather is moving. A backing wind, shifting anticlockwise from south-west to south to south-east, usually means a low is approaching from the Atlantic. Expect rain, strengthening wind and falling visibility. A veering wind, swinging from south-west to west to north-west, shows a cold front has passed: the sky will brighten, but squally showers, hail and sharp gusts often follow.

Strength matters as much as direction. In the car park, leaves and small twigs in constant motion suggest a breeze that will feel like a gale on a 600-metre ridge. Whole trees swaying, and a struggle to hold a map flat, means you should be thinking about a lower route. On a beacon hill, add a force or two for exposure; the summit is rarely the same as the valley floor.

There are local quirks too. On a sunny morning, anabatic winds draw air up the slopes, so a ridge can be breezier than you expect. At dusk, cooler air drains downhill as katabatic flow, bringing a sudden chill and a breeze through the valleys just as you are descending.

Pressure, mist and the hill fog question

If you carry a barometer, or a watch with one, use it properly. A fall of one hectopascal an hour is a hint of trouble; three in three hours is a genuine warning of a deepening low, and the hills will feel it first with strengthening wind and lowering cloud.

Hill fog is the quieter hazard. Check the gap between air temperature and dew point: if they are within a degree or two, mist, drizzle and cloud are close. A valley fog that burns off by eleven does not guarantee clear summits; frequently the cloud base sits stubbornly at ridge height all day, and the beacon stays inside it. If a weather report gives a cloud base below your summit height, plan for navigation rather than views.

A five-minute check at the trailhead

• Compare the cloud base with the height of your beacon. A base at 500 metres and a summit at 700 metres means the top is in cloud.
• Note the wind direction, then ask whether it has been backing or veering on the drive over.
• Ask whether the cloud is thickening or breaking. A sky in transition tells you more than a snapshot.
• Check the pressure trend, if you have a barometer or altimeter. If the altimeter reads higher than the map says, pressure has fallen since you set it.
• Set a turn-back rule before you start: cloud base below the summit, gusts that unbalance you on open ground, or thunder within a few miles.

Beacons, headlands and the walk back

Coastal beacons bring their own signals. Haze over the sea on a warm afternoon, a distinctly damp smell in the air and a freshening onshore wind can all precede sea fog rolling over the cliffs. Inland, watch for the light dimming behind you while the summit stays bright: the rain front is often behind, not ahead.

Remember that the descent is where most hill days go wrong. Tired legs, cloud that has closed in and a wet, greasy path are a poor combination. Take a bearing before you leave the top, keep the map handy rather than buried, and consider a lower or more sheltered route home. A last look at the sky from the car park costs nothing, and it is often the most useful forecast you will get all day.