Reading a Weather Forecast for Hillwalking

Why a Hill Forecast Is Not a Town Forecast

A forecast written for the valley floor tells you almost nothing about the summit. The atmosphere cools as you climb, at roughly one degree Celsius for every hundred metres of ascent, so a mild eight degrees in the car park can become two degrees with a hard frost above 900 metres. Wind behaves the same way: it strengthens with height and accelerates through cols, over ridge lines and around the shoulders of hills, which is why a breeze that barely moves the grass at the trailhead can stop you dead on a plateau.

When you read a mountain forecast, look for the level at which the numbers are quoted. If the figures apply at 500 metres and your route tops out at 1,100 metres, you are looking at a different day entirely. Add wind chill to that gap and the picture shifts again. This is the single most common mistake I see in hillwalkers preparing for a big day out.

Wind Speed: The Number That Decides Your Day

Most people scan the rain symbols first. Look at the wind instead. Sustained wind speed tells you how hard the day will feel; gusts tell you whether you can stay upright. A forecast of 30 mph gusting 50 mph is a serious day on exposed ground, and anything gusting over 60 mph on a ridge should have you seriously questioning the route.

• Under 20 mph: comfortable walking on most terrain, wind barely noticed in shelter.
• 20 to 35 mph: noticeable push on exposed sections, worth using poles and adjusting your line.
• 35 to 50 mph: hard going, difficult conversation, real risk of being knocked off balance on loose ground.
• Above 50 mph: consider a low-level alternative entirely. Gusts above 60 mph on a ridge are genuinely dangerous.

Wind direction matters as much as speed. A westerly airstream that has crossed the Atlantic brings moisture and cloud; an easterly in winter can be bitterly cold and dry. Check which way the wind is blowing relative to your route, and note where you will be fully exposed rather than tucked behind a spur.

Visibility and Cloud Base: The Quiet Risk

Cloud base is often given as a height in metres, and it is the figure that governs whether your navigation gets tested. A base of 900 metres with summits at 1,000 metres means you will enter the cloud near the top, probably on the steepest ground, with the least margin for error. A base of 300 metres means you may be navigating by compass from the first kilometre.

Visibility forecasts matter especially in winter, when the difference between clear air and a whiteout can be a matter of a few hundred metres of altitude. If poor visibility is forecast, plan your day around features you can follow reliably: handrails, obvious ridgelines, walls, streams. Prepare to walk on a bearing, and know your escape route down before you need it.

Precipitation: Timing Beats Totals

Rainfall is usually expressed as a probability and an amount in millimetres. Both are useful, but the timing is what shapes your day. Two millimetres falling steadily over six hours is a wet, manageable trudge. Two millimetres falling in twenty minutes during a squall line is a different proposition, especially on a descent.

• Read the hourly breakdown, not just the daily summary. Look for the window when the rain arrives and when it clears.
• Distinguish showers from persistent rain. Showers pass; fronts settle in. The forecast wording tells you which you are getting.
• Watch for snow line and freezing level. The same precipitation becomes a very different hazard a few hundred metres higher.
• Check the ground conditions. Heavy rain after a dry spell runs off fast; sustained rain on saturated peat bog makes every step an effort.

If the wettest hours fall in the middle of your day, start earlier, shorten the route, or pick a lower objective. There is no shame in changing the plan.

Understanding Uncertainty

A forecast is a probability statement, not a promise. When two reputable sources disagree, that disagreement is information in itself: it usually means the weather is finely balanced, and small changes in timing or track could swing the day either way. Treat a forecast issued yesterday with more caution than one updated this morning, and check again before you set off.

Pay attention to phrases that signal confidence or its absence. "Showers developing" means expect interruptions. "Persistent rain clearing eastwards" gives you a rhythm to plan around. When a forecast hedges, plan for the worst version of it and you will rarely be caught out.

Turning the Forecast into a Plan

Sit down with the numbers and make decisions rather than gathering information. Set a wind threshold above which you will not go high. Identify two or three turning points on the map where the route can be shortened. Pack for the conditions forecast at summit level, not in the car park, and carry spare gloves and a hat even in summer.

Tell someone your route and your expected return time, and agree what happens if you are late. Take a paper map and compass regardless of what your phone says. Then get out there, because the hills on a grey, windy day are often the ones you remember longest.