Why the forecast is your first piece of kit

You can own the best boots in the shop, carry a bothy bag and navigate like a mountain rescue veteran, and still have a thoroughly miserable — or genuinely dangerous — day out because you picked the wrong hill on the wrong day. The forecast is the cheapest safety decision you will make all week, and it costs you ten minutes on a Thursday evening.

The trick is not to read it as a weather enthusiast, but as a route planner. Every number in a mountain forecast translates into a decision: which ridge, which ascent, what time to turn around, and whether to swap the high tops for a low-level valley walk and a decent café. Here is how to read the three elements that matter most.

Wind: the number that decides your route

Wind catches people out because you cannot feel it in the car park. A still, mild morning at 200 metres tells you almost nothing about conditions on a 900-metre summit, where the same air mass can be blowing thirty miles an hour harder across an exposed ridge.

As a rough guide: 20 to 25 mph on the tops is a stiff, tiring breeze. At 30 to 40 mph you are leaning into it, your walking pace drops noticeably, and a conversation becomes a shout. Above 50 mph, most people cannot walk in a straight line on open ground, and on a narrow ridge with a big drop either side, that is the point where you should be somewhere else entirely.

• Gusts matter more than the average. A forecast mean of 35 mph with gusts to 60 mph is a fundamentally different day from a steady 35 mph. Gusts knock you off balance at exactly the wrong moment.
• Terrain magnifies wind. Cols, plateau edges and the shoulders of ridges funnel and accelerate airflow. A wind that feels manageable in a sheltered corrie can be ferocious the moment you step onto the skyline.
• Wind chill is the real risk. Around 5°C with 40 mph of wind feels closer to -4°C. Hands and face stop working first, and cold hands mean clumsy map work at the exact moment you need precision.
• Direction shapes your route. A headwind on a long ascent can add an hour to a planned traverse. A tailwind on the way out means a headwind on the way back, when you are tired.

Visibility: hill fog, and the ridge that vanishes

Forecasts often give a cloud base, something like cloud free above 900 metres or hill fog patches above 600 metres. Read that against your route profile. If your summit sits at 850 metres and the cloud base is forecast at 700, you should plan the day as a navigation exercise, not a viewpoint walk.

Visibility matters most when it drops below about 100 metres, and seriously below 50 metres. At that point you are navigating on bearing, timing and pacing, with a compass you can actually use and a map in a case. A phone screen is a useful backup, never the plan — cold, wet hands and a fading battery are a poor combination.

Two details worth watching: featureless plateaus where every direction looks identical, and edges or cornices you cannot see coming. In poor visibility, keep well back from any drop you know is there from the map. The corrie rim does not announce itself.

Precipitation: what falls, and what it does to the ground

Millimetres of rain tell you intensity; temperature tells you what it will actually be. A valley temperature of 3°C with heavy precipitation usually means snow above roughly 400 to 600 metres, depending on the freezing level given in the forecast. Sleet at the car park is a strong hint that the tops are white.

Think about the ground as well as the sky. After a wet week, peat is saturated, becks rise fast and small crossings that are usually a step become a genuine hazard. Wet rock — particularly the smooth, slabby grit and the polished volcanic rock of popular ridges — turns easy scrambling into something that deserves care and time.

• Time the front. If rain is due at 2 pm, plan to be off the exposed ground by then, not halfway along a committing ridge.
• Watch the trend, not just the snapshot. A forecast that improves through the day is very different from one that deteriorates after lunch.
• Add snow to the wind. Spindrift and drifting snow destroy visibility and bury paths and footprints, making a familiar route genuinely unfamiliar.

Turning numbers into decisions

The forecast only earns its keep when you have converted it into a plan before you leave the house. Compare summit wind speed against your route's exposure, cloud base against your high point, and precipitation timing against your estimated finish. If two of those three look marginal, pick a lower, shorter, more sheltered route — there is no shame in it, and the hills will still be there next weekend.

Check the forecast the night before and again on the morning, and look at the specialist hill forecasts and fell-top conditions reports rather than a town forecast for the nearest big settlement. Give yourself a hard turnaround time, based on daylight and the weather trend, and honour it even when the summit feels close. A ridge you turn back from is a good day out; a ridge you push on across when the wind and cloud arrive is how accidents begin.