Start with a forecast built for the hills

A general lowland forecast tells you what the weather will do in the valley, and the valley is where your walk starts and finishes. It is almost never where the risk lies. What you need is a mountain forecast that gives wind speed and temperature at altitude, a freezing level, and a cloud base in metres rather than a vague symbol of a cloud with two raindrops.

Look for the written summary as well as the numbers. Good mountain forecasters use it to flag the things that actually bite: severe wind chill, spindrift on the tops, persistent hill fog, or a front arriving earlier than expected. Read the whole thing before you look at the grid, because the sentence "rain easing by late morning" can change your plan more than any individual figure.

Wind is the number that decides your day

Wind speeds on hill forecasts are usually quoted for a specific height, often around 900 metres. Exposed ridges, cols and summits regularly see 50 per cent more than the quoted figure, so treat it as a floor, not a ceiling.

• 20–25 mph: noticeable but manageable; hold onto your hat on summits.
• 30–40 mph: hard going, especially with a heavy pack. Balance becomes an issue on rocky ground.
• 40–50 mph: difficult. Walking in a straight line takes effort and exposed edges feel genuinely serious.
• Above 50–60 mph: you will be pushed off your feet. This is not a day for a narrow ridge.

Gusts matter more than the average. If gusts are 25 mph above the mean, expect to be staggered sideways every few minutes. Wind direction is just as useful: it tells you which slopes are sheltered, and — more importantly — which ones will be exposed on the way home when you are tired.

Rain, and what the ground does with it

Rainfall totals are more meaningful than most people assume. Five millimetres over a day is showery weather you can mostly walk through. Twenty to thirty millimetres is a soaking, and on already saturated ground it turns paths into streams. Fifty millimetres or more in an upland area means spate conditions, and steep gills that were a hop on the way out can become impassable an hour later.

Timing is the practical bit. A band of rain arriving at two in the afternoon argues for an early start and a shorter route, or a lower one. Scattered showers with hail and the odd rumble of thunder are a classic afternoon build-up on summer hills — plan to be off the tops by early afternoon rather than caught on a summit plateau with nowhere to hide.

Check the freezing level too. If it is forecast to drop through your route height, expect snow, ice or wet slush on the path, and add an hour to your estimate.

Cloud, visibility and finding your way

Cloud base, or "cloud free heights", is the forecast you should read alongside wind. Bases at 900 metres mean you may well stay in clear air all day. Bases at 600 metres mean the summits are likely to be in fog, and if it has been wet, paths that look obvious in sunshine become a compass-and-pacing exercise.

Watch for the combination of a falling cloud base and a falling freezing level. That is how you get a whiteout — no horizon, no features, no distinction between snow and sky. It is disorientating even for experienced navigators, and it is the point at which many hill days go wrong.

Also check daylight. Sunset times change fast in autumn, and a slow, claggy descent in the dark is a very different proposition from the one you sketched over breakfast.

Deciding whether to change the plan

Most sensible days out on the hills come down to a handful of questions asked honestly:

• Can I name at least two escape routes off this route?
• Where is my turning point, and what time do I need to reach it?
• Could I navigate this route if visibility dropped to thirty metres?
• Is there a lower, sheltered alternative that keeps most of the pleasure?

If you cannot answer the first three confidently, change the route. If the forecast combines strong winds, heavy rain and a low cloud base, cancel the high-level plan and walk the valley instead. A bothy trip in foul weather is a fine thing in its own right, and a low-level walk to a quiet upland tarn can be just as memorable as a summit. The hill will still be there next weekend.

Check again in the morning, and again on the hill

Forecasts shift. Read it the night before to shape your plan, then read it again over breakfast — that is when the short-term updates matter most. Take a printed or saved copy with you, since signal on the tops is unreliable at best.

Once you are out, compare what you are actually getting with what was promised. If the wind is stronger and the cloud lower than forecast, believe your eyes and turn around early. That habit of calibrating the forecast against reality is what turns a cautious hill walker into a confident one, and it is the single best safety skill you can practise on an ordinary Saturday.