How to read a mountain forecast

Temperature drops by roughly 1°C for every 100 m you climb when the air is dry, and about 0.6°C when it's wet. A mild 12°C in Fort William is 2°C on the top of Ben Nevis before the wind. Wind speeds on summits are typically two or three times what they are in the glen, because there's nothing to slow the air down. And cloud sits at a height, so a town under blue sky can be below a lid that the summit is inside.

So a forecast for the town is a forecast for a place you're leaving. The summit forecast on every TRXO walk page is for the highest point of the route, from the Met Office's own model, and that's the one to read.

Rain is uncomfortable. Wind is what stops you. Up to about 30 mph you'll notice it; from 30 to 40 it's hard work and you'll be leaning into it on ridges; from 40 to 50 you'll be knocked about and it's no longer safe on narrow ground; above 50 mph most people can't stand up on an exposed summit, and above 60 you shouldn't be there. Gusts matter more than the average: a 35 mph wind gusting 55 is a 55 mph day.

Wind also multiplies the cold. A 0°C day in a 30 mph wind feels like −10°C on bare skin. The forecast's 'feels like' figure is the one your hands and face will believe.

Our rule of thumb: summit wind over 40 mph, or gusts over 50, and we pick a lower hill or a sheltered glen. Over 60, we don't go.

The cloud base is the height at which the hill goes into cloud. If it's forecast at 700 m and your summit is 900 m, you'll spend the top part of the day in mist with no view and no landmarks to navigate by. That's fine on a path with a cairn every fifty metres; it's a serious problem on a featureless plateau, which is why the Cairngorms are so dangerous in cloud.

Forecasts give a chance of cloud-free summits, often as a percentage. Thirty per cent means it will probably be in cloud. Ninety means plan for the view. The cloud base also moves during the day; a base that lifts after lunch is a reason to start late.

The height above which the temperature is below zero. Above it, rain falls as snow, wet rock turns to ice, and paths become something you need crampons for. In winter the freezing level is what decides whether a walk is a walk or a mountaineering day; if it's below your summit, assume ice and snow on the top and carry the kit or change the plan.

In summer it's usually above the hills, but not always. A freezing level of 1,000 m in June means sleet on Ben Nevis, and people have died of exposure on Scottish summits in August.

A 40 per cent chance of rain means that, on days like this, it rained at this spot four times out of ten. It doesn't mean it'll rain for 40 per cent of the day. For walking, the amount matters as much as the chance: a 60 per cent chance of 1 mm is a shower you'll barely notice; a 60 per cent chance of 15 mm is a soaking, swollen burns, and a river crossing you may not be able to make on the way back.

Rain on its own rarely ends a day. Rain with wind and a low cloud base is the combination that does.

Read the summit forecast the night before and again in the morning; mountain forecasts change fast. Look at the wind first, then the cloud base, then the freezing level in winter, then the rain. Decide what you'll do if it's worse than forecast, which it will be about a third of the time, and tell someone your plan.

Then go, and enjoy the day. A hill in cloud and wind, within your limits, is still a hill. The forecast's job is to help you pick which one.