Why mountain forecasts look nothing like your phone's weather app

You check the app, see a cheerful sun symbol for the town at the foot of the valley, and pack light. Three hours later you are bent double in horizontal sleet on a summit plateau, wondering where it all went wrong. It did not go wrong — you were simply reading a forecast for the wrong place.

Most consumer forecasts are generated for populated, low-lying places: towns, airports, farmland. A mountain forecast has to describe a three-dimensional landscape where conditions change with every 100 metres of height and every change of aspect. A summit 900 metres above the valley might be 6°C colder, in cloud when the valley is clear, and experiencing winds two or three times stronger than the car park you left from.

So the first habit to build is simple: read a forecast for the summit, the ridge and the valley separately. If you only have one number, use the highest ground you intend to visit.

Wind: the number that decides whether you go

Rain makes you wet. Wind makes you slow, cold and, occasionally, unable to stand up. It is the single most useful figure on any mountain forecast, and the one most often skimmed over.

• Under 20 mph: barely a factor on most ridges, though it will still bite on an exposed summit.
• 20–30 mph: noticeable. Walking becomes effortful on open ground, and hats, maps and loose kit start to disappear.
• 30–40 mph: genuinely hard going. Balance is affected on rocky ground and you will be leaning into the wind.
• Over 40 mph: difficult to walk in a straight line. Gusts will knock you off your feet on exposed ground.
• Over 60 mph: time to change your plan. Escape routes and navigation become very difficult indeed.

Always look at the gust figure, not just the average. A steady 25 mph with gusts to 50 mph is a different day from a steady 25 mph. Also check the direction: a westerly that has been blowing for days means saturated ground and full spate burns, while a cold easterly in winter brings hard snow and rimed rock.

Rain: understanding probability, amount and timing

A 60% chance of rain sounds vague, but it is only one of three rain numbers worth reading. The others are how much and when.

Two millimetres spread over a day is drizzle you barely notice. Twenty millimetres in three hours turns a path into a stream, fills the gullies and makes river crossings genuinely dangerous. Look at the hourly breakdown rather than the daily total, and note whether the heaviest rain arrives before, during or after your planned descent.

Ask yourself what the ground will do with that water. In the UK, upland peat and thin soil shed water quickly, so burns rise fast and drop fast. In Scandinavian fjells, where much of the ground is bare rock and scree, rain runs off almost instantly and paths can become slick slabs. A wet forecast is not automatically a no — but it does mean you should reconsider anything involving a ford, a gorge or a steep grass descent.

Temperature, feels-like and the freezing level

Mountain air cools roughly 1°C for every 150 metres of ascent, which is worth doing in your head before you leave the house. Valley at 8°C, summit 900 metres higher: expect about 2°C at the top, before wind chill is even considered.

Wind chill is not a gimmick. A 5°C day with 40 mph winds feels closer to −5°C on exposed skin, which is the difference between a comfortable walk and the early stages of hypothermia. Add rain and you lose heat roughly three times faster than in dry air.

In winter, the freezing level is the figure you need. If it is forecast at 700 metres and you plan to reach 1,000 metres, you are heading into ice, hard neve and possible windslab. If it sits above the summits, expect wet snow, slush and a thoroughly miserable descent through saturated heather.

Cloud, visibility and the cloud base

The forecast cloud base tells you where the mountain effectively ends. A base at 600 metres means summits are inside the cloud, and visibility there could drop to 20 metres or less. Navigation becomes a compass-and-pacing job rather than a case of following the path.

Cloud also removes the thing you came for. A clear valley with a summit in cloud is a walk with no view. If the forecast suggests cloud lifting through the afternoon, consider reversing your route to climb later rather than earlier.

Watch for convective language — showers, bright spells, heavy at times. That usually means cloud building through the morning and towering over the high ground by early afternoon, with sharp showers and possible thunder. An early start is your friend.

Building a simple check before you set out

You do not need to become a meteorologist. Work through the same five questions every time, and the forecast will start to read like a route plan.

• What is the wind speed, gust and direction at summit height? This decides whether the route is possible at all.
• Where is the cloud base relative to my highest point? This decides whether I can navigate and whether I will see anything.
• How much rain, and when? This decides my timings and my river crossings.
• What is the summit temperature and the freezing level? This decides my layers and my footwear.
• What is my escape route if the forecast is wrong? Because sometimes it will be.

Finally, read the forecast the morning you leave, not the night before. Mountain weather shifts quickly, and a plan that was sound at 9pm can be a poor idea by 6am. Take a paper copy or a screenshot, keep a spare layer you did not think you needed, and be genuinely willing to turn around. The best forecast in the world is no substitute for a flexible head on the hill.