A mountain forecast is not a valley forecast with a bigger number for the wind. It is a different product, written for a different atmosphere, and most of the disappointment we hear about “wrong” forecasts comes from reading the wrong one, or reading the right one badly. Here we go through the four numbers that matter most to a walker – freezing level, wind at altitude, precipitation probability and the forecaster’s own confidence – and explain what each is actually measuring, so that the evening before a long day we can turn a screen of figures into a plan.
Start with the right forecast, not the nearest town
General forecasts are issued for populated places, which sit in valleys. The air 1,500 m (about 4,900 ft) above that valley is colder, faster and wetter, and a village forecast of 18 C with a light breeze tells us almost nothing about the col we intend to cross. Dedicated mountain forecasts, produced by national weather services and a handful of specialist services in the main ranges, describe conditions at specific altitudes, typically in bands of 500 m or at named summit heights. They read differently too: instead of an icon and a temperature they give a short text discussion, a table by altitude, and increasingly a statement of how sure the forecaster is. The habit worth building is to read the synoptic picture first, then the altitude table for our own high point, then the confidence note. The icon comes last.
Freezing level: the number that changes what we carry
The freezing level is the altitude at which the free-air temperature crosses 0 C (32 F). Above it, precipitation falls as snow or graupel, wet rock freezes, and the puddle on the path at dawn is ice. It is quoted as a height, for example “freezing level 2,400 m, falling to 1,800 m by evening”, and the trend is often more important than the absolute value: a front that drags the freezing level down 600 m in an afternoon will turn rain on a 2,000 m pass into snow while we are still on it.
A rule of thumb links the freezing level to the temperature at our altitude. In the free atmosphere the air cools on average by about 6.5 C per 1,000 m (roughly 3.5 F per 1,000 ft); in dry, sunny conditions the rate is closer to 10 C per 1,000 m, in cloud closer to 5 C. So with a freezing level of 2,400 m and a high point of 1,900 m, we can expect something near 3 C there before any wind. That is the number to feed into the wind-chill calculation, not the valley temperature. The freezing level also describes the ground: above the summits for several days in spring, old snow is soft and rotten by afternoon; well below them for a week, north-facing gullies hold hard neve that needs more than trail shoes.
Wind at altitude: read the gusts, not the mean
Mountain forecasts quote wind for a given height, usually as a mean speed with a gust figure, for example “west 40 km/h, gusts 70 km/h at 2,500 m”. Three things need translating. Wind increases with height because there is less friction from the ground, so a 20 km/h breeze in the valley can quite normally be 50 km/h on the ridge. Terrain accelerates it: cols and the crest of a ridge funnel and compress the flow, so a free-air forecast can be exceeded by half again on a narrow saddle. And it is the gust that knocks us over, not the mean.
As working thresholds on exposed ground: mean winds around 40 km/h (25 mph) make walking tiring and conversation hard; gusts around 60 km/h (37 mph) will stagger an adult on a ridge and make a narrow crest genuinely unsafe; sustained 80 km/h (50 mph) and above is a day for the forest or the valley. Some regional forecasts add a “difficulty of walking” line derived from exactly these thresholds. If ours does not, we do the translation ourselves, and we plan the route so that the most exposed section is done early, when the wind is usually lighter and we still have the energy to fight it.
Precipitation probability: what the percentage is and is not
“60% chance of rain” is one of the most misread numbers in weather. Formally it is the probability that measurable precipitation (usually at least 0.1 mm) falls at a given point during the forecast period. It is not the fraction of the area that will be wet, nor the fraction of the day it will rain, nor a measure of how heavy it will be. A 60% figure is compatible with a two-minute sprinkle at four in the afternoon and with an all-day soaking; the amount, when given, is the separate line labelled precipitation total.
For planning we care about three things the percentage hides: timing (a front crossing at 14:00 is very different from showers from dawn), type (rain, snow or freezing rain, which follows from the freezing level) and amount (5 mm in a day is a nuisance, 30 mm turns stream crossings into a problem). Good mountain forecasts give these in three-hour blocks. With only a percentage, we treat anything above about 40% as “it will rain on us at some point”, and we use the synoptic discussion to guess whether the rain is frontal (steady, hours long, on schedule) or convective (short, heavy, in the afternoon, unpredictable in place).
Confidence: the line experienced walkers read first
Modern forecasts are built by running a model many times with slightly different starting conditions – an ensemble – and seeing how much the runs agree. When they agree, the forecaster writes “high confidence”; when they diverge, “low confidence” or “uncertain timing of the front”. This is not hedging. Some weather patterns are stable and predictable four days out; others sit on a knife edge where a 100 km shift in the track of a low turns a dry day into a wet one.
Our rule is simple. High confidence and a good forecast: commit to the long, exposed day. High confidence and a bad one: do not argue, change the plan. Low confidence: plan for the worse scenario and build in decision points, a col we can reach by 10:00 and turn back from, a lower alternative that shares the first half of the route. And we refresh the forecast on the morning of the walk, because a low-confidence forecast issued 36 hours earlier is worth little next to one issued at 06:00 with the overnight observations in it.
A checklist for the evening before
Read in this order, and write the answers where the whole party can see them:
- Which system is passing, and when does the front or the ridge of high pressure arrive?
- Freezing level at dawn and at 15:00, and the trend between them.
- Mean wind and gusts at our highest point, plus a third for cols and crests.
- Precipitation timing, type and amount, not just the percentage.
- Confidence, and the hour by which we must be off the most exposed ground.
Five lines take three minutes. They turn a forecast from something we look at into something we use, and on the days when the mountain and the model disagree, the person who wrote down what was expected is the one who notices first that the plan needs to change.