Reading a mountain weather forecast

Air cools as it rises, wind accelerates over and around terrain, and cloud forms where rising air reaches its condensation level. The consequence is that a summit a few hundred metres above a valley can be many degrees colder, in far stronger wind, and inside cloud, on a day the valley is pleasant. None of that is unusual weather; it is what mountains do to ordinary weather.

Mountain forecasts exist to describe those effects, which is why they are organised by altitude rather than by place. A forecast that gives one temperature for a region is answering a different question from the one a person going up a hill is asking.

Freezing level

The altitude at which the air is at zeroDecides whether precipitation falls as rain or snow, whether the ground is frozen, and whether snow will be firm or slushy. The single most consequential number on a winter forecast.

Wind speed at height

Not the valley windGiven for summit level, and often several times the valley figure. Above certain speeds walking on a ridge stops being possible regardless of fitness.

Wind chill or feels-like

What the temperature does to a personWind strips heat far faster than still air at the same temperature. This is the number that determines whether stopping is survivable.

Cloud base

The altitude cloud begins atDecides whether you will be navigating in visibility of a few metres. Compare it with the height of your route, not with the summit alone.

Chance of cloud-free summits

A probability, stated as oneUseful precisely because it is honest about uncertainty. A low figure is not a reason to stay in; it is a reason to plan for navigating blind.

Precipitation timing

When, not just whetherWeather arriving at two in the afternoon and weather arriving at nine in the morning produce completely different days on the same route.

The useful discipline is to read the forecast against the specific route rather than in the abstract. A strong wind matters enormously on a narrow ridge and much less in a corrie. A low cloud base matters on a route with complex navigation and much less on a path. The same forecast is a reason to cancel one plan and no obstacle to another, which is why parties with a second, lower option in mind lose far fewer days than parties with one plan.

The other discipline is checking again in the morning. Mountain forecasts are revised, they are least reliable furthest out, and a forecast read on Thursday for Saturday is a planning input rather than a decision. The decision is made on the day, with the current forecast, at the start of the walk-in — and then again at the point where committing to the route stops being reversible.