How do I read a mountain weather forecast?
The specialist mountain forecasts, the four fields that actually decide your day, and how to convert a summit wind speed and a freezing level into a decision.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 5 min
- Safety
- warning
Short answer
Use a mountain-specific forecast such as the Met Office mountain area forecasts or MWIS rather than a general app. Read four things: wind speed and gusts at summit level, the freezing level, the cloud base and the chance of cloud-free summits, and the visibility. Above about 40mph walking becomes difficult; above 50mph it is not sensible. Temperature drops roughly 1°C per 100 metres of ascent before you add wind chill.
General weather apps forecast for a settlement at a given altitude. Mountains make their own weather, and the difference between the valley and the tops is routinely the difference between a jacket and a genuine hazard. Both the Met Office and the Mountain Weather Information Service publish free, detailed forecasts for each British upland area, written for people who are going up.
Safety
Step by step
- Use a mountain forecast, not a town forecast.The Met Office publishes mountain area forecasts for the main British upland regions, and MWIS publishes area forecasts written in plain language. Both give summit-level values and both are free. A general app forecasting for a village 400 metres below the summit is answering a different question.
- Read the summit wind speed first.Wind is the field that most often decides whether the day happens. Roughly: below 25mph is unremarkable; 30 to 40mph makes walking on exposed ground tiring and unbalancing; above 40mph it is hard to walk in a straight line and hard to hear each other; above 50mph, on a ridge or a summit, people get blown over. Gust values matter more than the mean, because it is a gust that knocks you off balance.
- Convert the wind to a chill temperature.Wind chill is the reason people get into trouble in summer. A 3°C summit with a 40mph wind feels far below freezing to exposed skin and strips heat from wet clothing extremely fast. Every mountain forecast gives a feels-like or wind chill figure — read it, and dress for it.
- Find the freezing level.The height above which temperatures are below zero. If the freezing level is below summit height, expect ice on paths, rime on rock, and frozen ground — and above it in winter, that means axe and crampons ground and a different kind of day entirely. A freezing level dropping through the night is what turns yesterday's wet path into today's ice.
- Read the cloud base and the chance of cloud-free summits.Cloud base is the height at which you walk into fog. If it is below your route's high point, plan for navigation in poor visibility and be honest about whether the group can do that. MWIS gives a percentage chance of cloud-free summits, which is the most useful single number for whether you will see anything.
- Look at the precipitation type and timing, not just the amount.When it arrives matters more than how much. Rain arriving as you reach an exposed ridge is a different problem from rain in the last hour of a descent. Hail and sleet at altitude are far more chilling than the same amount of rain.
- Check the visibility figure separately.Forecasts give visibility in metres or kilometres. Under about 100 metres, navigation is by compass and pacing only, and route-finding on complex ground becomes slow and serious.
- Apply the lapse rate to work out summit temperature.Temperature falls by roughly 1°C per 100 metres in dry air and around 0.6°C per 100 metres in saturated air. A 900-metre summit is commonly 6 to 9°C colder than the valley before any wind chill at all.
- Read the whole text, not just the icons.MWIS in particular writes a short paragraph on how the day will feel and what the hazards are — that prose contains judgements that no icon carries. It is the most useful part.
- Decide before you leave, and again at the car park.Mountain forecasts are updated at least daily and often twice. Check the latest version the morning you go, and have an alternative lower-level route already chosen so that changing plan is easy rather than a defeat.
Tips
- MWIS and the Met Office sometimes disagree. Reading both and taking the more cautious of the two is a reasonable default.
- In winter, add the avalanche forecast from the Scottish Avalanche Information Service for Scottish areas — it also carries excellent snow and weather observations.
- Wind speeds are usually given for summit level. Cols and ridges funnel wind and can be substantially windier than the forecast figure.
- A forecast of 'hill fog clearing by mid-afternoon' still means navigating in cloud all morning. Plan for the conditions you will actually be in, at the time you will be there.
Common mistakes
- Reading the forecast for the nearest town — It is forecasting hundreds of metres lower, in shelter, and it will show a mild day when the tops are at freezing with a 45mph wind.
- Focusing on rainfall and ignoring wind — Rain is uncomfortable. Wind is what causes falls, hypothermia and the inability to make progress, and it is by far the more common reason a day goes wrong.
- Checking the forecast the night before and not again — Fronts move. The morning update is the one that matters, and it takes two minutes.
Questions people ask
At what wind speed should I not go up?
There is no single number, because it depends on the terrain, the group and how exposed the route is. As a general guide, over 40mph on exposed ridges is unpleasant and unbalancing, and over 50mph people are blown off their feet. Take a lower, sheltered route instead.
What is a temperature inversion?
A layer where temperature rises with height instead of falling, so the valley is cold and cloudy and the summits are warm and clear above a sea of cloud. Mountain forecasts flag them, and they make for the best days of the year.
How far ahead is a mountain forecast reliable?
The first 24 hours are good, two to three days are useful for planning, and beyond that treat it as a trend rather than a plan.