Why the Valley Forecast Won't Do

A forecast for the nearest town is a starting point, not an answer. Hills make their own weather, and the higher you go, the more the numbers change. As a rough rule, expect the temperature to fall by about one degree Celsius for every hundred metres of ascent, so a mild nine degrees in the valley can mean three or four degrees on a 900-metre summit before wind chill is even considered. Add a forty-mile-an-hour wind and the effective temperature drops well below freezing.

Wind is the other great divider. Forecast speeds usually describe open, exposed ground, and they rise sharply with height. A ridge, a col or a summit plateau can see considerably more than the valley, while a sheltered corrie may feel almost calm. When you read a forecast, ask yourself where the measurement is meant to apply and how exposed your route actually is.

Wind Speed and Gusts

Forecasts usually give a mean wind speed and a separate gust figure. The mean tells you how hard the day will feel; the gust tells you whether you can stay upright. On a British hill, gusts of 40 mph are enough to make walking difficult and conversation nearly impossible. At 50 to 60 mph you will be leaning into the wind, watching your footing on every step, and a stumble can become a fall. Above that, most hillwalkers should be thinking about a lower route rather than a summit.

Wind direction matters as much as strength. A westerly gale that batters an exposed ridge may leave the eastern corrie perfectly walkable. Check the direction against the shape of your route, and note that forecast directions often veer or back through the day. A tailwind on the way out becomes a headwind on the way back, which is rarely welcome.

Visibility, Cloud and the Look of the Day

Visibility is the forecast element people skip, and it is often the one that decides whether a route is enjoyable or an ordeal. Hill fog is not the same as valley mist. Cloud base forecasts in metres tell you roughly where the cloud sits, so a base of 600 metres means any summit above that will be in it. If your route tops out at 900 metres, assume you will spend the upper part of the day navigating rather than admiring the view.

Precipitation cuts visibility further, and so does wind-driven rain or spindrift. A forecast visibility of 20 to 30 metres on high ground means compass work, pacing and timing. Ask yourself honestly whether you are happy navigating in that, and whether the rest of your group is.

Precipitation: Amount, Type and Timing

• Probability versus amount. A 70 per cent chance of light rain is a very different day from a 70 per cent chance of heavy showers. Look for expected totals and intensity, not just the percentage.
• Timing. An hour of rain is a jacket on and off. Six hours of steady rain means wet ground, greasy rock and a much slower pace than you planned.
• Type. In winter, check the freezing level and how it changes through the day. Rain turning to snow halfway up a route changes everything underfoot.
• Ground conditions. After heavy rain, saturated bog absorbs nothing, streams rise quickly, and a crossing that was easy in the morning may be serious by mid-afternoon.

Putting It Together for Your Route

Before committing to a high route, run through a short set of questions. It takes five minutes and it will change your day for the better.

• What is the highest point on the route, and what is the forecast for that height rather than the valley?
• What are the mean wind speed and the gust speed, and is my route exposed to that direction?
• Where is the cloud base relative to my summits, and can I navigate if visibility drops to 30 metres?
• How much rain or snow is expected, and over what period?
• In winter, is there a separate avalanche report, and does it cover the slopes I intend to cross?
• How easily could I shorten the day if conditions turn out worse than forecast?

Adjusting the Plan Rather Than Abandoning It

Poor weather on the tops rarely means staying at home. It usually means choosing a different objective: a lower fell, a wooded valley, a route on a sheltered aspect, or a shorter circuit with an early turn-around point. Build that flexibility into the plan before you set off, and tell someone where you are going and when you expect to be back.

Weather on British hills also moves quickly. A forecast written at six in the morning can be overtaken by a front arriving early, so carry a map and compass regardless of what your phone says, and be willing to turn back. Reading a forecast properly is not about seeking certainty. It is about understanding the range of conditions you might meet and being prepared for the top end of that range. Do that, and you will make better decisions on the hill, come home with drier boots, and enjoy far more of your days out.