Why the Valley Forecast Won't Do

It's tempting to check the weather for the nearest town, see a dry day with light winds, and start loading the rucksack. But a valley forecast describes a place that could be 600 metres lower and sheltered by the very ridges you plan to walk. Conditions on a Highland Munro or a Welsh 3,000 can differ completely from the town at the foot of the glen: wind two or three times stronger, temperatures five degrees lower, and cloud sitting on the tops while the valley basks in sunshine.

Mountain-specific forecasts exist precisely because of this gap. Whatever source you use, read it with the terrain in mind. The useful question isn't "what is the weather doing?" but "what will it be doing at 900 metres on a north-east-facing slope at two in the afternoon?"

Wind: The Number That Decides Your Day

Wind is the most common reason hill days get cut short, and the figure that matters is the summit wind speed, not the one in the car park.

• Under 20 mph: barely noticeable on the move; only a nuisance on summits and cols.
• 20–30 mph: you'll feel it leaning into you. Worth checking how exposed your ridge section really is.
• 30–40 mph: hard going. Balance suffers on rough ground, and you'll be shouting to be heard.
• 40–50 mph: difficult to walk in a straight line on an exposed crest. Hands free, head down, and seriously consider a lower route.
• Over 50 mph: most people are knocked off balance. Avoid narrow ridges, exposed scrambles and anything with a big drop beside it.

Always check gusts as well as the average. In mountain terrain, gusts routinely run 50 per cent above the mean, and they arrive sideways on cols and through the gaps between summits. Then add wind chill: a still 5°C in the valley can feel closer to −3°C on a summit with a 40 mph wind. That's the difference between thin gloves and full winter kit.

Freezing Level: What's Underfoot

The freezing level is the altitude at which air temperature reaches zero. It tells you what state the ground is likely to be in, and it's the detail summer walkers ignore most often.

As a rule of thumb, temperature drops by roughly 2°C for every 300 metres of ascent, a little less in damp air. So if the freezing level is forecast at 900 metres and your summit sits at 1,100 metres, expect the top 200 metres to be at or below zero.

• Freezing level above the summits: rain everywhere, wet rock, saturated peat. Grip and speed matter more than warmth.
• Freezing level at or just below the tops: the classic UK hazard — rain in the valley, wet snow or freezing rain on the summit, and icy patches on slabs and stiles.
• Freezing level well below the tops: genuine winter conditions. Axe and crampons, and a route that suits them.

Think about aspect too. Snow and ice linger on north and east-facing slopes and deep in gullies long after the valley has thawed. A freezing level that climbs through the day also tells you the snowpack is softening — what was firm neve at nine in the morning may be slush by two.

Precipitation: Chance, Amount and Timing

A "60 per cent chance of rain" says nothing about how heavy that rain will be. It's a probability that measurable rain will fall somewhere in the forecast area. A 30 per cent chance can still deliver a soaking squall; an 80 per cent chance might be two hours of light drizzle. So read the expected amounts in millimetres, and the timing, alongside the percentage.

• Frontal rain usually arrives as a band with a clear start and finish. You can plan around it.
• Showers are hit and miss, but the heavy ones bring hail, sudden loss of visibility and a sharp drop in temperature.
• Upland enhancement is real: western hills can collect two or three times the valley rainfall over the same period.

Timing is where you gain the most. If rain is due at 3pm, an early start on a shorter route may have you back at the car before it arrives. If the front is coming in at 10am, the plan probably needs to change, or the day needs to move.

Turning the Numbers Into a Decision

Before you commit, run through the forecast in this order: summit wind, freezing level, precipitation timing, then cloud base and visibility. Then ask three practical questions.

• Is the route still appropriate? An exposed ridge in 45 mph gusts is a different proposition from the same ridge in 15 mph.
• Can I shorten or lower it? A glen walk, a forest track or a lower summit can give most of the day's value with far less of the risk.
• Where is my turnaround point? Decide it before you leave, based on the forecast rather than on how you feel at the col.

Finally, treat anything more than three days out as a rough sketch rather than a promise. Check again the night before and once more over breakfast, and stay willing to change your plan in the car park. Packing for the forecast you hoped for, rather than the one in front of you, is how capable walkers end up in trouble. The hills will still be there next weekend.