Reading Weather Forecasts Before Heading to the Hills

A cloud-with-a-raindrop icon tells you almost nothing about whether a day on the hills is sensible. The useful information is buried in the details: wind speed and gusts, cloud base, the amount of rain in millimetres, and the height at which the air is freezing. Learning to read those numbers takes ten minutes and will change how you plan a mountain day.

Start with the numbers, not the symbols

Most consumer apps give you a summary icon and a single temperature. Mountain forecasts go further. You want wind speed and gust speed at a stated height, cloud base or the level of the cloud-free zone, precipitation probability and expected amount per hour, the freezing level, and the visibility if it is offered. If a forecast gives you numbers without saying at what altitude they apply, treat them with caution: a wind speed quoted for sea level can be 20 to 30 mph weaker than what you will meet on a 900-metre summit. Always check the forecast is for the hills, not the nearest town in the valley.

Wind is the decision-maker

Wind stops more mountain days than rain ever does. Rain is uncomfortable; wind makes walking impossible and can knock you off your feet on an exposed ridge.

As a rough guide for an adult carrying a day pack:

• 10–20 mph: noticeable on ridges, but manageable. Add a windproof layer and carry on.
• 20–30 mph: hard work on exposed ground, especially against you. Expect to lean into it.
• 30–40 mph: balance becomes unreliable, and it is genuinely difficult to walk in a straight line or hold a conversation.
• Over 40 mph: for most hillwalkers this is a no-go on summits and ridges. Gusts are often nearly double the average, so a mean of 35 mph means gusts of 60 mph.

Check the direction too. A 35 mph westerly might be perfectly pleasant on the sheltered eastern side of a hill and brutal the moment you cross the shoulder. Plan a route that keeps the worst of it behind you, or drop your ambitions to a lower, wooded valley walk.

Visibility and cloud base

Visibility is measured in metres, and the thresholds that matter are practical ones. Above about a kilometre, distant features are hazy but you can still navigate by eye. At around 500 metres, you should be relying on map and compass rather than landmarks. Below 100 metres, you are in the sort of mist where you cannot see the next cairn, and navigation becomes a genuine skill requirement rather than a nice-to-have.

Cloud base forecasts tell you where the hills will be in the cloud rather than under it. A forecast of "cloud free above 700 metres" means the summits are clear. "Cloud base 300 metres" means you will spend the whole day in grey, damp, disorientating conditions. Low cloud on its own is fine for a confident group; low cloud combined with strong wind and steady rain is where incidents happen. Ask honestly whether everyone in your party can take a bearing, follow it and count paces when they cannot see more than a few metres ahead.

Precipitation: timing matters more than total

A 90% chance of rain sounds grim until you notice it amounts to 0.3 mm per hour, which is drizzle you will barely feel. A 40% chance of 8 mm per hour is a different proposition entirely: that is heavy rain, and it will soak you through in an hour. Look at the hourly breakdown rather than the daily total.

Timing is everything. A front passing through between 9am and 11am means an early start or a late one may keep you dry. Persistent orographic rain over western hills can last all day. Also think about what the rain does to the ground underfoot: saturated peat, greasy rock, and burns or streams that rise quickly and become awkward or unsafe to cross.

Temperature, freezing level and wind chill

The freezing level tells you the altitude at which the air reaches zero degrees. If it is forecast at 900 metres and you plan to be on a 1,000-metre summit, expect ice on the path, verglas on rock and frozen ground where you least want it. If the freezing level is above the summits, you will probably be wet rather than icy.

Temperature alone is misleading, because wind strips heat away from you. A still 5°C day is comfortable in a mid-layer. The same 5°C with a 40 mph wind feels closer to -3°C on exposed skin. Add rain and you have the classic conditions for hypothermia in a group that is moving slowly. Factor in wind chill when you decide what layers, hats and gloves everyone is carrying, and remember that the slowest member of the group is the one who gets coldest.

Turning the forecast into a decision

Decide your thresholds before the day, when nobody is invested in the summit. For a mixed-ability group, a workable rule is no summit plans above 40 mph gusts, no committing ridge routes when cloud base is below the ridge height, and a shorter, lower route if the rain is forecast to be heavy and persistent through the middle of the day.

Check the forecast the evening before and again on the morning, because mountain conditions change quickly and yesterday's picture is often stale. Compare two independent sources if you can. Look at daylight hours as well as weather: a route that takes eight hours is not viable if sunset is at 4.30pm.

Finally, give yourself permission to change the plan. A low-level walk in driving rain can be a brilliant day out, and it builds the skills and judgement that make bigger days possible later. The hills will still be there next weekend.