Weather for hill walking and the mountains
How Ireland's hills turn a mild day into a cold one, the wind and cloud on the tops, which months and mountains are wettest, how to read a valley forecast for a summit, and when to turn back.
Ireland's mountains are modest in height, from Carrauntoohil at 1,039 metres down to hills you can climb before lunch, and that modesty is the trap. The weather on top is not a slightly worse version of the weather in the car park; it is a different climate, and it changes in the time it takes to eat a sandwich. This guide explains how to turn the forecast for a town into a forecast for the summit.
Colder with every hundred metres
Air cools as it rises, by roughly 1°C for every 150 metres in damp Irish conditions, sometimes faster on a dry, sunny day. Starting from 12°C in Killarney, the top of Carrauntoohil is about 5°C before the wind is counted. The Wicklow summits, Lugnaquilla and the Dublin hills, run 5 to 6°C colder than the city, and the Mournes and the Galtees about the same. In winter that means rain in the valley is sleet at 400 metres and snow on the tops, and a thaw at sea level can still leave hard ice on the Reeks for a week.
Windier too
The ground slows the wind, so the higher you climb the less shelter there is. A summit typically has one and a half to two times the wind speed of the valley, and gusts stronger again, especially on ridges and in the gaps between peaks where the air is squeezed. A Force 5 at the trailhead, a breezy day by lowland standards, is a Force 7 on the ridge, which is hard to walk against and dangerous on a narrow edge. The wind and gust layers on the forecast map show the pattern across the country, and the feels-like guide explains what that wind does to the temperature: 5°C in a Force 7 on the summit feels like -2°C.
Cloud on the tops
The thing that most often spoils a day is cloud. In a moist Atlantic airflow the cloud base sits at 300 to 600 metres, which puts the top third of most Irish mountains inside it. Visibility drops to twenty metres, the path vanishes, cairns look alike and every slope leads somewhere you did not mean to go. The humidity and cloud figures in the hourly forecast are the clue: a forecast of 95% humidity and overcast skies, with the rain symbol for the afternoon, means cloud on the tops even if the valley is dry. The best mountain days come in high pressure with a dry easterly or northerly, when the cloud base is high and the air is clear enough to see from Mweelrea to Croagh Patrick.
The wettest ground in Ireland
Mountains make their own rain. Air forced up a slope cools and drops its moisture, so the western mountains catch two to three times the rainfall of the lowlands: the MacGillycuddy's Reeks, the Twelve Bens and the hills of Mayo and Donegal receive well over 2,000 mm a year, and parts of the Reeks more than 3,000, against about 750 mm in Dublin. It comes mostly from October to January, when the Atlantic depressions line up, and it comes in a day: a yellow rain warning for Kerry means streams that were stepping stones in the morning are waist-deep by the afternoon. The county rainfall pages show the month-by-month figures, and the records page the wettest days ever measured.
The best months
May and June give the most daylight, the driest ground and, in a good year, the clearest air; the midges have not yet arrived in the west. September and early October are often settled and the colours are at their best. July and August are warmer but hazier and showerier, with thunder possible on a hot afternoon, and the hills are busiest. From November to March the days are short, the ground is sodden, the tops are often iced, and a walk that takes five hours in June takes six in the mud and leaves no margin for a wrong turn before dark; sunrise and sunset for your start point are on its town page. The guide to the best months covers the year as a whole.
Reading the forecast for a summit
- Take the hourly forecast for the nearest town in the valley, say Killarney for the Reeks or Wicklow for Lugnaquilla.
- Subtract about 1°C for every 150 metres of height to get the summit temperature.
- Multiply the wind by one and a half to two, and allow gusts half as strong again.
- Work out feels-like from those two figures, and dress for it, not for the car park.
- Check the cloud and humidity: anything near 100% means fog on the tops.
- Look at the rain layer and the radar loop before you set off, and again at the last point with signal.
When to turn back
The mountain rescue teams of Kerry, Wicklow, Mayo and the rest are called out most often not for falls but for people caught by cloud and darkness in weather that was forecast. Turn back when the cloud comes down and you are not certain of the route, when the wind on a ridge makes you stagger, when a stream you crossed easily is rising, and always when daylight will not cover the descent. An orange or red warning for the county, explained in the warnings guide, is a day for the low-level loop or the pub. If something goes wrong, call 112 or 999 and ask for Mountain Rescue; there is no charge, and they would rather come early than late.