Welsh 
Mountain Hazard
British mountains are small compared with many others but are very accessible. Every year, millions enjoy days or weeks walking, climbing and camping among the peaks; and every year, some people get into trouble. Some, indeed, are killed, because they went into the mountains unprepared for the weather they met.
Some are unaware of the hazards of mountain weather; some underestimate the risks; some leave decisions too late.
The higher we climb, the colder and windier it usually gets, so that wind-chill increases. Why does the air get colder and windier as we go up mountains?
Air is compressible, so when it is stirred the up-currents expand and cool (like air expanding from a tyre valve), whereas down-currents contract and warm (like air compressed in a tyre pump). When air has been well stirred some time during the previous few days, as is often the case, its temperature decreases upwards – on average by about 6°C in every 1000 metres and sometimes by as much as 10 degrees. This means, for example, that on top of the Munros (the Scottish mountains with tops above 3,000 feet) it can be colder than in the valley bottoms by as much as 10°C. Warm clothing may be needed, even on a sunny day, and ice may be encountered, particularly in the clearer, fresher weather following a wet spell.
It is windy aloft partly because the effect of ground friction decreases upwards and partly because air temperature usually decreases poleward. The latter causes the west-wind component to increase with height, so that gale-force winds (and even stronger winds) are more common at the tops of high mountains than at sea level. Added to that, winds can accelerate around mountains as they do around tall buildings. Over Himalayan peaks, for example, winds over 150 km/h are not uncommon in winter. Hence, climbers are forced to plan their expeditions into narrow 'windows of opportunity’ – periods between winter winds and monsoon rains when the weather is likely to be good. So .. avoid the tops when strong winds at low levels are forecast, and keep away from the windiest spots, such as ridges and the gaps between summits.
Mountain tops are often in cloud, spoiling the view and leading to a risk of getting lost. Clouds are formed by condensation in rising air, and the cloud base is lower in moist air than in dry. Air is particularly moist when the wind blows from the sea or when there is persistent rain. In the British Isles, the cloud base is usually below 1000 metres, often much below, so it is no wonder that clouds envelop our mountains, especially as they are mostly in the west and the first to be affected by rain areas sweeping in from the Atlantic. Clouds can be supercooled, i.e. their water remaining liquid even when the temperature is below freezing. The droplets in such clouds freeze when they hit fence posts, rocks and vegetation – indeed anything that stands up into wind, including people! The soft ice can build up over hours or days into a thick deposit of rime that can cause as much difficulty as lying snow.
The rain and snow that fall from nimbostratus clouds –
and sometimes even passing showers from cumulonimbus clouds –
tend to be heavier and longer-lasting over mountains than over
nearby low-lands. This is because winds are lifted over the windward
sides of mountains, causing clouds to become more dense, and raindrops
larger, over windward slopes than over lee slopes. However, strong
winds may blow the falling drops over to the leeward side before
they can reach the ground, so the heaviest rain need not be over
the highest ground.
Persistent driving rain in strong winds requires adequately waterproofed clothing. Keep a look-out for deep depressions approaching from the west, as they bring rain and strong winds. Remember, too, that persistent rain makes boggy areas very unpleasant. Moreover, mountain streams swell rapidly, making them difficult or even impossible to cross, with implications for reversing the route if the weather becomes too bad.
Thunderstorms are certainly impressive among mountains, but there is a risk of lightning strikes in high places. Avoid the tops if thunder is likely. Watch for tell-tale clouds, particularly those approaching from upwind. If you are caught out, retreat to lower levels, or sit out the storm in a spot away from prominent rocks. Keep your hands off the ground, though, to reduce the risk of a current passing through your body.
Snow on the hills makes a pretty scene, but it slows walking and makes navigation more difficult. Because temperatures decrease upwards on average, snow becomes more likely than rain at greater heights. In fact, much of our rain starts off aloft as snow, melting as it falls. So, particularly in winter, do not be surprised to find snow falling on the tops when there is only rain in the valleys. Snow combined with strong wind (a blizzard) can be life-threatening. Drifting greatly reduces progress; and the risk of wind chill makes it unwise to wait 'until the worst is over'. Avoid the hills when a blizzard is likely. In winter, the left side of a depression tracking across Britain is a likely place for a blizzard over high ground.
Snow combined with poor visibility in cloud also causes problems because shadows disappear. Navigation becomes almost impossible and can lead to blundering into dangerous places ' even over the edges of precipices. This is a 'whiteout'. If there is already much lying snow and a risk of the cloud base descending onto the hills, it is wise to consider abandoning the trip.
Even very experienced mountaineers get caught out by the weather, even killed. To avoid unnecessary risks, listen to and understand the latest weather forecast, and modify it in the light of your own observations of changes. Take note of informed local knowledge. Change your plan if the weather is likely to get seriously worse than expected. Enjoy the weather among the hills. Go prepared.
On 6 March 1967, there was a gust of wind of 232 km/h at the Cairngorms Weather Station, Scotland, altitude 1,074 metres.
On 15 January 1968, the mean hourly wind speed at Great Dun Fell, Cumbria, altitude 857 m, reached 159 km/h. The same speed was reached at Lowther Hill, Scotland, altitude 736 m, on 20 January 1963.
This Weather as a Hazard feature was written by David Pedgley, author of Mountain Weather, a 150-page guide for all who go hill-walking or climbing in the British Isles. The book is fully illustrated with weather maps, satellite images and cloud photographs and is recommended not only for those who walk in the hills or climb mountains but also for outward-bound instructors and for teachers of GCSE and A-level Geography. Published by Cicerone Press, the book can be purchased from the Society.