Mountain weather

Liz Bentley, Chief Executive, Royal Meteorological Society

Every year millions of people head up into the mountains to trek, ski, and enjoy the wonders of the mountain scenery. However, for some the experience isn’t so pleasant, as they venture onto the mountain unaware of the weather they may meet. Some people will underestimate the severity of mountain weather, be unaware of the mountain conditions, or leave decisions on the weather too late when on the mountainside. As a result, every year, even in the UK, people die on the mountains as a consequence of the weather.

Something as simple as cloud cover can hugely impact climbing to the top of a mountain. Mountain peaks are often shrouded in clouds. As you walk up into that cloud, visibility rapidly decreases, which can result in people getting lost. Clouds are formed by the condensation of water vapour in rising air. In the UK, the cloud base (the bottom of the clouds) can be below 1,000m, so low that much of the time the clouds cover the mountain tops. Cloud cover on the mountains is particularly common in the west of the UK, where the moist air blows in from the Atlantic. Some clouds can be supercooled; that is, the water remains a liquid even when the temperature is below freezing (0°C). These supercooled water droplets freeze when they hit solid objects such as fences and even people. The ice can build up into a thick layer, known as rime, which can cause as much difficulty for walkers as lying snow.

Rain and snow falling over mountains tends to be heavier and longer lasting than over nearby low-lying areas. The windward side of a mountain tends to be wetter than the leeward, more sheltered side of the mountain. This is because the air is forced to rise over the mountains, causing the air to cool as it rises and condensing the water vapour from a gas to liquid water droplets. If the temperature is low enough, the precipitation will fall as snow rather than rain, and above the snow line – the boundary between a snow-covered and snow-free surface – the snow will settle and provide long-lasting snow cover. Snow combined with strong winds can lead to blizzards and very poor visibility, known as a ‘whiteout’, which can be life-threatening on a mountain top. Warm clothing on the top of mountains is definitely needed.

The higher up the mountain we climb, the colder and windier it usually gets and the wind chill factor increases. The wind chill factor takes into account the wind speed and humidity to provide a ‘feels like’ temperature of how cold it feels on the human skin. On the Munros, Scottish mountains with tops above 3,000m, it can be 10°C cooler at the top of the mountain compared to the valley bottom below. In fact, air can cool by 6°C in every 1,000m of altitude and sometimes as much as 10°C, and that’s before you factor in the wind chill.

A recent article published in Review of Geophysics (doi.org/ 10.1029/2020RG000730) looks at how climate change is impacting mountain weather. The report states that mountains have been warming around 25% to 50% faster than the global mean since around 1950. It also says that there is increasing evidence that mountain precipitation, caused by rising air up mountain slopes, is not as enhanced as it was in the past. Although precipitation is generally increasing in many mountain regions, it is not increasing as fast as expected, given a warmer atmosphere. Finally, there has been a change in precipitation type, with snow increasingly falling more as rain over recent decades.

These changes in temperature and precipitation have been detrimental to snow, ice, and glaciers, with nearly all mountain glaciers receding worldwide. This has accelerated in many regions in the past two to three decades. There has also been an uphill migration of climate zones, causing many species and their associated habitats to move upslope with it. In many parts of the world, mountains provide freshwater supplies to approximately 1.6 billion people. As our climate changes, these once-reliable water supplies are disappearing. Finally, as snow is replaced by rain, we see more mountain flash-flooding events and associated hazards such as landslides, which are likely to become more frequent.