Weather at Altitude

Mountains make it possible for snow to be found at the equator and for one side of a mountain range to experience one of the world's wettest climates, while the other side is arid. It has been said that rising 300m in altitude provides the equivalent of moving 350 miles towards the pole at sea level in terms of climate.

As a result, anyone planning to venture into a mountainous region, whether hill walking in the Peak District, or mountain climbing in the Andes or Himalayas, needs to know what type of weather to expect. It is vital that they check the forecast before they leave and, while in the mountains, that they pay constant attention to any changes in the weather.

Air pressure
Air pressure decreases with altitude. The exact reduction in pressure depends on the weather, but an average measure is 1 millibar for every 8 metres you ascend, up to about 2-3000 feet. Everest (8,848m) would have a standard atmospheric pressure at its summit of somewhere between 310 and 360mB, compared to 1013mb at sea level.

Temperature and humidity
As a result of the reduced air pressure, rising air expands and cools. Dry air cools at a fixed rate of approximately 2-3°C per 300m. Moist air behaves differently as once it starts rising, the water vapour starts to condense. This change of water state releases some heat and reduces the rate of cooling.

Precipitation
Mountain ranges act as a barrier to moisture laden winds. Air rising to pass over the mountains cools and the water vapour condenses into cloud, rain and, if it is cold enough, snow. The prevailing global wind patterns mean that rain is more likely to fall on west sides of mountains in mid-latitudes and on the east sides of peaks within the tropics.

Where particularly heavy rain falls on mountains with steep and narrow channels, it can cause some of the worst flash floods in the world. The opposite sides of the mountains to these wet areas are known as rain shadows.

The colder air found up high altitude mountains means that precipitation falls as snow much more often than in the surrounding valleys. For example, in the Rockies locations at high altitude (and therefore generally colder) can receive up to four times the amount of snow fall seen in the valleys.

The Highlands of Scotland receive on average 3000mm rainfall per year, compared to only 800mm near the east coast. Snowfall is similarly higher in the Cairngorms, where snow falls on average 100 days of the year, compared to 20 or less near the east coast.

In fact, as a result of the altitude, snow can be found near the equator on, amongst others, Mount Kenya (5,199m) in Kenya, Mount Kilimanjaro (5,895) in Tanzania and Cotopaxi (6,310m) in Ecuador.

Wind
The windiest place in the world is Mount Washington (1,918m) in New Hampshire, USA. On 12th April 1934 a surface wind speed of 231mph was recorded. As a rule, for example, at 900m wind speed is roughly double that at lower ground.

Not all downslope winds are warm. Even when air warms as it descends, it can still be cooler than the air it is displacing, which results in a biting cold wind. Katabatic winds occur when cold dense air drains down a mountain side into the valley below, such as the 'mistral' in France and 'bora' along the Adriatic coast.

Jet streams are fast moving winds in the upper atmosphere. They are found between five and ten miles above sea level and have speeds of up to 310mph. The height of jet streams means only the extreme mountain climbers are likely to have to consider them.

In fact, the jet stream is the main concern for those facing 'summit day' on Everest. It is only when the jet wind rises and gives a period of calm that a summit attempt is possible. This is normally prompted by the Asian Monsoon starting to move north into the Bay of Bengal, which pressures the jet to move north. Wind speeds on average in the Himalayas are 75mph and can reach more than 100mph.

But how does it feel?
How it feels in the mountains depends on a combination of factors, particularly whether it is cloudy or windy. It can feel relatively warm, even in sub-zero temperatures, if the sky is clear and the sun is shining, while a higher temperature can feel relatively cool if accompanied by a biting cold wind. The cold wind can bring the added problems of wind burn and dehydration in the dry air.

Of course the thinner, or less dense, air at altitude not only makes breathing difficult, but also provides less protection against UV radiation than is found at a lower altitude. Sun burn is therefore much more likely to those unprotected, even on cloudy days, and is even more likely in snow covered areas as the sun's rays are reflected back off the snow.

So, whether you are planning an expedition or a leisurely Sunday afternoon stroll, be aware that the weather can change quickly and dramatically. Its one of the few times where preparation for the unexpected can actually make the difference between life and death.