Mountain Weather

Ben Nevis

From the lofty heights of the Himalayas to the rather more modest peaks of the British Isles, mountains can have some fascinating effects on the weather.

Temperature, wind and rainfall are all heavily influenced by these geographical wonders, to the extent that mountain ranges can impact the climate of vast areas.

The impact mountains have on rainfall is one of the most significant. When moist air moves over mountains it is forced upwards, causing a rapid fall in temperature which condenses the moisture. This creates clouds and rainfall where there was none (called orographic rain), or adds to existing weather systems to intensify rainfall (similarly called orographic enhancement).

As the same air crests a mountain peak, it descends much drier because much of the moisture has already been forced out. Thus we see a classic pattern where one side of a mountain range (the side from which weather typically approaches) will be particularly wet while the other will be particularly dry. We see that here in the UK, with western areas such as the Lake District being much wetter than areas to the east. More extreme examples occur elsewhere, so the Andes in South America create a rainforest basin to the east and a desert-like landscape to the west.

The same effect also causes a phenomenon known as Foehn winds - where air which has lost much of its moisture after flowing over a mountain increases in temperature as it descends. This creates a warm, dry wind flowing off the mountains - sometimes boosting temperatures by as much as 30 °C. In the UK the impact isn't quite as large, but eastern areas such as Aberdeen can be significantly warmer than areas to the west of high ground.

Fohn effect

There are two other key types of wind influenced by mountains: anabatic and katabatic.

Anabatic winds happen because land warms faster than air, so when the sun beats down on the top of a mountain it heats up faster than the air at the same altitude around it. This causes the air over the mountain to rise, which in turn drags up air from lower down - creating an often warm wind which flows up the mountain.

Katabatic winds happen when cold air on top of a mountain or glacier descends because it is denser than milder air lower down. Gravity sees that cold air accelerate down the mountain to the point where, in places like Antarctica, these winds can reach hurricane force. They are generally cold for obvious reasons, but can be warmed if they flow over hotter surfaces on their way down - the Santa Ana wind in California is one example.

Anabatic and Katabatic winds

There's an even bigger factor to talk about when it comes to mountain weather - temperature.

We all think of mountains as cold, often snow-covered, environments - but why are they like this and do we ever see exceptions?

Let's start with the basic physics. The Sun's rays come in through the atmosphere and heat up the Earth's surface, so the lower part of the atmosphere is warmed from the ground up. This means the further up you go, the further away you are from that heating influence so you lose temperature with altitude - this is called 'vertical temperature gradient'.

It follows an easy to remember pattern in dry air: roughly speaking, you lose 1 °C for every 100 metres in altitude gained. So the top of Ben Nevis (at 1,344 metres) could be about 13 °C colder than at sea-level.

The temperature difference will be less in moist air, which we see a lot of in the UK, so we can't always rely on that formula to be exact. That said, you can expect that most of the time you will see a significant fall in temperature as you ascend - particularly when you include the impacts of more wind exposure increasing wind chill.

However, sometimes the vertical temperature gradient can be turned on its head, leading to what we call a temperature inversion - which means sometimes it will get warmer as you ascend. This happens when the air at ground level is colder than the air above and there are two potential causes for this.

Firstly, warm air can move in over colder air - because the colder air below is denser, it becomes trapped there. The second cause is when the ground cools rapidly, cooling the atmosphere from the ground up while the air above remains milder. This typically happens on clear, still nights, as the ground rapidly radiates its stored heat.

Both these causes of inversion trap cooler air below, which often leads to misty or foggy conditions. It can also mean clouds form and are trapped at lower levels.This means you can ascend through cloud to break through into the sunshine and have beautiful views of the cloud below - making any surrounding peaks look like islands in the mist.

Clearly this doesn't happen all that often, so always it's safer to always expect it to be colder up the mountain than down. Also, while inversions do mean it can get milder as you ascend, conditions can always change quickly so it's always advisable to pack your usual warm and waterproof layers just in case.

Last updated: 29 January 2014