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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 potentially much drier because much of the moisture has already been rained (or snowed) 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 of mountain effects on rainfall occur elsewhere, for instance linked to the South American Andes range. 

The same effect also causes a phenomenon known as Foehn (or Föhn) 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 North America, east of the Rockies, the Chinook wind is a powerful example of the Foehn phenomenon, known for its ability to "eat snow". 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

Other key mountain winds

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

Anabatic winds happen typically when the land is strongly warmed by sunshine. As the lowest layers of the air over the sunny slopes are warmed by the land, a current is set up 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 extreme 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

Mountain temperatures

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

We 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. Due to the interaction between gravity, pressure and temperature when the air is mixed in the vertical, you normally expect the atmosphere to get colder as you ascend - this is called the adiabatic lapse rate of temperature with height.

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

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