Scotland

What is the Foehn effect?

Flooding Image copyright PA

The west of Scotland is set to be hammered by heavy, persistent rain in the coming days because of weather fronts moving in from the Atlantic.

However, the east of the country will continue to escape largely dry, with the north east being bright and unseasonably mild.

The weather pattern which produces those higher temperatures in the north east is known as the Foehn effect.

It's very common in Scotland due to our landmass, our mountain ranges and our close proximity to the sea.

This pattern tends to develop in a warm sector - the air mass stuck between a warm front and a cold front.

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Media captionWatch BBC Weather's John Hammond explain the Foehn effect

High humidity air is created within this zone. When you add in strong winds, this lifts the air over high ground.

When air rises, it takes any moisture out of it and dries the air, turning it warmer - often by several degrees.

At the moment, we're seeing strong southwesterly winds bringing in moisture from the Atlantic.

That moisture will manifest itself in the form of heavy rain over west Highlands.

As the air crosses the Cairngorms, we lose the moisture. The air dries out, taking the cloud with it, and hence we see dry, sunny weather for the north east, along with higher temperatures.

That means that on Saturday, we'll see 10 or 11C in the west, as opposed to 15 or perhaps 16C in the north east.

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