Foehn effect

What is it?

Issued 07/03/2015

The Foehn (Föhn) effect has been discussed a lot recently. It is a rain shadow which brings higher temperatures to areas in the lee (downwind from the rain) of high ground. 

This occurs when moist air moves over higher ground. The air cools as it rises and the moisture condenses falling as rain. This dries the air out and as it travels down over the leeward slopes it warms up again.

The temperature of moist air varies less with altitude than dry air. So the temperature rises more as the air moves down over the leeward slopes than it falls when it climbs over the windward slopes.

A couple of charts below illustrate the effect. On the first one yellow and green shading shows heavy rain in western Scotland. Notice how to the east it's drier.

The second one shows temperatures. Across Scotland there's a big temperature gradient with values of 8C (46F) in the west and to the east of the Highlands they're reaching 16C (61F)! This is the Foehn effect in action.

The effect is enhanced by strong winds which help to lift the air over higher ground. In the UK our geography and climate means eastern Scotland is one of the favoured for this weather phenomenon to develop. If eastern Scotland is the warmest part of the country the Foehn effect is often the reason.

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