How the Foehn Effect works

11th March, 2015

Hopefully you all managed to find some time to enjoy the glorious early-spring weather we experienced over the weekend. In our blog last week we questioned whether spring had sprung and it is safe to say that Mother Nature gave us a definitive answer over the weekend. In fact the weather was so nice that Murlough in County Down experienced the warmest day of the year so far on Saturday, with temperatures reaching 17.5°C.

So given we are only a week into meteorological spring, what was the cause of these unseasonably warm temperatures? Forecasters reported the above average temperatures were bolstered by the “Foehn Effect”. In the UK our landmass, mountain ranges and close proximity to the sea mean this phenomenon is particularly common; especially across the Highlands of Scotland, the Pennines and the Welsh mountains.

How the Foehn Effect Influences Our Weather

  1. Moist air running perpendicular to a mountain range rises as it comes into contact with high ground.
  2. As the air rises it cools at a rate of 0.5°C for every 100 metres it rises. So if a mountain is 1000m high and the air is 10°C at sea level, it will have cooled to 5°C by the time it reaches the mountain's summit.
  3. As it rises the air will cool to such a point that clouds begin to form and rain falls.
  4. Once the air passes over the top of the mountain it begins to warm and dry out. The clouds evaporate and sunny spells break out.
  5. As the air flows down the other side of the mountain it warms up at a much faster rate than it cooled - 1°C for every 100m. So the 5°C air at the top of the mountain will have warmed to 15°C once it reaches the bottom.
  6. In this example we would therefore expect to see a difference in the air temperature of 5°C depending on which side of the mountain you live.

This phenomenon was demonstrated as recently as Saturday. In the north west of Scotland the air temperature coming off the Atlantic was approximately 10° or 11°C. Once it had travelled across the Cairgorms to the north east, the air temperature had risen to 15° or 16°C.

Protecting Our Clients with 24/7 Forecasting

It is differences in weather conditions such as these that the team at our operations centre are trained to identify. We know that conditions can differ dramatically, even across a relatively small area and whilst there may be no risk to one of our clients, another just a few miles could require our services. Using post code specific data we are able to monitor these differences and act where necessary to ensure that our clients' premises are suitably protected from any risk of snow or ice.

Click here to see BBC Weather's Chris Fawkes explain how the Foehn Effect can influence the weather across Scotland. Alternatively, if you would like to find out more about how we can assist your business, do not hesitate to get in touch to arrange a FREE consultation and no obligation quotation.

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