The warming Föhn wind

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  • Observed maximum air temperatures on Thursday, with several locations to the east of high ground making it past 20C.

    The synoptic pattern at midday on Thursday, with the isobars indicating a westerly flow across the British Isles.

    A high resolution satellite image from Thursday, showing a pile up of cloud across many windward slopes and wave-like, broken clouds to the lee.

  • The warming Föhn wind
    27.09.2014 12:49

     

    Originally a term used in the European Alps, the Föhn (or Foehn) is now used as a general term for a strong, warm and dry downslope wind occurring to the lee of high ground. Many parts of the world experience Föhn-type winds, with the effect spotted in several parts of the UK just a couple of days ago.

    So what exactly is it?

    A Föhn wind occurs when a relatively stable, moist and mild airmass comes up against a hill or mountain barrier and is forced to rise. As the air moves up the windward slopes, it cools at the dry adiabatic lapse rate, which is roughly 1C for every 100m gain in elevation. After a time the air will have cooled to such a degree that is becomes saturated and water vapour will begin to condensate out, forming clouds and commonly rain over the top and windward sides of the hill or mountain. The condensation process releases latent (hidden) heat from the air parcel which, while continuing to rise up the windward slope, will cool at a slightly lower rate than before – roughly 0.5 to 0.6C per 100m. By the time the air parcel has cleared the top of the hill or mountain, it will have lost much of its moisture and is no longer saturated, thus during its trip back down the leeward slope, it will warm up at the original lapse rate of 1C per 100m.

    The end result? 

    An air parcel will end up warmer and drier on the leeward side of a hill or mountain compared to a similar elevation on the windward side, and wind speeds will also increase. On Thursday, north-east Scotland, north-east England and south-eastern parts of Northern Ireland benefited from this warming effect. Respectively it was the Grampians, Pennines and Mournes acting as the mountainous barriers to the general westerly airflow, resulting in leeward areas recording maximum temperatures of 20 or 21C, compared with the mid- to high-teens in those areas west of the mountains.

    And outside of the UK?

    The Föhn effect in the UK on Thursday was far from remarkable, with the potential for temperatures to rise by 20C in a matter of hours and relative humidity levels to drop below 10%. The strongest Föhns are more likely to be seen around higher mountainous regions than those in the UK, such as the Alps and Rockies, where it is more commonly known as a Chinook wind. The effects of a Föhn can also become quite extreme: in winter, the warm and dry air can cause rapid snow melt, earning the nickname ‘Snow-eater’. And in summer, it can lead to drought and the spread of wildfires.

    By: Laura Caldwell