Seeing through fog

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  • Horses in the fog on the Gallops at Cheltenham Racecourse on Monday 12 March, ahead of the 2012 Cheltenham Festival. Photo: Tim Ireland/PA Wire.

    Fog starting to lift above the tower of Westminster Cathedral, London, on Thursday morning, 15 March, in a snapshot taken from the MeteoGroup Weather Room.

    High resolution satellite image at 08:00 on Thursday 15 March, revealing fog blanketing much of England and Wales.

    Shallow fog hugging the River Severn at Ironbridge, Shropshire. Photo: Nick Potts/PA Archive/Press Association Images

  • Seeing through fog
    16.03.2012 08:04

     

    NASA, the National Aeronautics and Space Administration, has developed goggles that can “see through” fog.

    Of course, these fog-goggles - "foggles, if you like - are nothing like X-Ray goggles or the like beloved of 1950s B-movie science fiction. Instead they employ augmented reality, which means that a heads-up display can overlay a virtual version of one’s surroundings.

    This would be particularly useful for airline pilots, for instance, who would then be able more safely to land in fog by being accurately and instantaneously shown the position of runways and aerodrome buildings. 

    It would be a long time before such technology could be used in vehicles, and until such time it behoves motorists to drive according to the conditions.

    Unfortunately, though, there were a number of traffic accidents, particularly in the early hours of Thursday morning when fog was especially dense across parts of England and Wales. Visibility was hampered to less than 50 metres in a number of places.

    How did it get so foggy? What developed during the past few nights was radiation fog, so called because it forms when the land cools through thermal radiation on clear, still nights.

    The high pressure that has dominated Britain recently provided ideal conditions. On Wednesday evening and night it produced clear skies in many areas with very light winds. The air mass was chilly yet quite moist.

    The ground’s daytime warmth readily radiated away in the absence of any cloud to reflect it back down, and the lower layers of the atmosphere cooled in turn.

    As the temperature dropped the air became saturated and its water vapour condensed out into fog. Although it was a calm night it was not completely still – for fog to form properly there has to be a breath of wind to mix moisture into drier air above; otherwise fog will be very shallow or not form at all, and heavy dew or frost will result instead. Too much wind, though, and the mixing will be too thorough and spread through too large a depth of the atmosphere.

    As the nights are still quite long ahead of the vernal equinox, there was plenty of time for the fog to thicken before it was slowly eroded through Thursday morning by the warmth of the rising sun.

    In summer the short nights, drier ground and daytime warmth mean that radiation fog is far less likely than between autumn and spring. However, another type of fog can then be more prevalent around the British Isles.

    This is advection fog, caused by the horizontal movement (or advection) of air that is warmer than the surface over which it is moving.

    A prime example is the low cloud and fog that can plague the east coast in summer when a breeze from the easterly quarter sets in. Warm continental air passes over the relatively cool North Sea and picks up moisture to boot.

    The bottom of the air mass is cooled to its saturation point, and low cloud and fog form and envelop the coast. If there is enough of a breeze it can be pushed several kilometres inland, and might even produce drizzle, especially when forced to rise over hills. For this sort of fog, wind speed is far less relevant.

    This is what is know as North Sea fret in England and haar in Scotland. To the west people may be basking in warm summer sunshine but anyone mistakenly heading for a grand day out at the beach at, say, Scarborough or St. Andrews will find themselves shivering under grey skies and gloom.

    Similarly in a south-westerly air flow areas such as southwest Wales, southwest England and western Scotland can find themselves stuck for days under low cloud, coastal and hill fog and drizzle.

    At least with radiation fog, when there is a bit of strength in the sun in spring and autumn it tends to lift and thin, just as it did on Thursday across the eastern half of England to leave a warm and sunny afternoon.

    By: Stephen Davenport