Islands in the sun

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  • Forecast chart for next Tuesday 03 April. High pressure has shifted west of the British Isles and a cold front is expected to be clearing southwards, leaving us in a much colder northerly flow. Image: MeteoGroup

    The urban heat island effect - cities showing darker on this infrared satellite picture overnight 28-29 March because they are less cold than the surroundings.

    Temperature profiles for High Wycombe (yellow line) and Chesham (red line), both in Buckinghamshire, illustrating a high diurnal ranges 27th-29th March - especially for the Chesham frost hollow, up to 10 degrees colder than High Wycombe (slight UHI effect). Image: MeteoGroup

  • Islands in the sun
    29.03.2012 15:59

     

    After more than a week of warmth for much of the UK there will be a sea-change through the weekend and early next week, with north to north-westerly winds bringing much colder conditions.

    Maximum temperatures will drop by about 10 or 12 degrees and there is even the chance of wintry showers over high ground in northern Britain, although nothing to get too exercised about. There will be rain at times elsewhere but not enough to make much of a dent in the rainfall deficit in the east and south.

    However, if you have been up and about early enough (or been up all night) you will have noticed a distinct chill through the hours of darkness and around dawn, and even a frost in places. At this time of year the conditions that are producing warm sunshine during the day can also be ideal for promoting cold nights.

    Under the high pressure dominance of the past week or more, atmospheric subsidence means that clouds won’t form and the sun’s warmth is increasing as we in the northern hemisphere get further past the vernal equinox. If there is a lot of moisture at low levels, though, then fog and stratus can develop overnight and still be reluctant to thin by day, as we saw at times last week.

    With clear skies the sun is strong enough now as April approaches to cause sunburn, and it can heat the ground nicely. This in turn warms the air above it, with more rapid warming when the ground is dry, as it is in so many places at the moment.

    Overnight the clear skies and calm air typical of an anticyclone allow virtually unfettered re-radiation of that warmth back upwards into the atmosphere, and at the Earth’s surface the air chills. It can do so rather rapidly in susceptible spots, leading to huge diurnal ranges in temperature. For example, the “frost hollow” at Chesham in Buckinghamshire, nestled in the Chilterns, saw a temperature range of 24.3 degrees Celsius between night and day on March 28th.

    There is less of a day-night contrast in cities. This is due to the “urban heat island” (UHI) effect, which mans that city centre temperatures can stay several degrees higher than nearby rural areas overnight. The effect can also be present by day but is far less noticeable.

    The main, although not only, cause is the high concentration of buildings that are constructed from materials that have quite a high heat capacity, so storing warmth by day, and which radiate heat less quickly than other materials. Rather like a storage heater they can absorb warmth while the sun is shining then gradually release it at night, keeping the surrounding air warmer than it would otherwise be.

    The morning of March 28th offers a good example. Chesham recorded a minimum air temperature of -3.3 degrees Celsius and Wisley in Surrey had -0.6 degrees while the temperature at St James’s Park in central London did not fall below 5.5 degrees.

    An infrared satellite picture is worth seeking out overnight. As infrared shows colder areas as whiter, in the absence of cloud you can make out whole cities, looming dark amidst lighter surroundings.

    By: Stephen Davenport