Hotter-bourne

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  • The jet stream (band of red arrows) across the western northern hemisphere on 22 March 2012. Note the trough-ridge-trough pattern. The deep trough over the central USA has developed a 'cut-off' low centred over Oklahoma/Texas - to its east is a commensurate ridge bulging northwards, producing record-breaking temperatures. Likewise the trough over the Atlantic means a ridge over Europe, with the jet well to the north. Image copyright MeteoGroup.

    This graphic starkly illustrates the heat wave over North America. It shows land surface temperature anomalies over a week, based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite. These are temperature anomalies compared to the average (2000-2011) of the same eight day period of March. Image courtesy of NASA Earth Observatory

    A high resolution true-colour image from the MODIS instrument aboard NASA's Aqua satellite on 22 March 2012. It clearly shows a veil of suspended dust across the southern North Sea and much of England and Wales that most likely has its origins in North Africa. image courtesy of NASA/MODIS

  • Hotter-bourne
    23.03.2012 09:39

     

    On Thursday the temperature exceeded 20 degrees Celsius for the first time this year in the UK, the mercury creeping above that mark at Otterbourne in Hampshire

    The maximum temperature was 20.1 degrees Celsius, which is also just higher than the national March 22 date record, last set in 1918.

    Many parts of the country had temperatures rising at least to the mid teens but there were exceptions. Showery bursts of rain over south-western parts of England and Wales suppressed temperatures such that some locations struggled towards 11 or 12 degrees; and towards eastern coasts of England and Scotland, although the weather was dry it was decidedly chilly and locally cloudy and misty. Temperatures didn’t breach double figures in a few spots, due to the southeast and easterly winds cooling over the North Sea, the waters of which are at about their coldest point of the year.

    The situation will be similar on Friday and Saturday, although showers in the western half of the country will die away, but then a subtle change of wind direction should mean that eastern coasts will be a bit less cold on Sunday. Meanwhile temperatures in a few spots across Central Southern England and the Southeast could rise to 20 or 21 degrees Celsius.

    That’s the sort of maximum temperature we can on average expect in mid to late June. However, there are a couple of reasons why it won’t feel as warm as June: the sun is lower in the sky so its radiation is weaker than in June and doesn’t feel as warm on the skin; and it will be colder early and late in the day than one would ordinarily expect in June, with the peak temperature rather short lived.

    Even so, to get above 20 degrees in March is noteworthy. However, the record March temperature won’t be challenged; that’s 25.6 degrees Celsius which was logged at Mepal in Cambridgeshire on 29 March 1968.

    North America has been even more remarkably warm with several locations in Canada and the northern USA breaking their March high temperature records, in some places on successive days.

    Pellston, Michigan – a notably cold spot in the state - was an astonishing 27 degrees above average on Wednesday; but perhaps most extraordinary was St John, New Brunswick, where the temperature rose to 25.4 deg C on Wednesday. That not only smashed the previous March high temperature record for the city of 17.5 deg C but also easily bettered the highest April temperature on record, which is 22.8 deg C.

    This warmth has been present for one week to ten days, and the weather historian Christopher C. Burt suggests that such a heatwave in early spring is unprecedented. The cause is a deep trough of low pressure over the western USA where it is far colder with rain and mountain snow, which has forced a strong bulge of high pressure and warmth northwards over the central and eastern states.

    Similarly, although less extravagantly, a low pressure trough in the mid-Atlantic has forced the Azores High to ridge strongly NE across the British Isles and the European continent, developing a separate high pressure centre over the North Sea and northeast Europe which is feeding a south-easterly flow of winds towards the UK.

    To complete the picture, there has been little rain to clear the atmosphere and scrutiny of satellite pictures reveals a thin layer of suspended dust over the southern half of Britain that has most likely drifted up from the Sahara. Look out for dusty deposits anywhere it rained in the southwest.

    By: Stephen Davenport