Longer days fail to bring spring warmth to Caithness
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WEATHER statistics for April 2012 revealed it was Wick’s coolest and wettest April since 2000.
The longer days and strengthening sunshine failed to deliver a single day that could be deemed as being seasonally warm.
North-to-north-easterly winds featured prominently; these greedily imported raw polar and biting Arctic air masses, firmly coshing any prospect of warmth while also generating frequent showers or longer outbreaks of rain, sleet and hail.
The miserable weather was due to a weakening of the (NAO) North Atlantic oscillation – a large-scale atmospheric pressure seesaw that effectively determines the position and configuration of the polar-front jet stream.
The jet stream is a narrow corridor of fast-flowing westerly winds present in the upper atmosphere that steers the weather systems that affect our day-to-day weather.
Normally the core of very strong winds associated with the jet stream follows a rapidly moving, fairly straight line from west to east driving the depressions and fronts swiftly across the North Atlantic.
However, there was a very significant departure from this more typical regime that had become firmly established during March that consolidated its control over the Far North’s weather as April progressed.
Most importantly, the pathway followed by the jet stream during March and April began resembling a river with a slow-moving flow meandering from north to south.
In March the jet stream strayed far to the north of the British Isles allowing high pressure to develop, effectively "blocking" the usual west-to-east movement of the Atlantic depressions. Consequently in March Scotland enjoyed record-breaking warmth and exceptional dryness for the time of the year.
However, all this had changed by the start of April when crucially the meandering pattern shifted westwards and instead the UK found itself under the southern bend or meander of this "river".
The jet stream now followed a track far to the south of the UK over Iberia and France.
The unfortunate consequences that now arose from this change in the ongoing "buckling" of the jet stream were copious amounts of rain coupled with below-average temperatures caused by the areas of low pressure that had become stuck close to or over the UK because there was no west-to-east jet stream to push them along.
WICK’S mean temperature for April was 5.6C (42.08F). The long-term mean for the month is 6C (42.8F).
The town’s average daytime maximum temperature was 8.59C (47.46F), down 0.41C on the long-term slot for April. The highest maximum – 10.7C (51.3F) – recorded in a moderate west-south-westerly wind during a brief incursion of relatively mild Atlantic air on the 8th was nevertheless still very disappointing for the time of the year.
Remarkably this was to be the sole occasion during the entire month that the thermometer registered double figures in Celsius.
The lowest daytime maximum occurred on the 2nd when the mercury toiled to peak at a miserly 4.1C (39.4F) in the cold and showery north-to-north-easterly breeze that affected the area on that date.
The burgh’s overnight minima averaged out at 2.61C (36.7F), down 0.39C on the established value for April. The highest overnight minimum was 6.8C (44.2F) recorded on the 25th. The lowest overnight minimum reading was minus 1.2C (29.8F) recorded on the 25th.
The town also experienced an air frost on the 16th, 28th, 29th and the 30th. Ground frosts featured on 13 dates. Lowest temperature over the grass was minus 5C (23F) noted on the 29th and 30th respectively.
Precipitation was logged on 23 dates. By the end of the month the rainfall total had added up to 83.1mm or 184.7 per cent of the long-term average amount for the time of year. Wick’s wettest day was the 20th when 11.1mm (0.44 of an inch) filled the rain gauge. Snow was visible on the grass at dawn on one date – the 3rd.
The sluggishness of the NAO became even more pronounced during April.
Throughout the month the pressure difference between the Azores high and the Iceland low was consistently small hence winds were less strong over the North Atlantic. The corresponding and striking absence of strong winds that affected Wick during the month reflected this anomaly.
April 2012 hosted just three days when the wind reached or exceeded gale-force eight (39 mph/33.9 knots).
Windiest day was the 26th when a force-seven north-to-north-easterly wind gusted up to 48 mph/41.7 knots and severe gale-force nine on the Beaufort scale.
