Wick Weather: Pressure zones over North Atlantic have big influence on our conditions
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Weather Watch by Keith Banks
The North Atlantic Oscillation (NAO) is an atmospheric phenomenon that wields a profound influence on the climate of the Northern Hemisphere.
The types of weather experienced over the British Isles, Europe, together with the north-eastern part of the US and Canada, particularly during the winter, are determined by the significant fluctuations in the pressure differences that frequently arise between two quasi-permanent weather systems.
One is known as the Azores High, and the other is called the Icelandic Low. In very simple terms, the physical process that occurs has been described as behaving like an enormous atmospheric pressure see-saw.
Crucially, the NAO affects the strength and the configuration of the jet stream that is responsible for steering surface weather systems the from west to east across the North Atlantic.
It has two phases – one is positive the other is negative. When the pressure difference is greater than average, the NAO Index has a positive number. This situation arises when the pressure of the Azores High is more intense than the average, or alternatively when the Icelandic Low is deeper than usual.
Conversely, when the pressure rises over Iceland, and/or falls over the Azores region, the pressure gradient between both is lower than the average; hence the NAO Index has a negative number.
In terms of weather, when it is positive, the jet stream is more powerful and flows more directly from west to east. This phase allows moist and mild south-westerly winds, associated with fast moving depressions, to affect the British Isles, and to penetrate far into northern and eastern regions of Europe.
During the negative phase, the jet stream tends to be displaced much further southwards than usual. In winter, this steers the mild moist air associated with the Atlantic depressions towards the Mediterranean basin, leaving the British Isles to face northerly or easterly air streams with much drier but bitterly cold Arctic or continental air masses.
July 2021 was Wick's most warm since that of 2019. Closer scrutiny of the town's historic record for mean air temperature for July showed that it is the 11th warmest in a series stretching back to 1910.
In terms of precipitation, July 2021 was Wick's most dry since 1955, and that it is the royal burgh's second driest in a series commencing from 1910.
Wick's mean air temperature for July 2021 was 13.95C (57.11F). The long-term average, in terms of the averaging period 1981-2010 is 13.0C (55.40F).
Wick's average maximum daytime air temperature for July 2021 was 16.38C (61.48F). The long-term average for July, in terms of the value for this parameter, for the 30-year average period is 16.0C (60.80F).
The highest maximum was 24.6C (76.3F), observed on July 16. This is Wick's fourth-equal hottest day since 1910. Maximums of 19.9C (67.8F), and 21.4C (70.5F), recorded on July 14 & 17 respectively, were also notably warm temperatures for the town. The lowest maximum was 13.2C (55.8F), noted on July 4.
The burgh's average overnight minimum air temperature for July 2021 was 11.52C (52.74F). The long-term average for July is 10.0C (50.0F). Highest overnight minimum was 15.2C (59.4F), recorded on July 17. Lowest ambient temperature for the month was 4.7C (40.5F), logged on July 1.
The lowest temperature at 5cm over the grass was 2.6C (36.7F), witnessed on July 1.
Precipitation was logged on 24 dates. The total for July 2021 was 12.0mm (0.47 of an inch), or 19.5 per cent of the long-term average amount for July.
The wettest day was July 6. The quantity recorded for the 24 hours commencing 9am (GMT) was 2.8mm (0.11 of an inch).
There were no "days of gale" experienced during the month. The strongest wind velocity was witnessed during the hour ending 15 hours (GMT), on July 17, when a force 5 westerly wind gusted up to 38.0mph/33.0 knots, force 7 on the Beaufort scale.
