Pressure zones in the North Atlantic have a big influence on our conditions


Weather watch by Keith Banks

St Fergus Church on July 16. This day turned out to be Wick’s fourth hottest day since 1910 with the afternoon temperature peaking at 24.6 ° C (76.3 ° F). Photo: Keith Banks

The North Atlantic Oscillation (NAO) is an atmospheric phenomenon that has a profound influence on the climate of the northern hemisphere.

The types of weather encountered over the British Isles, Europe, as well as the northeastern part of the United States and Canada, especially during winter, are determined by the large fluctuations in pressure differences that frequently occur between two quasi-permanent weather systems. .

One is known as the Azores high and the other is called the Icelandic depression. In very simple terms, the physical process that occurs has been described as behaving like a huge swing at atmospheric pressure.

Crucially, the NAO affects the strength and pattern of the jet stream that is responsible for steering surface weather systems from west to east across the North Atlantic.

It has two phases, one positive, the other negative. When the pressure difference is greater than the average, the NAO index has a positive number. This situation arises when the pressure of the Azores high pressure is more intense than average, or when the Icelandic low pressure is deeper than usual.

Conversely, when the pressure rises in Iceland, and / or falls in the Azores region, the pressure gradient between the two is lower than the average; therefore, the NAO index has a negative number.

Meteorologically, when it is positive, the jet stream is more powerful and flows more directly from west to east. This phase allows humid and gentle southwesterly winds, combined with rapid depressions, to affect the British Isles and penetrate far into the northern and eastern parts of Europe.

During the negative phase, the jet stream tends to move much further south than usual. In winter, this directs the mild, moist air associated with the Atlantic depressions towards the Mediterranean basin, leaving the British Isles to deal with northern or eastern drafts with arctic air masses or continental much drier but extremely cold.

July 2021 was Wick’s hottest since that of 2019. A closer look at the city’s historic record for average air temperature for July showed it to be the 11th hottest in a series dating back to 1910.

In terms of precipitation, July 2021 was the driest in Wick since 1955, and it was the second driest in the Royal City in a series starting from 1910.

The average air temperature in Wick for July 2021 was 13.95 ° C (57.11 ° F). The long-term average, in terms of the 1981-2010 average period, is 13.0C (55.40F).

The average maximum daytime air temperature at Wick in July 2021 was 16.38 ° C (61.48 ° F). The long-term average for July, in terms of the value of this parameter, for the average 30-year period is 16.0C (60.80F).

The highest maximum was 24.6C (76.3F), observed on July 16. This is Wick’s fourth hottest day since 1910. Highs of 19.9C (67.8F) and 21.4C (70.5F), recorded on July 14 and 17 respectively, were also particularly warm temperatures for the city. The lowest high was 13.2 ° C (55.8 ° F), noted on July 4.

The town’s average overnight minimum air temperature in July 2021 was 11.52 ° C (52.74 ° F). The long-term average for July is 10.0C (50.0F). The highest night-time minimum was 15.2 ° C (59.4 ° F), recorded on July 17. The lowest ambient temperature for the month was 4.7 ° C (40.5 ° F), recorded on July 1.

The lowest temperature 5 cm above the grass was 2.6 ° C (36.7 ° F), observed on July 1.

Precipitation was recorded on 24 dates. The total for July 2021 was 12.0 mm (0.47 inch), or 19.5% of the long-term average amount for July.

The wettest day was July 6. The amount recorded for the 24 hours starting at 9 a.m. (GMT) was 2.8 mm (0.11 inch).

There were no “galey days” during the month. The strongest wind speed was observed during the hour ending at 3 p.m. (GMT) on July 17, when a force 5 westerly wind blew up to 38.0 mph / 33 , 0 knots, force 7 on the Beaufort scale.

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