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Jet streams shift with the seasons and help control our weather patterns


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Weather Watch by Keith Banks

Snowfall, Rumster Forest, January 31. Picture: David G Scott
Snowfall, Rumster Forest, January 31. Picture: David G Scott

Winds show considerable variation between the upper and lower regions of the atmosphere. The most prominent features of high altitude winds are the jet streams – ribbons or cores of very fast moving air.

The name "jet" is not given to the winds at these altitudes because it is the level where jet aircraft fly (although they regularly do fly at these levels), but because the ribbon of strongest winds occupies a very narrow zone, rather like a jet of water gushing from a hosepipe.

Arguably, the most powerful and important jet streams occur in the temperate latitudes, in both hemispheres. The dynamics of the polar front jet (PFJ) stream, that influences the day to day weather across the UK, are very complicated.

However, in brief and simple terms the PFJ is caused by the sharp contrasts in horizontal temperatures that occur when the warm, moist sub-tropical air encounters the cold and the dry polar air.

This convergence of air masses, where the less dense, warm air is forced to rise over the more dense polar air, occurs near the boundary between the troposphere and the stratosphere, at an altitude of between seven and nine miles above the Earth's surface, where the wind velocities increase dramatically.

The polar jet stream can be thousands of miles long, a couple of hundred of miles across, and have a vertical depth of several miles. In both hemispheres, the PFJ moves north and south with the seasons, as the horizontal temperature gradients shift with the areas that are in receipt of the strongest solar energy.

In the stratosphere, in the autumn and the winter , there is another fast flowing westerly jet, that develops, called the polar night jet. This jet flows around the polar vortices that are present in both hemispheres.

January 2021 was Wick's coldest since that of 1985. Closer examination of the burgh's record for mean air temperature for January affirmed that it is currently the 15th most cold in a series stretching back to 1910.

The town's precipitation archive for January showed that January 2021 was the wettest since that of 2016, and that it is presently the 48th driest in a series commencing from 1910.

In terms of wind, there was one "day of gale" logged. Indeed, one of the most striking features of the month was the virtual absence of wind.

Wick's mean air temperature for January 2021 was 2.20C (36.0F). The long-term average, in terms of the averaging period 1981-2010, is 3.70C (36.66F).

Wick's average maximum daytime air temperature for January 2021 was 4.20C (39.56F). The long-term average is 6.20C (43.16F). Highest maximum was 6.8C (44.2F), recorded on January 16. Lowest maximum was 0.7C (33.3F), on January 23.

The town's average overnight minimum air temperature for January 2021 was 0.20C (32.4F). The long-term average for the month is 1.20C (34.16F).

The highest overnight minimum air temperature for the period was 4.9C (40.8F), observed on January 14.

There were 20 air frosts. Lowest minimum air temperature for the month was minus 3.8C (25.2F), on January 28.

The temperature fell to 0.0C (32.0F), on 22 dates. The lowest temperature recorded at 5cm over the grass was minus 6.9C (19.6F), noted on January 6.

Precipitation was measurable on 29 dates. The total for January 2021 was 70.6mm (2.78 inches), or 97.6 per cent of the long-term average for amount for the month. Wettest day was January 20. The quantity for the 24 hours commencing 9am (GMT), was 14.8mm (0.58 of an inch).

Wind velocities reached or exceeded gale force 8 (39.1mph/33.9knots) on just 2 dates.

Windiest day was January 21, when a force 8 north-westerly wind, associated with the circulation of Storm Christoph, gusted up to 53mph/46.1knots, severe gale force 9 on the Beaufort scale.

Snow was visible over the grass at dawn on nine dates.


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