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Not all clouds are the products of natural processes


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WEATHER WATCH: Keith Banks with his monthly round-up of the highs and lows of Wick's weather

Sunrise at Staxigoe harbour, March 23. Note the scattering of the different-sized wavelengths of the visible light emitted by the Sun. The physical process is known as Rayleigh scattering. Picture: Alan Hendry.
Sunrise at Staxigoe harbour, March 23. Note the scattering of the different-sized wavelengths of the visible light emitted by the Sun. The physical process is known as Rayleigh scattering. Picture: Alan Hendry.

Clouds are ubiquitous, and manifest in many different types, species and varieties. However, it is worth noting that not all clouds are the products of natural processes: some are man-made.

The most common are the so-called contrails (condensation trails) that form in straight lines behind aircraft, usually at very high altitudes. Contrails are usually the products of supercooled water vapour that freezes instantly when it encounters ice crystals that may already be present, and/or the products of combustion ejected by aircraft. Supercooled water is water that exists in a vapour or liquid state, at temperatures below its usual freezing point.

When the environment is very dry, contrails disperse very rapidly. But when the air already has a high humidity the cloud can persist, and on occasions may even merge together, and form a fibrous-looking sheet of high-altitude cirrostratus or perhaps cirrocumulus (both cloud species composed entirely of ice crystals) that can obscure the Sun.

Distrails (dissipation trails) are paths or patches of clear air, evaporated from cloud by the passage of aircraft, when the heat from the engines tends to be a more important factor, in terms of their formation, than the emission of exhaust products and water vapour – although in certain situations both can make a contribution to their development. Distrails can be described as the visual antithesis of contrails.

A species of convectional cloud called pyrocumulus, which occurs naturally, is associated with volcanic activity. However, this can be also be created by the intense heat generated by deliberate or accidental fires. Pyrocumulus can evolve into spectacular but potentially very dangerous cumulonimbus flammagenitus clouds during extreme wildfire events.

March 2021 was Wick's mildest since 2014. Closer scrutiny of the town's historic record for mean air temperature for March attested that it is currently the 10th most mild in a series starting from 1910.

Wick's precipitation record for March 2021 showed that it was the wettest since 2019, and that it is presently the 42nd most wet in a series stretching back to 1910.

Wick's mean air temperature for March 2021 was 6.47C (43.65F). The long-term average, in terms of the averaging period 1981-2010, is 4.90C (40.82F).

Wick's average maximum daytime air temperature for March 2021 was 9.08C (48.34F). The long-term average for March, for the current 30-year average period, is 7.80C (48.04F).

A seasonally mild 10.0C (50.0F) was logged on seven dates. The highest daytime maximum air temperature was 14.6C (58.3F), recorded on March 30. Lowest daytime maximum was 4.6C (40.3F), observed on March 4.

The town's average overnight minimum air temperature for the month was 3.86C (38.95F). The long-term average for March is currently 2.0C (35.60F). Highest overnight minimum for the month was 7.6C (45.7F), noted on March 30.

There were two air frosts. The lowest ambient temperature for the month was minus 2.1C (28.2F), recorded on March 6.

The temperature fell to 0.0C (32.0F) or lower at 5cm above the grass on seven dates. The lowest temperature observed over the grass was minus 4.5C (23.9F) on the 6th.

Precipitation was measurable on 16 dates. The total for March 2021 was 62.8mm (2.47 inches), or 94.4 per cent of the long-term average amount for March. Wettest day was March 7. The amount recorded for the 24 hours commencing 9am (GMT) was 21.0mm (0.83 of an inch).

There were no "days of gale" witnessed during the month. However, wind velocities reached or exceeded gale force 8 (39.0 mph/33.9 knots) on eight dates. The strongest wind velocity was observed during the half-hour ending 8.30pm (GMT) on March 9, when a force 7 south-easterly wind gusted up to 48 mph/41.7 knots, severe gale force 9 on the Beaufort scale.


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