Weather Watch: Frost can be a hindrance – or a help – to gardeners
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Weather Watch by Keith Banks
A hazard of great importance to farmers, gardeners, and to some extent, building contractors, is frost. Air frost occurs when the air temperature drops to 0C (32F), at 4 feet (1.2 metres), above the ground.
When this happens, and the humidity of the air is very low, plants and other vegetation may be adversely affected, and sometimes they can be completely killed off.
Gardeners call this potentially insidious invisible type of frost a black frost, because the moisture contained in delicate plant tissues expands when frozen, ruptures, and becomes blackened, shrivelled with a burnt appearance after they have been destroyed.
In the case of the construction industry, prolonged very low temperatures can severely impact on length of time that is required for concrete to set properly.
If the air temperature does not fall as low as this, the ground temperature can because the ground usually cools more rapidly than the air. When this occurs a ground frost, often referred to as a hoar frost, results.
This indicates that the air is saturated, and as a consequence, the water vapour that has become super cooled is then deposited directly onto an ice surface. A hoar frost is composed of very delicate needle-shaped ice crystals.
Air frost is the real enemy of plant tissue, rather than hoar frost crystals. Indeed, they may even provide a significant degree of protection in cold weather.
A review of Wick's historic record for mean air temperature for December showed that December 2021 was the coldest since that of 2017. However, it is currently the town's 45th equal mildest in a series commencing from 1910. The royal burgh's average air temperature was identical in December 1998.
In terms of precipitation, December 2021 was the least wet since that of 2018, and is presently the 11th driest in a series that stretches back to 1910.
Wick experienced three days of gale during December 2021.
Wick's mean air temperature for December 2021 was 4.76C (40.57F). The long-term average for December, in terms of the averaging period 1981-2010, is 4.0C (39.2F).
Wick's average maximum daytime air temperature for December 2021 was 6.51C (43.72F), 0.09C lower than the long-term average for the month. A seasonally mild 10.0C (50.0F) was reached or surpassed on six dates. The highest ambient temperature was 11.8C (53.2F), recorded on the 24-hour recording period, commencing 9am (GMT) on December 12.
Lowest daytime maximum for any 12-hour recording period ending 6pm (GMT) was 3.0C (37.4F), observed on December 1.
Wick's average overnight minimum air temperature for December 2021 was 3.0C (37.4F). The long-term average for December is currently 1.4C (34.5F). Highest overnight air minimum was 8.9C (48.0F), witnessed on December 15.
There were four air frosts. The lowest ambient temperature was minus 2.8C (27.0F), logged on December 28.
The temperature fell to 0.0C (32.0F), or lower at 5cm over the grass on 14 dates. The lowest temperature recorded over the grass was minus 4.8C (23.4F), recorded on December 28.
Precipitation was measurable on 26 dates. The total for the month was 43.6mm (1.72 inches), or 58 per cent of the long-term average for December. The wettest day was December 7. The amount recorded for the 24 hours commencing 9am (GMT) was 8.0mm (0.31 of an inch).
Days of gale, by definition, occurred on December 7, 8 and 13 respectively. However, wind velocities reached or exceeded gale force 8, (39.0mph/33.9knots) on nine dates. The strongest wind velocity observed was during the hour ending 3am (GMT), on December 13, when a force 7 west-south-westerly wind gusted up to 62.2mph/54.1knots, storm force 10 on the Beaufort scale.
