COLUMN: Weather Wise with Gordon Currie

We can expect more thunderstorms in the week ahead. (S)

We can expect more thunderstorms in the week ahead. (S)

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More musings from Gordon, plus weather prospects for the week ahead:

The story is familiar

Each facet yearly told,

Tho’ time can n’er bring changes

In this pictured harvest gold,

Of grease and grime and cat-mint –

These stubble scents arise,

A sudden snapping shear-bolt,

The baler man’s surprise!

The eve’ning shadowed combine

A shaft of sunlight dust

Symbolise the harvest wheat

And fortify our trust

Of faith in Nature’s handiwork,

Man’s labours young and old,

Accord such relished pride and joy-

The glory of the gold!

This countryman’s frontispiece for our Augusts of modern times remind us of the reverence which is created within the human work lust by the spectacle of mature, standing grain ready for the harvest.

To be able to understand just why the honour and respect of the cornfield is so timeless, one has to savour for themselves the tremendous wealth of wholesomeness in the life that exists between the earthly roots of the cereal plant and those golden heads of mature grain.

Harvest time in the days of yore used to be associated with a season of myths and legends concerning the weather, long before the world of science began to influence our thinking.

Critical indeed, becomes the picture of ripened corn standing razor edged against the dubious blueness of August’s skies, slightly hazed and tarnished by the colours of thundery cumulonimbus clouds fouling the horizon.

No wonder the ancients imagined that lightning ripened the crops at the time of Lammastide, (August 1) coupled with the lengthening nights: “After Lammas corn ripens as much by night as by day.”

Another adage once declared: “Rain about the Lammastide when the corn begins to fill, is worth a plough of gold.” This saying is now quite obsolete in our 21st century, due to earlier harvesting varieties of cereals, although perhaps, it may just be valid in 2013, with a later seasonal year.

There are many theories about the signs and powers of an atmosphere that has become overloaded with electricity, some of which are sound enough, but others are completely mythical.

An interesting piece of rural lore states that: “the abundance of harvest depends upon sour milk.” This saying, originating from the ancient ideas about lightning, implies that thunderstorms will aid crops from the point of view that lightning on its way through the atmosphere produces ammonia and nitrogen oxides.

The ammonia, dissolved in rain, becomes ammonium hydroxide, a plant food. The nitrogen oxides react with the atmospheric moisture to form nitric and nitrous acids, which, as soluble nitrates, fertilise the soil.

The relationship between the ripening harvest and sour milk was once based upon the erroneous belief that thundery weather was responsible for turning milk sour.

Scientists put this theory to test a century ago by stimulating in the laboratory, the electrical conditions of a thunderstorm and recording their effect on milk. The result proved that lightning tended to preserve milk, rather than turn in sour.

The atmospheric temperature, combined with high humidity, ideal conditions for thunderstorms – were found to promote the rapid multiplication of lactic acid bacteria.

It is an accepted fact that summertime “heat” thunderstorms are triggered by vertical convection currents from the ground, but there can be certain exceptions to this rule.

Sometimes, in hot weather, high pressure systems weaken only very slowly and it becomes quite difficult to discern just when the actual breaking point in the weather will be.

Air mass changes become diffuse, while, because of small scale temperature inequalities creeping in aloft, the general stability becomes slowly unbalanced, sometimes with independent convection currents starting in the middle or high levels of the atmosphere.

The visual result of an unstable layer of air developing high above a warm, stable layer near the ground can be observed in a spectacular and very significant cloud formation known to the meteorologist as altocumulus castellanus.

In simplified form it appears as a sheet of stratiform cloud at its base, anywhere between 8,000 and 14,000 feet, with well-built turrets, forming small towers of rock-like cumulus on its upper surface.

An excellent illustration of altocumulus castellanus occurred in the spectacular sunset sky of Monday evening, July 22.

The display was very unique, with the formation breaking into singular cloudlets, maintaining their sharp upward sprouting which signified the onset of instability in the upper air.

These vertical sproutings became more pronounced as the sunset approached indicating that the instability was becoming quite vigorous. In Victorian times, these clouds were recognised as “post boys” for their distinctive role in being prophets of approaching thunderstorms.

Theoretically, and indeed in very practical terms, thunderstorm activity follows within 12 to 24 hours. In this particular instance, sharp thunderstorms broke out just ten hours following the sunset display, on the morning of the 23rd. These storms had moved up from the south overnight and coupled with our local early morning cover of low stratus “ Haar ‘’ cloud from the North Sea, their immediate onset was made even more sinister by the sudden ominous darkening of the sky at breakfast time.

Large hailstones were reported to have fallen in the Rainton area, while here at Wath near Ripon, 7.8 millimetres of rain and small hailstones (0.31 inch) fell in ten minutes. The accuracy of the post boys in this instance was faultless and the presence of large hailstones indicated very severe instability, shooting up well above the freezing level of the atmosphere, which, in the July heat-wave could well have exceeded 16 – 18,000 feet.

PROSPECTS FOR THE WEEK AHEAD:

A very warm and thundery start to August, as our fresher westerly winds are expected to back towards southerly by today, pulling hot, humid air off the continent. A low pressure system with its cooler Atlantic front will affect us this weekend, followed by a return of cooler, fresher Atlantic conditions for the coming week.

Starting warm, humid and thundery with showery outbreaks of rain and thunderstorms, clearing to brighter, fresher conditions with sunshine and scattered showers for the weekend. Sunny days next week, interrupted by a few showers. Temperatures 75 – 80F. (24 – 26.6C.) dropping to slightly cooler levels during the weekend and thereafter, 70 – 73F. (21 – 23 C.).