WEDGE-SHAPED ISOBAR PROGNOSTICS.
Though the bulk of British weather is made up of cyclones and anticyclones, there are two other distributions of pressure marked out by wedge-shaped isobars and straight isobars respectively, which have never before been described, but which are associated with many well-known sayings. The chief interest in these prognostics consists in the contrasts which they present to cyclone prognostics, as in many cases they are associated with fine or dry weather, as opposed to the damp of an approaching depression. We shall first describe the prognostics which characterize the wedge-shaped area of high pressure that is frequently formed between a retreating and an advancing depression, as it is by far the most common.
All these prognostics owe their value to the fact that as the new depression comes on, rain may be expected. By reference to Fig. 3 it will be seen that in the rear of the retreating depression the weather is beautifully fine, of the sort of which we should say that it was “too fine to last,” or, if it lasted a whole day, we should call it a “pet day.”
Fig. 3.—Wedge-sharped Isobars.
An unseasonably fine day in spring is called a pet day in Scotland. The fate of pets they say awaits it, and they look for bad weather on the morrow.
During the day the sun is burning hot:
When the sun burns more than usual, rain may be expected.
During the night white frost is formed, owing to calm radiation:
A white frost never lasts more than three days; a long frost is a black frost.
Frost suddenly following heavy rain seldom lasts long.
As the day advances after a white frost, the air becomes dull from the influence of the on-coming depression; whence the saying—
When the frost gets into the air it will rain.
During the very fine weather, on the east side of a wedge-shaped area there is often great visibility with a cloudless sky:
The further the sight the nearer the rain.
When the Isle of Wight is seen from Brighton or Worthing, rain may be expected.
When to the people about Arbrouth the Bell-Rock light is particularly brilliant, rain is expected.
About Cape Wrath and along that part of the coast when the Orkney Islands are distinctly seen, a storm or a continuation of bad weather is prognosticated.
When from Ardersier and the adjoining parish on the southeast side of the Moray Firth the distant Ross-shire hills are distinctly seen in the morning rain is expected that day.
To the people in Eaglesham, in Renfrewshire, when the Kilpatrick hills appear near a change to wet is looked for; but when they appear remote, dry weather will continue.
If the old moon embraces the new moon, stormy weather is foreboded.
Great confidence is placed in this old prognostic:
I saw the new moon late yestreen
Wi’ the old moon in her arm,
And if we gang to sea, master,
I fear we’ll come to harm.
The reason of “visibility” is uncertain; the old idea that it is due to excess of vapor is certainly erroneous. The dry and wet bulb hygrometer always indicates a considerable amount of dryness when it is remarked, and Mr. Cruickshank has shown by long observation at Aberdeen that visibility is greatest at the driest season of the year.
At the extreme northwest edge of a depression there is often unusual “refraction,” a well-known sign of rain. This seems to be due to the cold air in the rear of a depression being much below the temperature of the sea. If so, it is a sign of rain, for the reason that one depression is usually followed soon by another, which also explains the saying—
A norwester is not long in debt to a souwester.
Refraction and visibility combined also explain the following curious local prognostic:
When Ailsa Craig is distinctly seen, and seems near at hand, the people of Cumbræ look for change. When the weather is going to be finer it lies flat; but when rain is coming it assumes the form of a mushroom.
Ailsa Craig is an isolated rock standing in the Firth of Clyde, about thirty miles from Cumbræ. It has a conical top, with precipitous sides, so that in ordinary weather only the top is seen lying flat on the horizon; but sometimes in the rear of a depression it appears lifted by refraction, and the light encroaches at the edges, as is often seen with projecting promontories, giving the whole a fanciful resemblance to a mushroom. The prognostic is mistrusted by the inhabitants during frost, and we have proved it by our own observation. On the west side of the wedge-shaped area, as the new depression comes on, the blue sky gradually assumes a dirty appearance, accompanied by a halo, and gathers into cloud, and later on rain begins to fall; while in the southern portion the rain is often preceded by strips of cirrus, either lying in the direction of the wind, or sometimes at right angles to it:
Cirrus at right angles to the wind is a sign of rain.