STRAIGHT ISOBAR PROGNOSTICS.
Now we come to the consideration of some very interesting rain prognostics associated with straight isobars. While those in a depression are preceded by an almost ominous calm, and a dirty, murky sky, these are associated with a hard sky and blustery wind, of which it would be ordinarily remarked “that the wind keeps down the rain,” or “that when the wind falls it will rain.” While also the prognostics which precede cyclone rain hold good for the reason that they are seen in front of the rainy portion, those associated with straight isobars hold good because, though there is little rain actually with them, the area which they cover to-day will probably be covered by a depression to-morrow—the conditions being favorable for the passage of depressions.
On turning to Fig. 4 it will be seen that while the pressure is high to the south, it is generally low to the north, without any definite cyclonic system, and that the isobars run straight nearly east and west.
Fig. 4.—Straight Isobar Prognostics.
Near the high pressure the sky is blue, then as we approach the low pressure feathery cirrus, or some form of windy sky, makes its appearance, while a blustery wind whirls the dust, or blows the soot down; and animals turn their tails to the wind to avoid its force on their faces:
When chimneys smoke and soot falls, bad weather is at hand. The whistling of the wind heard within doors denotes rain.
When sheep, cattle, or horses turn their backs to the wind, it is a sign of rain.
Getting still nearer the low pressure, the sky is found to be gathering into hard stratus, at first with chinks between its masses, through which divergent rays stream down under the sun, which is spoken of as “the sun drawing water.” Sometimes, especially in winter, these rays are lurid, and are referred to in the following prognostic:
Along the north shore of the Solway, from Dumfries to Gretna, a lurid appearance in the eastern or southeastern horizon, called from its direction “a Carlisle sky,” is thought a sure sign of coming rain. They describe it as lurid and yet yellowish, and the common saying is:
The Carle sky
Keeps not the head dry.
At the same time there is often great “visibility” with a hard overcast sky, and moderately dry air, in which the stratus seems to play the part of a sunshade, for as soon as the sun comes out the clearness of distant objects diminishes. This visibility must not be confounded with the visibility already described with a cloudless sky, which occurs with wedge-shaped isobars. Simultaneously we often find “audibility.”
If the noise of a steamer or railway train is heard at a great distance, rain is predicted.
This distinctness of distant sounds must be carefully distinguished from sounds which are not usually heard, being brought up by the wind coming from a rainy quarter. For instance, the whistle of a railway train to the south of a house will not be usually heard with the normal southwest wind of this country; but when the wind backs in front of a depression to the south, then it will be heard, and although this will be a good prognostic, still it is not true audibility.
When people of Monzie (Perthshire) hear the sound of the waterfalls of Shaggie, or the roar of the distant Turret clearly and loudly, a storm is expected; but if the sound seems to recede from the ear till it is lost in the distance, and if the weather be thick a change to fair may be looked for speedily.
In Fortingall (Perthshire) if, in calm weather, the sound of the rapids on the Lyon is distinctly heard, and if the sound descends with the stream, rainy weather is at hand; but if the sound goes up the stream and dies away in the distance, it is an omen of continued dry weather, or a clearing up if previously thick.
The course of Turret and Lyon is from west to east.
True “audibility” is best described by the saying:
A good hearing day is a sign of wet.
Much sound in the air is a sign of rain.
This last exactly conveys the kind of sound referred to. The reason why audibility is produced is unknown, but the old idea that it is due to excessive moisture in the air is certainly erroneous; in several instances we have observed that the upper current of the wind appeared to be moving much faster than the lower, and perhaps that may have something to do with it. When the gradients are very steep, a little rain sometimes falls with straight isobars, generally in light showers, with a hard sky.
Though as a matter of convenience we have described the sequence of weather as we proceed from high to the low pressure, it must be clearly understood that it does not represent the sequence of weather to a single observer, but rather what the weather will be simultaneously in different parts of the country; for instance, that if there are cirrus clouds in London there may perhaps be a lurid sky in Edinburgh. The prognostic value of all is due to the fact that a depression will soon form which will probably extend over the whole country.