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.