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.
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.
