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have hitherto described, the lines enclose an area of high or low pressure, while in straight isobars the lines only mark the position of what may be called a barometric slope.

On turning to Fig. 8, it will be seen that while the pressure is high to the south, it is generally low to the

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north, without any definite cyclonic system, and that the isobars run straight nearly east and west, with a slope towards the north. The wind is from the south-west or west, and usually strong and gusty, but short of a gale. On the high-pressure side 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. The falling of soot refers to blacks falling out-of-doors and coming into windows from being blown about. Sometimes, in very damp weather, soot seems to fall from condensation of vapour on itself, and at other times masses of soot fall down a chimney from the action of hail or very heavy rain.

Getting still nearer the low-pressure, the sky is found to be gathering into hard strato-cumulus, 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 the appearance of the sky is then very striking. This prognostic is common all over Northern Europe, and in Denmark takes the form of "Locke is drawing water." Loki is a well-known demi-god in the Scandinavian Eddas, so that we have here a direct survival of mythic speech. This hard strato-cumulus is especially characteristic of straight isobars in Great Britain.

At the same time there is often great "visibility," with a hard, overcast sky and moderately dry air, in which the cloud 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." 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 south-west wind of Great Britain; but when the wind backs to the south in front of a depression, then the noise will be heard; and though this will be a good prognostic, still, it is not true audibility.

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 the 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 is cirrus in London, there may perhaps be a lurid sky in Edinburgh.

But now audibility, visibility, whirling dust, and lurid chinks with divergent rays are well-known signs of rain almost all over the world, so we have to explain why the appearance of the sky in straight isobars is a sign of rain. It is found by experience that straight isobars are never persistent, and that, practically, the district which they cover one day will be traversed by a cyclone the next day. It does not follow that the cyclone is necessarily in existence when we observe the straight isobars; but, from the nature of weather-changes, straight isobars seem to be an intermediate form of atmospheric circulation which precedes the formation of a cyclone.

We cannot, therefore, draw a section across straight isobars and say that it will give the sequence of weather at any place, for we are not dealing with a moving form of pressure, but with a transitional state of things which cannot last long. The chief interest of these rain prognostics lies in the contrast which they present to those associated with a cyclone. While those in a cyclone are accompanied 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 of such a depression, 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 cyclone to-morrow-the conditions being favourable for the passage of depressions. Another point of contrast lies in the comparative dryness of the air in straight isobars, as compared with the excessive amount of moisture which precedes cyclones. The same remarks apply to these as to the fine-weather prognostics associated with wedge-shaped isobars.

All the prognostics we have discussed under this heading fail when the straight isobars are formed during a general rearrangement of the whole distribution of pressure over the northern hemisphere, because a cyclone may not then traverse the district where the well-known signs of rain had been observed.

The other fundamental forms of isobars-V-shaped

depressions and cols are not associated with any distinctive prognostics, so we will defer our consideration of these shapes till a subsequent chapter.

GENERAL REMARKS.

We are now in a position to take a general survey of the whole principle of prognostics, and to answer the questions which, we mentioned at the commencement of the chapter, were formerly considered insoluble. What is the place of prognostics in meteorology, and how has modern research developed their utility? Why do prognostics sometimes fail? Why are not all prognostics associated with increasing damp? Why is rain or fine weather not always preceded by the same prognostic?

The details which we have already given abundantly show that every portion of every shape of isobars has a characteristic weather and look of sky, and that prognostics simply describe these appearances.

Theoretically, then, when the isobars are well defined, we ought to be able to write down the prognostics which might be visible everywhere, but practically we cannot do so completely; and also, theoretically, all that any prognostic does is to enable a solitary observer to identify his position in any kind of atmospheric circulation. Thus the associates of the front of a cyclone or secondary are signs of bad weather; while those of the rear of a cyclone, or of any portion of an anticyclone, are signs of fine weather. The word “front” implies not only the idea of motion, but also of the direction of that motion. But here comes in the reason why prognostics can never

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