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pressure, it is an indication of a sudden decrease of the same, with rain and wind;' and so Shelley, in his Ode to
the West Wind,'has described it as 'the locks of the approaching storm,
verge Of the horizon to the zenith's height.' It is the cloud known to seamen, in less poetic language, as
goats' hair' or 'mares' tails.' Another cirro-form cloud, which Mr. Ley calls cirro-velum, the veil cloud, but which is more commonly spoken of as cirro-stratus, marks differences not only of the velocity but of the directions of the air-currents at a great height.
It appears to consist of numerous interlacing threads of cirro-filum, which are so closely matted together as to form an almost uniform sheet. This sheet may be either so finely woven and so thin as to form an almost invisible veil of cloud, or may be comparatively thick, and form a dense canopy covering a large area.' In reality the threads are not interlaced, but cross each other's lines at different levels. Such a cloud coming in from the west is, in this country, a sure prognostic of bad weather following at a short interval of time; or, as the nautical adage has it :
• If clouds look as if scratched by a hen,
Stand by to reef your topsails then.' In some rather rare instances,' says Mr. Ley, 'the sheet of cirrovelum is so thin and so evenly distributed that the particles do not seem to exist in any particular formation, and the whole sheet presents a faint milky appearance in the sky, which may be only just discernible. This very fine and thin sheet is not common in the British Isles. It nearly always indicates the worst type of weather, for the very existence of a great shallow layer of ice-dust with no structural arrangement shows that the currents which carry it are too rapid and too variable to admit of such.'
Such a cloud produces halos round the sun or moon, or gives them that pallid appearance which experience has denominated 'watery;' and thus Longfellow describes his old sailor as foretelling a hurricane, because
Last night the moon had a golden ring,
And to night no moon we see.' The meaning of these indications is that cirro-filum and cirro-velum are formed in front of a cyclone, by the air
which, in the upper regions of the atmosphere, overflows, or is thrown out from the centre. The cirro-filum extends to the region of the high barometer, which-as already mentioned-precedes the cyclone; the cirro-velum belongs to the greater disturbance where the barometer has begun to fall, and is closely followed by the nimbus, the black rain-cloud, and the violent wind. As the cyclone passes, strips and patches of cirrus are seen, hopeful signs of clearing weather.
It must, of course, be understood that in these latitudes, as everywhere else, bad weather is always a concomitant of a cyclonic disturbance; but changes of weather may accompany other systems of isobars. Of these, the most important is the wedge' of higb pressure which forms in the space between two cyclones, one following the other at a little distance. Hence the point of the wedge is usually towards the north. The front of it is really the rear of the leading cyclone, and is marked by fine, clear weather, with north-westerly winds; a hot sun by day, by night calm radiation; in summer, heavy dew; in winter, white frost. As the wedge advances, the barometer rises, to fall again as the wind changes to south-west, and cirro-filum ushers in cirro-velum, marking the near approach of the second cyclone. Hence the usually short duration of a white frost. “A white frost,' says the popular adage,' never • lasts more than three days; or, again, 'frost suddenly ' following heavy rain seldom lasts long.'
Visibility of distant objects is another concomitant of the eastern side of such a wedge, and has thus come-in this country-to be considered an indication of bad weather. • The old moon in the new moon's arms' is a mark of this visibility, and a prognostic of the advancing cyclone, as the old ballad-monger properly expressed it :
I saw the new moon, late yestreen,
Wi' the auld moon in her arm;
I fear we'll come to harm.' But the portent is special to these latitudes; further south --at Malta, for instance-the phenomenon is seen regularly every month through long spells of fine weather.
Much of our proverbial weather wisdom--we might almost say all of it that has any value-can similarly be referred to the cyclone, anti-cyclone, or wedge. A rainbow, for instance, can only be seen when rain is falling opposite to a bright sun. Hence in the morning it must be in the west, and tells of rain to the westward which will probably reach the observer before long; in the evening, it tells of rain in the east, and a clearing sky in the west-of the cyclone passing and the fine weather in its rear coming on. A great many of the popular indications really tell only of moisture in the air: they are natural hygroscopes; such as a piece of dry seaweed, which becomes damp; a piece of catgut, which contracts and pulls the figure of a woman back into her cottage; drains and ditches, which smell offensively; soot, which falls down the chimney; corns and old wounds, which become painful, and many such like. They indicate moisture, which is generally, but not always, followed by the nimbus of a cyclonic system. It is possible for the air to be very damp without an approaching cyclone, a condition which an isobaric chart reveals to the meteorologist, but which our forefathers were unable to verify; it is still more possible for a cyclone, after it has come sufficiently near to betray its approach, to fill up and die out, or to turn off in some other direction. It is this uncertainty of the path of a cyclone which so often invalidates not only the popular prognostics but the forecasts of a trained observer. Mention has already been made of some of the irregularities to which the track of a cyclone is liable under the influence of an area of high pressure to the eastward. After the event, it is commonly not difficult to understand what has happened ; but beforehand it is not easy to forecast it, and many cases occur in which a correct forecast is, at present, impossible. According to Mr. Abercromby,
“When a cyclone takes an unusual path, the general character of the weather will remain bad, but the direction of the wind and the details in different districts will be wrongly forecast. . . . Sometimes the path will describe a complete circle of no very great diameter ; but the commonest case in western Europe is when the path of a cyclone takes the form of the letter V. For instance, a cyclone comes in from the Atlantic from about due west, and after it has gone as far as England, it moves back again in a north-westerly direction, as it has not been able to pass the area of high pressure which would then be lying over northern and central Europe. In another common case, the cyclone comes down from the north-west on to England, and then passes off in a north-easterly direction, towards Norway. . . . But the tracking of well-defined depressions forms but a small portion of the forecaster's business. On the larger number of days he has to estimate how or where cyclones will form in an ill-defined area of low pressure, or how far an area of low pressure will encroach on another region of high barometer. In this he must rely on his own opinion
and experience alone ; that must be fallible sometimes, but better results are obtained by trusting to personal skill than by attempting to use any mechanical rules or maxims.' A frequent cause of error is the sudden and unexpected formation of a small cyclone, or, as it is properly called, a secondary, in the southern rim of the primary, by which the whole forecast is upset, and the weather is necessarily much worse than was anticipated.
Secondaries and non-isobaric rains,' says Mr. Abercromby, are the forecaster's bugbear : they form so quickly, show so little on a synoptic chart, and move so irregularly, that rain, in general terms, is all that the forecaster can usually say. ... Sometimes, too, secondaries are so small that they do not show at all on a synoptic chart, which is constructed on reports received from stations often a hundred and fifty or two hundred miles apart. The whole loop of a secondary need not be nearly so large, and then a depression of that class might lie between two stations, and yet be indicated at neither. The weather, however, would be profoundly modified, and the forecasts would pro. bably be erroneous. There is also the important difference between wind and rain, that the former is always in the main determined by the steepness of the gradients, while the amount of precipitation bears no relation to any known meteorological element.'
Mr. Abercromby rightly dwells on the peculiar difficulty of forecasting in this country. Being on the advanced outpost of Europe towards the west, we have no knowledge of what is coming in from the sea till its actual indications arrire; we have no knowledge of the probable track till our own observation has revealed it. In Germany, on the other hand, the forecaster has the benefit of our previous experience. It is not often that a storm can reach the German coast without having been reported from this country. But it is not only that our geographical position is unfavourable; our meteorological position is still more so. When, as is frequently the case, a large anti-cyclone lies over the Scandinavian peninsula, the North Atlantic is an area of low pressure and bad weather; when this is reversed, the anticyclone forms over the ocean, the low pressure and bad weather over Sweden and Norway. In either case, Great britain is the debateable ground between the two systems; on it their struggle for the mastery is fought out-and, as n other battlefields, the inhabitants suffer the inconveni
of the forecasts daily issued o
does not seem very hopeful. It is more so to find that, with all disadvantages, about eighty out of
advantages, about eighty out of a hundred its daily issued by the Meteorological Office in
London are wholly or partially correct. For the year ending March 31, 1894, the percentage of success, complete or partial, was 84, and for the south-eastern district, including London, was as high as 89, while for this same south-eastern district the percentage of complete success was 65, and for the whole of the British Isles was 59, or 10 greater than the average of the previous ten years. Such a statement is, perhaps, not in accordance with the popular idea. This is because the partial failures are, for the reasons already given, generally as to the rain, about which the ordinary citizen is the more inquisitive. About the wind he cares comparatively little ; and if he has got drenched by an unforetold shower, he writes to the Times' to say that the forecasts are worthless, and the money voted for the Meteorological Office is thrown away. To the fisherman or the small coaster, to whom a wetting more or less is a thing of little consequence, but whose property and life may depend on a correct forecast of the wind, the work of the office, imperfect as it admittedly is, is a priceless boon. There is probably no one in all England more conscious of the shortcomings of the forecasts than the Secretary of the Meteorological Office, under whose name they are issued; but it is because, better than any one else, he knows where, and how, and why they fail, often without a gleam of hope that he may be able to avoid similar errors in future. In the United States, where storms, forming near the Rocky Mountains, traverse a country thickly studded with telegraph stations, it is not surprising to learn that about 90 per cent. of the forecasts issued from the Bureau in Washington are more or less correct. In Canada the percentage is about
At Hamburg it is about 85. Mr. Abercromby believes that, with more experience, better results will be obtained, but that the work will be carried on on the same lines as at present; he sees no probability of any radically different method being discovered.
Forecasting,' he says, depends neither on any theory nor on any calculation. The whole science, from beginning to end, rests solely on observation. The shapes of isobars, and the relation of wind and weather to them are matters of experience only. We find that certain kinds of weather are associated with different portions of each fundamental form of isobars, and we classify accordingly. We give each shape of isobars a conventional name, but that does not bind us to any theory of atmospheric circulation. . . . It is impossible to suppose that we have yet nearly reached the highest perfection of which forecasting is capable ; but still we know enough of the nature of the