CONCLUSION.
We have endeavored in this paper to deal with such prognostics as can be readily classified, but besides these there are many more, what may be termed unclassified prognostics. Our object has been to show the relation between certain prognostics and certain forms of isobars, and by this means to assign them their proper value. It cannot be doubted that if careful attention were given to the observation of the aspect of the sky, the different forms of cloud and local signs, these would prove, even in conjunction with a single barometer reading, of great assistance in predicting the weather. Of course those persons who live in the neighborhood of large towns labor under a great disadvantage, as the sky has nearly always a dirty appearance, and is frequently obscured by smoke. For this reason London is about the worst place to reside in for studying the weather.
We have only been able to give the rudiments of the new method of considering prognostics, for the method is capable of great extension, and we hope that we may succeed in interesting some of the Fellows sufficiently to induce them to observe prognostics in conjunction with the daily weather charts.
In conclusion, we would venture to express our opinion that the observation of local signs of weather ought to form a most important element in all arrangements for telegraphic reporting for the purpose of forecasts, and that the duty should be impressed upon the observers of reporting at once important changes in the local and general signs of the weather. Great attention should also be paid to the observation of the forms and motion of clouds, and as considerable lack of knowledge prevails, even on the part of good observers, regarding the different forms and modifications of clouds, we are glad to see that the meteorological office has already commenced systematic observations of cirrus clouds. Telegraphic observers should be specially instructed in cloud observations and prognostics by some one thoroughly well versed in the subject, so that there may be strict uniformity among all the observers, but we admit that the proposal presents serious difficulties in the way of realization.
Theoretically, when the isobars are well defined we ought to be able to write down the prognostics which might be visible, but practically we cannot do so. Besides, there are sometimes cases of isobars which have no well-defined shape, but with which thunder-storms or heavy showers often occur. These, as is well known, do not affect the barometer, but are abundantly forewarned by the commonest prognostics, and as the rainfall is usually heavy in them the failure of the forecast which omits to notice them is very conspicuous.
The scope of this paper precludes us from entering into the complicated question of the non-cyclonic rainfalls in this country. It will be enough to state that the prognostics which precede them are rather those associated with broken weather, such as bright sunrises or heavy clouds banking up without the barometer falling, than the muggy, dirty weather of a cyclone front. The warning they give is also much shorter, rarely more than three or four hours, if so long.
The result of this paper may be summarized as follows:
The authors explain over one hundred prognostics by showing that they make their appearance in definite positions relative to the areas of high and low atmospheric pressure shown in synoptic charts. The method adopted not only explains many that have not hitherto been accounted for, but enables the failure as well as the success of any prognostic to be traced by following the history of the weather of the day on a synoptic chart. The forms discussed are cyclones, anticyclones, wedge-shaped and straight isobars. The details of weather in the last two are now described for the first time. They also point out that prognostics will never be superseded for use at sea and other solitary situations, and that prognostics can be usefully combined with charts in synoptic forecasting, especially in certain classes of showers and thunder-storms which do not affect the readings of the barometer.[[1]]
DISCUSSION.
Dr. Tripe said that some of the Fellows might think the paper hardly suitable for reading at a meeting or printing in the journal of a scientific society, but many of them took an interest in such papers, because they were comparatively simple. In reference to the table of solar and lunar halos in connection with rainfall, it appeared that rain was pretty sure to fall within three days after the occurrence of a lunar halo, and to the extent of 80 per cent., as regards solar halos, on the first or second day.[[2]] He considered visibility was a good prognostic, and afforded a more certain indication of the speedy occurrence of rain than even mare’s tails. What caused visibility was unknown to him. The prediction of rain from damp walls depended chiefly on the previous weather; a sudden change from cold to warm would have the effect of making walls condense the moisture contained in the air without approaching rain. The falling of soot down a chimney was, he thought, hardly a good prognostic; it was caused to a great extent by the direction and force of the wind, and also the angle at which the wind struck the chimney-pot. He considered the paper a good one, and a step in the right direction.
[1]. The prognostics quoted in the paper have been mostly taken from the following works:
“Popular Weather Prognostics of Scotland.” By Arthur Mitchell, M. D., Edinburgh, New Philosophical Journal. “Weather Lore.” By Richard Inwards, F. M. S., London, 1869. “A Handbook of Weather Lore.” By Rev. C. Swainson, M. A., Edinburgh, 1873.
[2]. The calculation on which his remark on halos was founded is as follows:
Rain occurred in connection with—
| Solar halos. | Lunar halos. | ||||||
|---|---|---|---|---|---|---|---|
| Direction of wind | S. | SW. | W. | S. | SW. | W. | |
| Number of observations | 22 | 28 | 59 | 8 | 11 | 21 | |
| Rain within forty-eight hours | per cent | 79 | 78 | 81 | 50 | 91 | 76 |
| Rain on third day | do | 9 | 3 | 7 | 40 | 9 | 0 |
Dr. Marces said, with regard to visibility being a sign of rain, he had himself observed on the borders of the Lake of Geneva, that if the mountains on the opposite side of the lake could be seen very distinctly on a cloudy day, rain was likely to fall within a short period. On such occasions the coast appeared much nearer than usual. The late Professor de la Rive, of Geneva, ascribed this phenomenon to the atmospheric dust being hygrometric and becoming transparent in damp weather from the moisture it absorbed.
Mr. Stanley remarked that solar and lunar halos depended upon the moisture in the air, and that a sign of rain could be better assured by reference to the hygrometers. He thought that fine weather in front of a cyclone was due to increase of pressure, and therefore of temperature, caused by the onward march of the cyclone. This increase of temperature was known to render sound more audible, and by making the air clearer distant objects became more visible. It was known that condensed vapors disappeared under a slight increase of pressure, just as, conversely, under the ordinary air-pump, vapors appeared on a diminution of pressure. He considered that the fall of rain in the centre of a cyclone was caused by the increase of pressure due to the tangential action of the air in the cyclone. The straight isobars described in the paper, were, he thought, in segments of a very large cyclone, which was general in all large displacements of air.
Captain Toynbee thought that the great clearness of the air experienced when isobars took the wedge-shaped form, was due to the fact that such isobars represented a ridge of high barometrical pressure, which lay between two cyclonic systems, and that the high pressure of the ridge was maintained by air which had previously risen in front of the advancing cyclonic system, when it had lost most of its moisture, and now came down as a dry, pure, clear, northwesterly wind. Hence the clearness of the air was, where wedge-shaped isobars were being experienced, a sign of rain, because it indicated the fact that a cyclonic system would soon advance over the same position. This explanation was based upon the Rev. W. Clement Ley’s theory of the motion of air in cyclonic systems.
Mr. Scott thought that it might have been desirable to give the authorities or sources from which some of the principal sayings had been derived, in order to show which were general and which were of more local import. Reference had also been made to the prevalence of bad smells with a falling barometer, and to the fact that rheumatic affections and neuralgia were felt especially at such a time; but no attempt had been made to explain the connection of those phenomena. It was a well-known fact that a sudden reduction of pressure, such as that experienced by divers when they returned to the surface of the water, produced neuralgic affections. The authors had described straight line isobars and their accompanying prognostics, but only for west winds, for pressure lowest in the north. He should be glad to see the list of prognostics extended so as to include those for straight isobars with east winds when the pressure was lowest in the south. With regard to refraction being a prognostic of rain, in some cases it was a prognostic of easterly winds, and was recognized as such in the west of Cornwall. In the past summer he had himself verified this latter statement on several occasions at Scilly and the Land’s End. The authors, he thought, spoke a little too strongly when they said the barometer gave no sign of thunder-storms. The type of isobar which accompanied these was well known; it exhibited small undulations or incipient bights representing imperfectly formed secondary depressions. At the same time he must say that no one yet, in Europe at least, had been able to forecast correctly the amount of rain for a given day in a given place. The probability of some rain could be recognized, but no attempt could be made to estimate its amount. This was apparently owing to ignorance of the conditions of the upper atmosphere.
Mr. Dyason expressed an opinion that the formulaters of weather prognostics in the past must have been color-blind, an affliction from which he was not sure that the authors of the paper were exempt. “Where are the scarlet, orange, green or gold harmonious, and the general glow?” He had endeavored in a series of sketches of skies and clouds to portray the colors as they presented themselves to his mind. In relation to visibility he referred to the Lakes of Geneva and Lucerne, quoting the lines used in the locality of Mont P
Si Pilate a un chapeau, le temps se mettra au beau;
A-t-il un collier, on peut la montée risquer;
Mais s’il porte son épée, il y aura une ondée.
Mr. Dyason exhibited a sketch of the Matterhorn taken south of Zermatt during intense visibility. He did not admit “that London was a bad place for skies;” those he now exhibited were sketched in a northwestern suburb of the metropolis.
Professor Archibald considered that some of the prognostics mentioned in the paper required more explanation. For instance, it was stated that candles burned with an unsteady light in damp weather, but no reason was given why they should do so. One very good prognostic in use in Scotland appeared to have been omitted, namely: “The northwest wind is a gentleman, and goes to bed.” He would also like to know why fires burned with a blue flame in frosty weather. He considered that visibility was often a local phenomenon connected with temperature. He had noticed intense visibility and a fog in juxtaposition on that very day, when travelling from Tunbridge Wells to London.
Mr. Symons pointed out that although it might be useful to trace the origin of some of the weather proverbs, it would be extremely difficult and perhaps impossible, as some of them were ancient. With regard to the connection between lunar halos and rainfall, he thought that it would be curious, considering the large number of wet days in this country, rain falling on an average every other day, if some of them did not happen to follow lunar halos. Respecting doors and windows sticking in damp weather, this was rather the result of existing damp than a forecast of more damp to come; for if existing damp foretold coming damp it was not easy to see how dry weather could ever occur. With regard to visibility, he was glad the authors had referred to Mr. Cruickshank’s observations at Aberdeen, extending over twenty-one years, and Mr. Symons thought that much might be learned from a thorough discussion of similar observations.
Mr. Wilson directed attention to Professor Tyndall’s theory, that audibility was due to the homogeneity of the atmosphere.
Mr. C. Harding thought that the paper would be useful to an isolated observer, whose forecast was dependent upon his own observation. It appeared to him that in classifying the prognostics, the authors of the paper had rather begged the question—given a certain occurrence which was admittedly a prognostic of bad weather; this seems to have been consequently classed as belonging to the front half of a cyclonic disturbance. He suggested that with each occurrence of the prognostic in question it would be better to note the existing conditions of atmospheric distribution, and, finally, to classify by actual observation. He stated that the form of isobars described by the authors as wedge-shaped had been referred to many years ago in various publications, by Captain Toynbee, as a “ridge” of pressure. Some distinction should have been made between the weather of anticyclone in summer and in winter, as it differed materially. Audibility had been referred to as a sign of bad weather; he might say that it was notorious in ballooning that for a given distance sounds could be heard more distinctly than on the earth, and in a recent ascent he had noticed that the shrill voices of children were much more audible than the voices of grown persons.
Mr. Abercromby, replying, said that Doctor Tripe’s analysis of halos was interesting, but that if rain did not fall within twenty-four hours after a halo, any rain after that would not be due to the cyclone which produced the halo, but to a new one. Damp walls were certainly due to excess of vapor, not to rapid changes of temperature. Soot was doubtless blown down by wind, but when used as a prognostic it rather referred to soot falling out of doors, and was to be attributed to excessive damp. Halos were unquestionably due to the presence of a thin film of ice-formed cloud, and not merely to the amount of moisture in the air. This ice film was only formed in front of cyclones or thunder-storms, and for that reason was a sign of rain. With regard to visibility alluded to by several speakers, he could not admit that it was due to excessive moisture, for the hygrometer showed that it was not so. He agreed with Captain Toynbee, so far as believing that there was a descending current of dry, clear air with a northwest wind on the front side of a “wedge,” but he doubted if that was the whole explanation. In the centre of anticyclones there was also a descending current of dry air, but no “visibility.” The diagram of straight isobars was given for westerly winds, as that type was by far the commonest. Like every other shape of isobars, the details varied with the type of weather in which they occurred. In an elementary paper it was considered inexpedient to go into so much detail. In the case mentioned by Mr. Scott, isobars trending east and west, but sloping towards the south, the same broad features as given in the diagram would be reproduced, but with a harder sky and an east or northeast wind. The prognostics would, however, be much more likely to fail, for cyclones in the northerly or easterly types, to which such isobars would belong, did not follow with the same regularity as in the westerly type. The refraction, which was a prognostic of east winds on the southwest of an anticyclone, was very different from the refraction on the northwest edge of a cyclone, which portended a fresh storm. The chief difference was the haziness of the horizon in the former case as compared with its visibility in the latter. The latter kind was hardly known on the south coast of England. He strongly suspected that the condition common to both kinds was a relatively cool air over a comparatively warm sea. The type of isobar which exhibited incipient “bights” or imperfect secondaries, was doubtless that most frequently associated with thunder-storms, but the kind of thunder-storms alluded to in the paper as showing no trace on the isobars undoubtedly occurred. Any reference to cloud or sky colors was intentionally omitted in this paper, as also to prognostics relating to diurnal winds. Candles burned badly before rain, probably owing to a stagnant, damp air. Blue flame was owing to the formation of carbonic oxide when a fire burned very red on a frosty night. Doors and windows sticking in damp weather indicated rain because damp preceded the rain area of a cyclone.
The idea suggested by Mr. C. Harding, that because certain prognostics were known to be associated with bad weather, therefore they were classed as belonging to the front of a cyclone, was entirely erroneous, and exactly opposite to what had been done. The method Mr. Abercromby had adopted for the last twelve years had been, when he observed a good example of any prognostic, to make a note and put it by with the nearest synoptic chart for the day, or often with both the preceding and succeeding charts. When a sufficient number had been collected, the charts were examined, and the necessary deductions drawn from them. Thus the charts were classified according to the prognostics associated with them. The results of all these researches had been embodied in the paper, and the important fact deduced was that every portion of every shape of isobars had a characteristic weather and appearance. The general fact of a “wedge” being associated with fine weather had been noticed many years ago by Captain Toynbee, but the details of weather in different portions, and their relations to prognostics, had a considerable amount of novelty.
Mr. Marriott said that it would be impossible to give the authorities or sources from which all the sayings in the paper had been derived, but the foot-note on page [23] contained a list of the works from which the quotations had been made. The authors on the present occasion had only dealt with certain classified prognostics; and this would explain why many others had not been noticed. They hoped, however, to deal with these in a future paper.