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.