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Popular Science Monthly/Volume 2/December 1872/Weather Prophecies

WEATHER PROPHECIES.

THE science of the weather may be said to have sprung up within the last half century, and we must not therefore wonder that, until very recently, meteorological science has rather been concerned with the weather as it has been, than in prophesying what kind of weather may be expected. Indeed, this is almost the case at the present day; for, were it not for the telegraph, storm-signals would be of little avail. Much was gained when, from the conclusions drawn from a large number of observations, a storm could be telegraphed from any place as coming, instead of as happened. This stage of the science is perhaps as far as can be usually attained in the present day; in some future time, from the careful study of the laws, it may be possible to predict, with average certainty, the state of the weather from day to day, or even for several days to come. It remains to be seen how far this power has been attained; and it may not be uninteresting to notice, in passing, the very unstable ground upon which weather predictions were founded before meteorology included this second division.

Whether we take as type the old dame's faith in the gambols of her cat, the high flight of some birds and the low flight of others, the "camel" in the clouds, or the chirruping of grasshoppers, we have much the same arbitrary system, or, rather, want of system, although these signs may not be without some definite cause, more or less remotely connected with coming changes in the weather. In many country places it is common to hear it remarked that "the rain will soon clear up, for the birds are singing;" the coming change is perhaps already sensible to their more delicate organization. There is also the appearance of the clouds; and to this indication even the lamented Sir John Herschel attached somewhat of a reliance, in that "anvil-shaped clouds" portended a gale of wind. But, as a rule, the moon may be considered to hold the first place of influence upon weather predictions. Halos round the moon are the phenomena most commonly observed, and are readily explained by the laws of the reflection of light from the particles of aqueous vapor suspended in the atmosphere. When these halos are colored, we may infer the presence of watery particles in the higher regions of the atmosphere; when the halos are white, we may conclude that the particles are frozen, and expect cold weather. Crossed halos, mock moons, or highly-developed phenomena, indicate larger crystals of ice, and probably frost, hail, snow, or heavy rain, after three or four days, according to the season of the year. Similarly the laws of reflection of light indicate that the cause of a deep-purple morning or evening sky is the large amount of moisture present in the atmosphere. Another effect of the moon, when at the full, is to clear the sky of cloud, traceable, says Sir John Herschel, to a distinct physical cause, the warmth radiated from its highly-heated surface; though, why the effect should not continue for several nights after the full, remains, in the opinion of the same accurate observer, problematic. Other lunar prognostics, founded on arbitrary rules, as to the time of the day or night at which the changes or quarterings take place, are worse than useless, for they are calculated to mislead, and are generally included in almanacs or note-books intended for sale only, being in some cases attributed to an eminent meteorologist or astronomer—Sir W. Herschel or others.

It is of course far from our purpose to enter here into a disquisition on the theory of the trade and anti-trade winds, and their barometrical indications—subjects that can be usefully discussed only in a treatise on meteorology: we limit ourselves to the present position of weather prognostics, although it must be admitted that any advance yet made or likely to be made in prognosticating the weather arises from the study of such recurrent phenomena, the investigation being much aided by the highly-developed character of the laws of the expansion of gases, upon which laws the theory of the wind is founded. Thus we know that a rise in the barometer, together with a fall in temperature, as shown by the thermometer, indicates the approach of a cold, northerly current of air; while a fall of the mercury in the barometer, with a rise of that in the thermometer, indicates that a southerly or warm air-current is on its way. Northerly currents may include winds from the northwest and northeast, as well as from the north; similarly, southerly currents may include winds from the south, southeast, or southwest. When the barometer rises while a northeast wind is blowing, with prevalent hail, rain, or snow, there may be no change. Of barometrical indications alone, it is generally known that a rapid rise portends changeable weather; a slow rise, the contrary; a rapid fall, heavy wind, rain, and snow; while a fluctuating height of the column of mercury indicates unsteady weather. With a heavy gale of wind in the east or southeast, changing south, the barometrical column may fall until the wind shifts its quarter. Upon such observations did Admiral Fitzroy base his code of instructions, now to be found by the side of every barometer, his forecasts depending on the indications of the barometer and thermometer, with observations as to the direction and force of the wind with regard to time and place, and its previous course taken altogether. These indications are thus not absolute, but relative to the preceding state of the weather. But also these indications are valid for only a short interval before the actual advent of the storm; and in some instances, as in the Hyperborean storm of the 2d and 3d of October, 1860, the interval is too short for any advantage to be taken of the notice. The particulars of this storm, which present in true character the difficulties which the meteorologist must encounter, are too interesting to be omitted, and we shortly recount them from the complete and admirably-conducted investigation published by Prof. C. Piazzi Smyth, in the "Annals of Scottish Meteorology for 1856 to 1871." The term Hyperborean has been employed to prevent confusion with tropical hurricanes; it has also been called, from its essential locality, the Edinburgh storm. We have to consider only the practical lessons to be deduced from the observations of this storm; the account of the actual observations must be read from the before-mentioned report of Prof. Piazzi Smyth. First, then, the barometric notice was insufficient and too local to be of service, while the storm was too quick in its movements. St. Hilda is the most westerly station; and, even if the storm could have been telegraphed thence, the message would have allowed only two hours for preparation, and would have arrived while the eastern men were sound asleep. If a message could have been sent from Iceland the day previous to the arrival of the storm, many wrecks would have been prevented. So that we see the present system of meteorology necessitates not only diligent but earnest watching of the signals that should be afforded by a net-work of cables and overland wires, for it is by a series of connected observations, extended over a large area, that the usefulness of this branch of meteorology is alone likely to be advanced.

But, it may be asked, what definitive knowledge can be gained, say not of storms, but of average weather for some future period? Here we must again refer to Prof. Piazzi Smyth's report on the rock-thermometers at the Royal Observatory, Edinburgh, and to the Proceedings of our own Royal Society for the 2d of March, 1870, in which predictions of the weather during the winters of 1871–'72 are attempted. The rock-thermometers have by their readings shown some well-marked supra-annual cycles, the relation of which to the sun-spot cycles will be known to our readers. And on this point it may be stated that the Radcliffe astronomer announces, in his report for 1871, that the mean azimuthal direction of the wind at Oxford, rigorously computed from automatic records during the last eight years, varies year by year through a range of 58° on the whole, between maximum and minimum of visible sun-spots, the tendency of the wind to a westward direction increasing with the number of spots, and with such west wind, it is to be presumed, the amount of rain also. "The most striking and positive feature of the whole series of observations," continues Prof. Piazzi Smyth, "is the great heat-wave which occurs every eleven years and a fraction, and nearly coincidently with the beginning of the increase of each sun-spot cycle of the same eleven-year duration. The last observed occurrences of such heat-wave, which is very short-lived, and of a totally different shape from the sun-spot curve, were in 1834.8, 1846.4, 1857.8, and 1868.8; whence, allowing for the greater uncertainty of the earlier observation, we may expect the next occurrence of the phenomenon in or about 1880.0. The next largest feature is the extreme cold close on either side of the great heat-wave: this phenomenon is not quite so certain as the heat-wave, partly on account of the excessive depth and duration, of the particular cold-wave which followed the hot season of 1834.8. That exceedingly cold period, lasting as it did through the several successive years 1836, 1837, and 1838, was, however, apparently a rare consequence of an eleven-year minimum, occurring simultaneously with the minimum of a much longer cycle of some forty or more years, and which has not returned within itself since our observations began. Depending, therefore, chiefly on our later observed eleven-year periods, or from 1846.4 to 1857.8, and from the latter up to 1868.8, we may perhaps be justified in concluding that the minimum temperature of the present cold-wave was reached in 1871.1; and the next similar cold-wave will occur in 1878.8." Between the dates of these two cold-waves there are located, according to all the cycles observed, even including that earlier one otherwise exceptional, three moderate and nearly equidistant heat-waves, with their two intervening and very moderate cold-waves, but their characters are quite unimportant. With regard to all the waves, it may be just to state that there has been in observation more uniformity, and will be therefore in prediction more certainty, for their dates than for their intensities.

We have thus very briefly surveyed the position of meteorology, and little remains to be said beyond that the results are highly in favor of the hopes of physicists to render meteorology an exact science.—Quarterly Journal of Science.