Popular Science Monthly/Volume 16/November 1879/Ocean Meteorology I
| OCEAN METEOROLOGY. |
By Lieutenant T. A. LYONS, U. S. N.
WERE the captain of a ship to contemplate making a passage in a sea he had never before traversed, he would find it desirable to be supplied with charts of two different kinds: one kind showing the rocks, shoals, and other dangers scattered throughout its expanse, the contour of its islands and bounding shores, and the soundings of its shallow waters; the other kind giving full and reliable information regarding its winds and weather, storms and currents, barometric and thermometric fluctuations. The first is essential to safe navigation; the second an invaluable auxiliary to a speedy passage. It is of this second kind—meteorological charts—that this article is to treat.
And, first, partly to introduce the subject, partly to illustrate it, I will very briefly touch upon a similar work for the land—a work which has now become familiar to all—I mean the daily synopsis and forecast of the weather published by the United States and several European Governments for the benefit of their people.
The value of an extensive organization for observing atmospheric phenomena was early appreciated in Europe, and as long ago as the year 1780 the Society of the Palatinate was established under the auspices of the Elector Charles Theodore, who entered with spirit and ability into its pursuits, and furnished it with the means of defraying the expense of instruments of the best construction, which were gratuitously distributed to all parts of Europe, and even to America. Some idea may be formed of the comprehensive scale of the journal of this society, when it is known that it contains observations three times in the day of the barometer, thermometer in the shade and in the sun, hygrometer, magnetic needle, direction and force of the wind, quantity of rain and of evaporation, the height of any neighboring water, the changes of the moon, the appearance of the sky, and the occurrence of meteors and of the aurora borealis. To these must be added, in some places, observations upon the electrical state of the atmosphere, upon the progress of vegetation, the prevalence of disease, changes of population, and migration of animals. The field of observation extended from the Ural Mountains in the east to Cambridge, in the United States, in the west; and from Greenland and Norway in the north to Rome in the south. This range included also stations upon three high mountains in Bavaria and upon the summit of St. Gothard. The observations of each year are summed up and compared with those which precede, in copious and most laborious tables of mean and extreme results, and many very interesting essays upon various branches of meteorology are interspersed throughout the volumes of the society.
Unfortunately for science, the secretary, Hemmer, died in the month of May, 1790, and from that time the society appears to have languished, and finally to have become extinct amid the troubles and the wars of the French Revolution.[1]
It might be of interest to trace the progress of meteorology since the days of the Palatinate Society—to recount the many improvements in the instruments, the new auxiliaries impressed into its service, the successive unfolding of its laws as immense masses of data came into view, and the gradual passing of the subject from the care of amateurs, who pursued it mostly as a pastime or matter of curious inquiry, into trained hands and organized bodies maintained by liberal government support. But this is not my purpose here: with a passing glance at an important guide-post erected about the year 1840 on the highway of this science, I will make a single stride over all this field and come at once to the problem proposed to the meteorologist of the present day, and the means at his command for its solution.
The writer of this guide to the way beyond gives in clear-cut outline all that has since been realized both in this country and England. After stating the necessity of making observations on land coördinate with those at sea, in order to study the atmosphere in its entirety, he uses these prophetic words: "This extension of the system landward was proposed in the beginning as a part of the original plan. I have never ceased to advocate it since, and to couple with it a system of daily weather reports through the telegraph. As much as we have accomplished at sea, more yet can be accomplished through the magnetic telegraph on the land. With a properly devised system of meteorological observations to be made at certain stations wherever the telegraph spreads its meshes, and to be reported daily by telegrams to a properly organized office, the shipping in the harbors of our seaport towns, the husbandman in the field, and the traveler on the road, may all be warned of every extensive storm that visits our shores, and while yet it is a great way off. The laurels to be anticipated from such extension of our beautiful field of research would crown the results already obtained, and probably entitle the whole to be regarded as among the most splendid achievements of the age. With this system established, and conducted as it ought to be, no ship need ever put to sea from any of our seaports in ignorance of the approaching storm. A like system for the British Islands and the Continent would lead to like results there; many storms, after visiting our shores, travel across the ocean and carry devastation there. Should the sub-Atlantic telegraph be laid, and, when laid, should it answer its ends, warnings of all such storms may be sent across the ocean several days in advance."—("Sailing Directions," by Lieutenant M. F. Maury, U. S. N., vol. i., p. viii. of Introduction.)
To recur to the problem of the meteorologist of to-day, it assumes three distinct phases: first, from a number of observations extending through many years and over a large expanse of land or sea, to discover the laws of atmospheric phenomena; second, from an unbroken series of observations at any one place, through a sufficiently long period to eliminate all merely adventitious changes, to determine the climate of that place; and third, from a number of simultaneous observations at different points of any circumscribed area, to predict what the weather will be over that area for any short time.
The solution of the first phase is all but complete: the great governing principles of our atmosphere are now quite well known—it is only the details that need defining. That of the second phase can scarcely be said to be more than begun: in only a few places on the globe have accurate observations been continued for a long enough period to reliably define their climates; but of late years, especially during the last twenty, such an interest has been awakened in this subject, that ere the century closes very many cities will possess the data for thoroughly describing the atmospheric changes to which they are subject, not according to the recollection of the oldest inhabitant, but by accurate records—figures that never deceive.
The effect of the weather upon mankind is only too well known: with the invalid or convalescent it is often a matter of life or death; with us all, how different our feelings on a fresh, genial day, when the air is dry and bracing, and a bright sun illumines an azure sky—how elastic, how full of vigor are we, compared with the lethargy that seizes us in somber, bleak weather, when dense, misty clouds hang in heavy folds around us, and shade even our very sensations with their gloom!
In Boston, the east wind of spring and autumn is a source of annoyance to its inhabitants; it comes laden with moisture and—coughs. In California, it is an equally unwelcome visitor, but for a very different reason: it parches and all but cracks the skin. In both places, the relative prevalence of this wind is a fact important to know. In Buffalo, the storms that sweep in from the lakes are disagreeable in the extreme; in Texas, the fierce blast of the norther is often very destructive. But all these instances are peculiarities singled out from a variety of items, highly interesting to any one contemplating either a temporary or permanent residence in a place new to him. The storms, the rain, and the snow he has to encounter; the average humidity and tenuity of the air he has to breathe; the variety and character of the winds that are to blow upon him; the mean and extreme of the daily, monthly, and yearly temperature to which he will be subjected; the relative number of cloudy and clear days—all this, constituting the climate of a place, should be known to one ere he hazards his comfort, his good feeling, or, it may be, his health, by a change of residence. And it is probable that, with the great number of observers now carefully noting and recording these items in various cities, the day is not far distant when their laborious experience of long years will be classified, reduced, and published in such a compendious form, that a stranger to any given place may, by half an hour's study of this publication, inform himself correctly as to its climate.
The solution of the third phase of the problem is the one productive of most immediate benefit to all, how much soever their callings may differ; and this universal interest warrants my stating its conditions more at length than I have done with the other two. This phase may be likened unto an algebraic equation—a combination of known and unknown quantities, which, being operated upon according to certain rules, gives a desired result.
First, to determine the known quantities, a variety of instruments must be read and recorded at stated periods. These are, anemometers, to indicate the direction and velocity of the wind; barometers, to measure the pressure of the air; and hygrometers, its humidity. Suppose sets of these, standard in quality, to be furnished a corps of trained observers stationed at various points throughout a given area, say a thousand square miles; let each observer note his instruments at predetermined hours, or, better, let the observation be automatic and continuous, which is now often done by means of mechanical contrivances; let a network of telegraphy connect all the stations with some central point: then, at any moment he wishes, a person at this point can ascertain the prevailing weather all over the area, or, in other words, the known quantities of his equation. Now, the atmosphere that encircles our globe is but an ocean of less density than the watery element that surges upon its surface; like that, it moves, contracts, and expands according to well-known physical laws, and these laws constitute the rules whereby the person at the central station operates on his known quantities, solves the equation, and obtains for a result the forecast of the weather for the next few hours.
Having a due regard for the conformation of the ground over which his prognostics extend, he well knows that, according to the relative variation of pressure, temperature, and moisture, there will be a corresponding variability of weather: that if the pressure is great at one place and slight at another, the air will as naturally flow from the former toward the latter as water will down an inclined plane; and as the velocity of the water will depend on the inclination of the plane, so will the violence of the wind be chiefly due to the difference of pressure: hence the direction and force of the wind are predicted. Again, whether the day will be warm or cold depends mostly on the temperature of this wind; and, furthermore, if it contains much vapor and blows toward a point where the temperature is lower than that from which it started, clouds, or rain, or snow will follow, according to the difference of temperature and the supply of vapor; but if a saturated wind blows toward a place where the temperature is high, and air dry, its moisture will be licked up by the thirsty air, and a mere haze will ensue, or clear weather continue.
This problem in its ever-varying conditions is the one daily solved by the Weather Bureaus of several Governments, in the interest of agriculture, comfort, and commerce; and perhaps nowhere more successfully than by our own. With its large corps of trained observers, its military discipline, variety of standard instruments, extensive field of operations covered by telegraphic lines, liberal Government support, and educated intelligence to guide the whole, there is every reason for the confidence so generally felt in the weather prognostics of the Signal Office of the United States Army.
With this preliminary glance at meteorology on the land, I shall now pass to a consideration of it as regards the ocean—the subject proper of this article; and as I have already divided the problem into three phases, it will be convenient to maintain this distinction—only, that for the ocean, the cases reduce to two: first, to seek out the hidden cause of the winds, whether as the gentle trades that scarcely ruffle the waters over which they glide, or as the violent hurricane that lashes the waves into a tempest of confusion; and, secondly, to determine the many items that, together, make up the climate of small areas of every sea.
The third phase of the problem on land is entirely excluded from the ocean. There we can not establish fixed stations and spread a web of electric wire over them, with some guiding genius ensconced in the midst. We can not (as is done every morning in the United States, England, France, and Germany, for the limits of each country) say what the weather will be, and how the winds will blow, for the ensuing twenty-four hours, in the Indian Ocean, the South Seas, or the North Atlantic. But we can give information yielding in no degree in importance to this purely ephemeral benefit; and the manner in which this is obtained and published is what I shall fully describe hereafter.
To the late Commander M. F. Maury, of the United States Navy, is due the credit of having given to ocean meteorology that vigorous impulse that placed it in the foremost rank of pursuits, and justly obtained for its advocate his distinction in this branch of physical science. He planted a germ which, under his own assiduous care, grew and overspread the globe: its seed fell in every maritime nation, and to-day they are producing meteorological charts of the ocean—all modifications or elaborations of his useful idea. It is therefore but proper that I should here give a short sketch of both himself and his great work.
Matthew Fontaine Maury was born in Virginia, January 4, 1806. He entered the navy as midshipman in 1825, and was promoted to the grade of lieutenant in 1836, having in the interval been attached to various cruising-vessels, on which he performed the customary duties of a sea officer. It was during one of these cruises that the outline of his future work acquired form and shape in his brain.
In 1839 an accident permanently incapacitated him for further service at sea, and he was therefore given charge of the depot of charts and instruments in Washington: this was soon afterward united to the Naval Observatory, and he became superintendent of both, retaining the position uninterruptedly until 1861—a period of more than twenty years. Later still, the scope, character, and importance of the chart department grew to such dimensions as to necessitate its separation from the observatory: this was done, and it became the Hydrographic Office, which it continues to this day, under the management of a naval officer. At present, it has no closer intimacy with the observatory than being under the guidance of officers of the same branch of the Government—the navy.
Maury was promoted to the grade of commander in 1855, and it was then also that he attained the height of his scientific fame: he had written his "Physical Geography of the Sea"; he had been chiefly instrumental in bringing about the Brussels Conference, whereby the civilized nations of the world entered into his plan of "ocean meteorology"; he had prepared his ponderous volumes of "Sailing Directions"; he had received the encomiums of numerous scientific bodies both native and foreign; and, with the constant aid of a large number of naval officers, he had compiled, with incredible labor and pains, that series of charts that has made his name so familiar to sailors, whatever the flag they sail under.
On the 1st of February, 1873, after having done more than any other man that preceded him toward tracing the wind in its circuits, and showing the navigator how to take advantage thereof, he died at Lexington, Virginia, in the sixty-eighth year of his age.
I will now give an outline of the charts compiled under Maury's direction. A full description would necessitate the reproduction of specimen-sheets, and that is impracticable here.
First and most important are the Pilot Charts. These give for small areas of ocean—every five degrees square—the relative frequency of different winds during each month. The following figure is a sample square of the whole series, and a few explanatory words will disentangle this web of figures. The radii extending from the inner to the outer circle inclose sixteen points of the compass—as north, north-northeast, northeast, etc. Every two concentric circles contain the data for each season. The problem being, then, to compare the relative prevalence of the same wind in different months, it is done as follows: suppose it a northerly wind; looking at the figures between
the two radii opening toward the top and between the outer and second circles, we see that, of periods of eight hours each, there were 32 in December, 21 in January, and 29 in February; the figures between the same radii and the second and third circles show that there were 41 periods in March, 33 in April, and 6 in May; and similarly, for each wind between every two radii. To compare different winds for the same month, say December, we look at the first figure to the left in each space between the outer and second circles, and find that, of periods of eight hours each, the wind was 32 times from the north, 29 times from north-northeast, 56 from northeast, and so on round the compass.
The figures 416, 385, and 408, in the upper right-hand corner, denote the total number of observations in December, January, and February, respectively; and similarly for the other months in the other corners. The figures in the center express the periods of calm in the several months.
Though this arrangement is compact and ingenious, still, when we come to make the comparison that is the real object of the chart, viz., the relative frequency of different winds in several adjoining squares, we find the task a little irksome.
Second, the Thermal Charts. These show the temperature of the sea-water at the surface for every month, isothermal lines bring drawn at every 10° from 40° to 80° Fahr, By using three colors, and a different arrangement of the figures for each season, all the observations of each month are made separately visible on one sheet in the spot where taken. The sheet thus appears to the eye a continued intermingling of curves and figures—blue, red, and black—generally open and easily traced where the observations are moderate in number, but an inextricable tangle where frequent.
As difference of temperature in adjacent portions of the sea indicates difference of density, which in turn denotes a mobility of the waters, that is, oceanic currents, these currents are therefore indirectly shown by this series of charts.
Third, the Track Charts. Such a quantity and variety of information is crowded into these, that I despair of giving any intelligible idea of them.
Imagine an artist perched a thousand feet above the center of New York and provided with canvas on which to delineate the city below—to trace in outline every street, house, and tree; every railway and telegraph line; all the moving objects, man, horse, and vehicle—what a complicated picture it would make! Yet this would by no means represent the intricacy of the network on the Track Charts. On them the experience of a large number of all the vessels that sailed the ocean for a period of fifty years is spread before us. Most prominent are their jagged courses from port to port; along these are symbols to represent the direction and force of the wind: roman numerals to express the magnetic variation; arrows and figures to indicate the set and strength of currents; figures to show the temperature of the sea-water; great circle routes; trade-wind limits; the name of each ship and date of making the passage—and all this in distinctive colors and peculiarity of line, so that each item can be determined with great exactness as regards both time and space.
Indeed, this profuse interweaving and crossing of lines and figures taxes the patience of even the most painstaking mariner. What the charts show forcibly at a glance, are the great ocean highways, but this chiefly by the multiplicity of tracks through the beaten paths, compared with their sparseness over less frequented routes.
Fourth, the Storm and Rain Charts. For every five degrees square and each month, they clearly show the relative prevalence (compared with the whole period of observation) of the following phenomena: gales from eight cardinal points of the compass, calms, fogs, thunder and lightning, and rain (including hail, snow, and sleet).
The arrangement of these charts is excellent, and they are easily understood.
Fifth and last of the entire set, the Trade-Wind Charts of the Atlantic Ocean. By a judicious use of colors, figures, and lines, the limits of both trades, of the calm belt between, and of the calm zone on the outer border of each system of trades, together with the several observations by which these limits were determined, are all clearly and distinctly shown for each month, on a single sheet.
In 1863 the publication of Maury's charts was discontinued; but in 1846 other charts, similar in nature, though entirely different in the method of compilation, were begun, are now in progress, and will be continued, until sets for all the navigable waters of the globe are completed; and it is a description of these new charts that will constitute the second paper of this article.
- ↑ For these particulars of the Society of the Palatinate I am indebted to the valuable treatise on meteorology by the late Professor John Frederick Daniell, of England.