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MEAN DIRECTION IN THE SUMMER (JUNE, JULY, AUGUST) BETWEEN 80° N. LAT. AND 56° S. LAT.
This map shows the mean direction of the wind for the time in which the Northern Hemisphere is highly heated, while the southern has its winter. Very prevailing sea-winds (S. W., S., S. E.) along all the southern and eastern coast of Asia (the summer monsoon) are the principal features of the
In Australia, especially on the northern coast, land-winds prevail. They are S. E. in the latter region. In other parts of the globe the difference between the direction of the wind in summer and that for the year is smaller. Yet, in the United States, there is a monsoon region north of the Gulf of Mexico, between the Rocky Mountains and the Mississippi. Southerly winds from the gulf are largely prevailing there. In Northern Africa northerly winds prevail to a larger extent than in the mean of the year. In the Atlantic the belt of the N. E. trade-winds has the most northerly position in the year, while north of it there are prevailing N. and N. W. winds to and beyond 40° N. lat. In Western Asia W. and N. W. winds prevail, this being a flow of air towards the barometric depression in N. W. India. Iu Southern Russia we see westerly winds at this season, the air flowing towards Central Asia.
MEAN DIRECTION IN THE WINTER (DECEMBER, JANUARY, FEBRUARY) BETWEEN 80° N. LAT. AND
56° S. LAT.
At this season the direction of the wind is nearly opposite to that observed in June, July and August in the monsoon region of Asia. N. E. winds prevail in Southern India and the IndoChinese Peninsula, N. and N. W. in the interior of India, and in China, Japan, and the Russian Amoor Provinces. The N. E. monsoon crosses the equator, appearing as a N. W. wind on the heated continent of Australia. In North America, Texas and the States to the north of it have prevailing N. and N. W. winds—a direction nearly opposite to that of summer. The S. W. winds which prevail the whole year in the temperate latitudes of the Northern Atlantic have now reached the maximum of their frequency and strength, blowing also in a great part of Europe. Southern Russia has prevailing east winds in winter. In Western Asia the westerly winds are not so largely prevailing as in summer. The trade-wind belt of the Northern Atlantic and Northern Pacific Oceans has receded to the southward.
DIRECTION OF THE WIND IN THE FOUR SEASONS BETWEEN 800 N. LAT. AND 56° S. LAT.
The general arrangement of this table is the same as for Plate 4. It will be noticed that generally the direction of the wind in spring is nearer to that of summer, and that of autumn to winter. This is especially the case in monsoon regions. In Northern Europe and the eastern part of the Atlantic Ocean the proportion of northerly winds is greatest in spring, giving, sometimes, a mean direction N. of W., while the other seasons have a mean direction S. W. or W. In other places the larger proportion of north winds has influence only in so far as to lessen the ratio of resultant, which is yet S. of W. In Southern Russia and Asia Minor the autumn has the largest proportion of N. E. winds, especially the months of September and October. Many places there have a mean direction nearly E. N. E. in autumn, while it is somewhat S. of E. in winter and W. N. W. in summer.
MEAN DIRECTION IN THE FOUR SEASONS IN THE UNITED STATES.
[See Explanation of Plate 4.]
Here, also, the mean direction of the wind in spring is nearer to that of summer, and that of autumn to winter. West of the Apallachian Chain, and north of 42° N. lat., there are more northerly winds in spring than in ammer and autumn, while further south, and west of the Missis pi, southerly winds prevail already in spring. In the Southern Atlantic and Gulf States there are more northerly winds in autumn than in other seasons.
MEAN DIRECTION IN THE FOUR SEASONS IN EUROPE.
[See Explauations of Plate 4.]
The great extension of northerly winds in the Mediterranean in summer must be noticed. On this Plate are placed a few arrows, whose shafts are divided into twelve portions, corresponding to the successive months, beginning with March (spring), and ending at the barb with February (winter). Here, as in the preceding plate, may be observed the peculiar “S” shape of the curves, so regular a feature in the movement of the wind in the successive seasons, that it was the occasion that led Prof. Coffin to his investigation of the monsoon influences delineated in the following Plate.
Monsoon INFLUENCES IN THE FOUR SEASONS BETWEEN 80° N. LAT. AND 56° S. LAT.
On this map there is a graphic representation of the forces which deflect the mean direction of the wind from its annual value at each season. Taking, for illustration, the monsoon influences at Easton, Pennsylvania, the manner of their representation is the following:
Spring being designated by I, as the first season, summer by S, autumn by A, and winter by W. The opposite directions of the deflecting forces for both sides of the Atlantic Ocean is especially to be noticed. It was first pointed out by Prof. Coffin in a report to the American Association for the Advancement of Science, in 1848, and then embodied in his work on “the Winds of the Northern Hemisphere.” The direction of the deflecting forces is from the S. E. on the coast of the United
States, and from N. W. on the Atlantic Coast of Europe in summer. Similar monsoon influences are at work on the coast of the White Sea and Arctic Ocean. In the real monsoon regions the deflecting forces are very powerful.
The mode by which these forces are ascertained is explained in the introduction to this work, and also fully illustrated in Plate 26.
As the opposition of these forces, however varied they may be in their directions and intensities, must ever represent a state of exact mechanical equilibriuin, some apparently abnormal cases found on the ocean, and mostly south of the equator, must be accounted for; that they be not attributed to erroneous computation. For instance, Zone 24, serial numbers 10 to 21 et seq., and preceding zones, in reference to which foot-notes have usually been appended to the respective pages of the Tables. They are to be explained by the fact that the observations in those localities were not numerous enough to be taken as the basis of a reliable annual resultant, and, therefore, the monsoon influences were obtained by comparing the separate seasons—110t with the meagre yearly resultant that they would have afforded—but with an annual resultant that was obtained by combining all the observations taken on that ocean, and within the limits of the zone.
MONSOON INFLUENCES IN THE UNITED STATES.
[See Explanation of Plate 10.]
The remarkable constancy of the winds between the Apallachian range and the Mississippi, and 34° and 42° N. lat., is especially to be noticed here. Hence the monsoon influences are extremely weak. The most powerful monsoon influences are seen in Texas, the region of the United States which is most like Eastern Asia in the course of its winds.
MONSOON INFLUENCES IN EUROPE.
In winter a monsoon infiuence from the S. E. is seen in Europe; it is a reaction of the high pressure in the interior of the continent. In summer, on the contrary, except in a part of the Mediterranean region, the monsoon influences are from the west.
ANNUAL MEAN DIRECTION OF THE WINDS IN THE UNITED STATES, SHOWING THAT CALCULATED
WHEN THE VELOCITY IS TAKEN INTO ACCOUNT IN COMPARISON WITH THAT FOR TIME ONLY.
The first is expressed by broken arrows, the last by full arrows. It will be seen that the mean direction varies but little, if the velocity is taken into account, from that calculated from the time only. Generally in the first case the ratio of resultant is somewhat greater (the arrows longer). For a more extended view of this topic, compare with this map the diagrams found in Plate 25, and also the introduction to the Velocity Tables, in Series C.
MAPS OF ISOBARS OR LINES OF EQUAL ATMOSPHERIC PRESSURE AT SEA-LEVEL FOR THE YEAR,
JANUARY AND JULY.
These maps are inserted from the treatise of Buchan, “Mean Pressure and Prevailing Winds of the Globe," published in the Transactions of the Royal Society of Edinburgh, vol. xxv., which was the first attempt to do for the pressure of the air what Humboldt and Dove had done for temperature.
95 July, 1875.
A knowledge of the atmospheric pressure is of the greatest importance for the explanation of the courses of the winds. The explanation of these maps is found in the “Discussion and Analysis of Winds," where constant reference is made to it.
PLATES 15 TO 20 INCLUSIVE.
RELATIVE PREVALENCE OF WINDS, IN SUMMER AND WINTER, EXPRESSED IN PERCENTAGE.
PLATE 15. Arctic Regions.
PLATE 20. South Temperate Regions, between latitude 25° and 60' south.
Monsoon influences of marked character are vividly depicted in Plate 17 (Hakodade, Nangasaki and Pekin), Plate 18 (Celebes Sea and China Sea), and Plate 19 (Sween Island, Australia), the belts of shading far outstripping their limits, and even overlapping one another in the cases of Port Blair and Colombo, Ceylon. On the contrary, when the bands are symmetrical for the two seasons, these windroses show the absence of any noticeable monsoon influence, as on Plate 16, for Europe, in the cases of Dublin, Greenwich, St. Petersburg, Vladimir, Debreczin and Gorki.
PERCENTAGE OF WINDS FOR THE FOUR SEASONS.
This Plate differs from the preceding only in containing windroses for spring and autumn, and illustrates the general similarity of the former to winter and of the latter to summer.
PLATE 2 2.
RELATIVE PREVALENCE OF WINDS IN THE UNITED STATES, IN SUMMER AND WINTER, EXPRESSED
This Plate, somewhat more compact in form, exhibits facts of the same nature as those contained in Plates 15 to 20, the percentage of winds at any place being represented in horizontal widths measured across the vertical bands. It enables one readily to find at what place wind from any particular direction is prevalent, by simply tracing down the column until great breadth is reached.
This Plate was drawn by the author as an early attempt to illustrate the connection between the rise and fall of the barometer and corresponding changes in the direction of the wind. The width
of the shading at the several points of the compass shows the average rise or fall of the barometer per day while the wind is from those points, the + indicating a rise, and the - a fall; the two arrows starting from the centre are directed toward the points of maximum and minimum pressure; and a light line indicates the mean of the two. The arrow that springs from the circumference shows the mean annual direction of the wind. In order to compensate for the rare occurrence of winds from some directions, at several of the places, and make the shading more symmetrical, without affecting the principle of the illustration, the mean rise or fall for each point is combined, in several instances, with the two contiguous ones on either side, and the shading is proportioned to the new means thus found.
A METEOROLOGICAL CHART FOR OGDENSBURG, N. Y., 1838.
This plate is a suggestive presentation of meteorological facts. Drawn by the author, in January, 1839, it is believed to be the earliest American effort to connect and vividly illustrate the mutual relation between the results of a minute record of the winds, made by the aid of a self-registering vane, and so many as five of the points chiefly noted in the registers of meteorological observers, viz., amount of cloudiness, fall of rain and snow, and fluctuations in the barometer and thermometer. Deductions from this chart occupy pp. 220–227 of the Report of the Regents of the University of the State of New York, for the year 1838. Each of the circles gives a synchronous view, the shading corresponding in position with the wind then prevalent, and by its width indicating the amount of the contrasted element. From each month, arrows radiating from the centre denote the point of compass from which the wind came that was accompanied by a maximum or minimum of rainfall, thermometric fluctuation, etc.
PLA TE 25.
This illustrates minutely the general results of a series of observations, covering 700 years, and taken at 418 places on the American continent, from 1854 to 1857. The object was to determine what relation the average velocity of the winds, as a whole, and the varying and separate velocity of each particular wind, has to the results, as to direction and prevalence, that are obtained when the variation in velocity is disregarded. The solution of this question was viewed as vital to the correct study of the winds, and therefore of no small importance in the search for the laws of atmospheric circulation.
This plate shows that the resultants computed by assigning to each wind its own separate velocity differ from those in which the variation in velocity is disregarded, in being about go more northerly, and baving a magnitude of 26 instead of 23 per cent.; and, further, that the velocity of all winds in the United States, north of latitude 33°, is a little more than seven miles per hour, resulting in a transfer of air in the mean direction of the main current at the rate of 2.0 or 1.7 miles per hour, according as velocity is counted or omitted.
The arrows represented as flying with the atmospheric current indicate the direction of the winds when only the time of their continuance is taken into account; the dotted lines show the result when the element of Velocity is also regarded. The height of the ordinates in the middle column is proportioned to the average velocity of the wind at each season of the year. In the right-band vertical series of diagrams, the ordinates that terminate in a continuous line show the velocity of the wind in the mean direction, on the suppositiou that the entire current moves with the foregoing average velocity; while, in contrast, those ordinates that end in the broken (dotted) lines exhibit the result, as to velocity in the mean direction, when to each wind is assigned its own special velocity; when the latter class of ordinates is longer than the former, which is usually the case, the intervening space contains the sign t.