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DESCRIPTION OF MAPS AND DIAGRAMS.

THE direction of the wind on the maps, Plates 1 to 13, is indicated by arrows.

For example, to indicate a N. wind, the head of the arrow is turned towards the south, and the tail towards the north. The direction indicated is not that of the prevailing wind, but the mean direction, the manner of calculation of which was explained in the beginning of this work. The length of the stem of the arrow, exclusive of its barb, is proportionate to the ratio of resultant, the greatest length being when the ratio is equal to 100, or when all winds come from the same direction. These maps were originally all drawn to a scale, in which one hundred per cent., as found in the tables, was intended to be represented by an arrow an inch in length; but, by the process of engraving adopted, it was found practicable to diminish the size of the maps somewhat, so that 100 per cent. equals two-thirds of an inch; for instance, on Plate 1, Zone 10, serial number 196, representing Eastern Pennsylvania, the arrow is 20 one-hundredths of an inch in length, corresponding to the tabular percentage .30 given on page 320. The more equally the winds are distributed around the horizon the smaller is the ratio, and also the shorter the arrow on the map. Where it is very small it indicates that there is no really prevailing wind. This is generally found on the boundaries of two systems of winds.

PLATE 1.

ANNUAL DIRECTION OF UPPER AND LOWER CURRENT IN THE UNITED STATES.

The mean direction of the wind, as observed by the wind-vane, is indicated by full arrows, and the direction of the motion of clouds by broken arrows. It will be seen that they very nearly coincide in nearly all regions of the United States. Generally the upper current is more purely west in all the regions east of the Mississippi, while the lower current has a more W. S. W. direction between the Mississippi and Apallachian Chain, as well as in the Southern Atlantic States, while in New England the winds are rather W. N. W. Near the Gulf of Mexico the arrows have a very different direction, but it will be seen that the arrows are very small, thus indicating an undecided prevalence of any wind. In some parts of Texas, also, the upper and lower current seem to come from different directions. In this map, as well as in the others, the figures relate to the serial number in the zone, and, by reference to the Numerical Index to Stations, given on pages 52 to 66, it is easy to find the name of the place indicated by each figure.

PLATE 2.

MEAN ANNUAL DIRECTION IN THE ARCTIC REGIONS.

It will be noticed that the mean direction of the wind is from the north in Greenland and Arctic America, and that the arrows are long, thus indicating very prevailing winds. On the northern coasts of Europe and Asia the winds are from the south, while Bear Island, between Norway and Spitzbergen, as well as Iceland, have prevailing easterly (polar) winds. In this map a dotted line is traced, and called "Southern limit of polar system." This is the same boundary as that traced by ( 749 )

the late Prof. Coffin on the maps of the "Winds of the Northern Hemisphere." In the mind of the deceased author this was the boundary between the prevailing polar winds of the Arctic regions and the equatorial (westerly) winds of the middle latitudes of the Northern Hemisphere. He traced it at a distance of 28° 20' from an imaginary point which he called the "Meteorological Pole," and located in 84° N. lat. and 105° W. long.

PLATE 3.

MEAN ANNUAL DIRECTION BETWEEN 80° N. LAT. AND 56° S. LAT.

The general prevalence of westerly winds will be seen here in the middle latitudes of the Northern Hemisphere. Yet they are not always true equatorial winds, but incline somewhat to the north in some regions. On the tropical seas easterly winds largely prevail, as indicated by the length of the arrows. This is the region of the trade-winds which prevail more largely in the Southern Hemisphere than in the Northern. In the middle latitudes of the Southern Hemisphere westerly winds again prevail, and this to a large extent, while further south there are again easterly (polar) winds. In some parts of the globe, where monsoon winds prevail, the length of the arrow showing the mean annual direction is rather small (as in India, China, Japan). This does not come from an undecided character of the winds, but is caused by the nearly opposite direction of the winds in winter and summer. As they counteract one another in the yearly resultant, the ratio of the latter is small. A reference to Plates 5 and 6, giving the mean direction of the wind in summer and winter, shows that at each season the arrows in China, India, Japan, and the surrounding seas, have a great length, showing largely prevailing winds at both seasons.

Monsoon comes from the Arabic word Mausim, or wind of the season. We call monsoon regions those that have winds of nearly opposite character in winter and summer, each of these winds prevailing during some month of the year nearly to the exclusion of all others. On the greatest scale we see such winds along all the southern and eastern coast of Asia, and on the surrounding seas, the winds in the tropical part of this country being N. E. in winter and S. W. in summer, while further north, in the interior of India, China, Japan, and the Russian Amoor provinces, the winds are rather N. and N. W. in winter, and S. and S. E. in summer. Monsoon winds are caused by the mutual reaction of great continental masses and the ocean, and thus they are most prevailing where the greatest continent-Asia-approaches the greatest oceans-the Pacific and Indian. In winter the pressure of the air is high on great continents, and thus air flows out from there, while in summer, on the contrary, the land-masses being highly heated, an ascending current is produced and the air from the surrounding oceans is drawn in to supply the deficiency. In all parts of the earth, where continents and oceans adjoin, we see a tendency to produce monsoons. This is what Prof. Coffin has called monsoon influences, but not everywhere monsoon winds are dominant. Monsoon influences may be considered as small deflections from the mean annual direction in regions where no great differences in the mean direction of the wind in the different seasons are experienced, and thus this relative influence of land and sea is small. In monsoon regions, on the contrary, this influence is experienced on the largest scale.

PLATE 4.

MEAN DIRECTION IN THE FOUR SEASONS IN THE ANTARCTIC REGION.

The direction of the wind is here represented by broken arrows, thus:

Α

The

straight line drawn from the tail to the head of the arrow gives the mean annual direction. sequence is always-spring, summer, autumn, winter. Thus the nearest part to the tail of the arrow indicates the mean direction and ratio of resultant in the spring, and the nearest to the head that of winter. As before stated, June, July and August are denominated "summer," etc.

PLATE 5.

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 season. 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. In Southern Russia we see westerly winds at this season, the air flowing towards Central Asia.

PLATE 6.

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.

PLATE 7.

DIRECTION OF THE WIND IN THE FOUR SEASONS BETWEEN 80° 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.

PLATE 8.

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 West of the Apallachian Chain, and north of 42° N. lat., there are more northerly winds in spring than in summer and autumn, while further south, and west of the Mississippi, southerly winds prevail already in spring. In the Southern Atlantic and Gulf States there are more northerly winds in autumn than in other seasons.

PLATE 9.

MEAN DIRECTION IN THE FOUR SEASONS IN EUROPE.

[See Explanations 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.

PLATE 10.

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:

W

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 equilibrium, 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-not 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.

PLATE 11.

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.

PLATE 12.

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.

PLATE 13.

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.

PLATE 14.

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.

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