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DISCUSSION AND ANALYSIS OF WINDS.

THE aim of Prof. Coffin in this work on the "Winds of the Globe," the reason why he did not write the text, and how I came to take charge of this part of it, has been already explained in the preface.

In what way the ideas of the deceased author would have been modified by the progress of theoretical meteorology in the last twenty years, as well as by the much more extended knowledge of facts we possess now, it is impossible to say. It is very likely, however, that he would have continued to rely principally on the inductive method, would have avoided hasty generalizations, and would have shown the same caution and candor as in his other works, omitting explanations of what our present knowledge did not give sufficient data, rather than mislead his readers.

Before drawing the conclusions which seem to follow from the tables and maps of this work, some explanations are necessary.

The object of this work is to ascertain the movement of the air over different parts of the earth's surface. For this purpose the mean direction and rate of progress of the wind were calculated according to the formula of Lambert. It is easy to see that to accomplish this with precision, we should know the velocity of the wind at all places at which calculations are to be made. Now we know the velocity of the winds in a somewhat accurate manner only for a very small number of stations. For many more the velocity of the wind was merely estimated, and for a majority of places, the direction of the wind alone is known. Now the progressive movement of the air over a certain place, even taking into account the direction only, without considering the number of miles travelled, can be ascertained from the number of observations alone if we make the supposition that all winds have the same velocity; but this is obviously not the case. In nearly all known instances where the velocity of the winds has been ascertained, it has been found to vary considerably; generally, the more accurate observations with self-registering anemometers give a greater difference between the velocities belonging to different directions of the winds than mere estimates have given, the difference being seldom less than 1 to 2, and sometimes even 1 to 4 or 1 to 5.

In considering attentively the observations for the stations where the number of observations, for hours, and the velocity are given, it is seen that generally the most frequent wind is also the strongest, or, comparing the mean direction of the wind calculated from the number of observations only, with that obtained by

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taking into account the velocity of the winds, it will be found that in the last case the mean direction generally approaches nearer to the actual direction of the prevailing wind.

Besides it is seen that generally the ratio of resultant is greater in the second case than in the first.

Thus it follows that, when we have the mean direction of the wind at a given place, calculated from the number of observations only, we may infer that, if the velocity was known, it would modify the result in so far as to make the mean direction nearer to that of the prevailing wind, and the ratio of resultant greater. Unfortunately most of the above deductions apply only to the United States, as it was the only country for which Prof. Coffin made his calculations from the original journals. As to printed meteorological journals, they were very scarce until within a late period, and many of these were not to be had in the United States. Therefore published means and abstracts had to be relied upon, and these gave only the number of observations for each wind. It might be thought that the results of the self-registering anemometers now in use in so many meteorological stations would give abundant material for the answer to this question, but, owing to the recent introduction of these instruments in some cases, and to discontinuity of record in others, comparatively few tabulated records of velocity of winds have been printed. Yet it seems that the angle between the mean direction calculated, taking into account the number of observations only, and that in which the velocity is considered, seldom exceeds 15°. In case of a very small ratio of resultant it can be much greater, but this small ratio itself shows that the mean direction is not much to be relied upon.

All this leads to the conclusion that it is possible to calculate the mean direction. of the wind from the number of observations only without incurring a large error. The map, Plate 13, shows the resultant direction for the number of observations only, as also for velocity, in the United States. I must also explain in what sense I use the words " polar" and "equatorial" winds. Polar designates a wind blowing from a higher latitude towards a lower; and equatorial, a wind blowing from a lower towards a higher latitude. I use these terms in the way which is most generally admitted, to avoid confusion. This agrees also with the manner in which winds are generally designated, so far as we call north wind one that blows from the north towards the south, and not vice versa.

It will be remembered that in the "Winds of the Northern Hemisphere" Prof. Coffin used the words "polar" and "equatorial” in the opposite sense.

Another question, to my mind, more difficult to answer, is as to the value of the observations on the motion of clouds. They may serve two ends: 1, to ascertain the motion of an upper current of the air; 2, to observe the lower current, free from the irregularities often found immediately above the surface of the earth.

Naturally enough, in this case all depends on the height of the clouds observed. Very seldom, if ever, in discussing observations from a journal, can even the approximate height of the clouds observed be ascertained, This alone detracts very much from the value of such observations. Besides this, the cases must be taken into account when there were no clouds, or, the clouds being very high, no

appreciable movement could be observed. All this lessens the value of the observations on motion of clouds.

Generally it is seen, that the clouds move from the same direction as the air near the surface of the earth, which would lead to the conclusion that the lower strata of clouds were those observed.

As to the higher clouds, the cirri, as far as known, they move generally from the west, except in the polar regions.

Considering all this, as well as the fact that the motion of clouds is recorded in this work for very few places outside of the United States, I shall not consider the subject in the further deductions, leaving to every one interested to draw his own conclusions from the tables and the map, Plate 1.

The most important works in meteorological science in the last twenty-two years are devoted to the proof of the mutual dependence of atmospheric pressure

and winds.

It has for a long time been admitted that in the belts of the trade-winds the air moves from the regions where pressure is high (the polar limits of the trades) towards the low pressure of the equatorial regions. The phenomena here were so simple and regular that the explanation was very easy. In the case of the tropical hurricanes it was also generally admitted that the wind blew towards the low pressure in the centre of the storms. The meteorological phenomena of the temperate and polar regions are much more complicated, and the causes of them less easily detected.

It was Prof. Buys-Ballot who proved the general dependence of the winds on the pressure of the air. In its original enunciation, his celebrated law of the winds declares that the winds will blow from the region where the barometer is above the mean towards that where it is below, and will be deflected 60° to 80° towards the right, owing to the rotation of the earth. He subjected this law to a severe practical test in using it in the system for prediction of storms which had been established at that time in the Netherlands. Buys-Ballot's law of the winds is now very generally accepted, though in a somewhat modified form, viz.: the wind blows from a region of high pressure towards one of low pressure, anl is deflected to the right owing to the rotation of the earth. In 1853, Prof. Coffin arrived at a very similar conclusion, saying, "that in the northern hemisphere a wind arriving from its mean direction always finds the point of maximum pressure on its left, and the minimum to its right; while the reverse is true in the southern hemisphere. There seem to be no exceptions to this law." He further states (Proceedings of American Association, 1853, p. 88) that the deflection in this case is 65°; that is, very near to that found by Dr. Buys-Ballot. Even before Professor Coffin, Espy expressed similar views, as seen in his "Philosophy of Storms" and "Meteorological Reports." Very likely the views of the American meteorologists were too much in advance of their time to be generally accepted. When Dr. Buys-Ballot published his law of the winds, meteorology had made much more progress, so as to render such views more easy of acceptance.

This law applies to storms as well as gentle winds, to single hours of observations as well as to monthly and yearly means.

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Buchan has rendered a great service to meteorology by extending Buys-Ballot's law to the general phenomena of the winds of the globe. He collected a great deal of information as to the mean pressure of the air, and drew isobaric lines, i. ., lines of equal pressure of the air reduced to sea-level, and by considering the prevailing winds he proved that they generally followed Buys-Ballot's law. As this work, "Mean Pressure and Prevailing Winds of the Globe," is very important in the discussion of the winds, I make the following extracts from it:—

"Distribution of Atmospheric Pressure in December, January, and February.

"In these months the highest pressures are grouped over the land of the Northern Hemisphere, and the larger the extent of land, the greater the pressure. The area of high barometer (thirty inches and upwards) embraces nearly all of Asia, all Europe south of the North and Baltic Seas, the North Atlantic between 15°-45° N., the West Indies, North America except the North and Northwest, and the Northern Pacific between 8° and 24° N. There are also two regions of high pressure of comparatively small extent, the one in the South Atlantic, the other in the South Pacific.

"The regions of low pressure are: the northern part of the North Atlantic and North Pacific, including portions of the continent adjoining; the belt of low pressure in the equatorial region, towards which the trade-winds blow, and the remarkable depression in the Antarctic region which is probably subject to little change throughout the year.

"In March the pressure diminishes over Asia, the middle and south of Europe and the United States. Everywhere else except in the tropics it is rising. This rise of pressure is most apparent in the temperate regions of the southern hemisphere. In the north of the Atlantic it is rapidly rising, the average pressure in Iceland now being 29.609 inches, thus showing an increase of 0.34 inch in comparison with January.

zone.

"In April, the heavy lines indicating a pressure above the average have all but left Asia, Europe, and the United States, and the isobars of 30 inches bound a belt of high pressure, which completely encircles the globe in the south temperate Pressure continues to rise in the north of the Atlantic, and to the north of North America. And it is probable And it is probable that a space of high pressure (at least 30 inches) completely encircles the north pole. In this month pressure is more equally distributed than in any other month; for, except the Antarctic Ocean, it scarcely rises anywhere above 30.1 inches nor falls below 29.8. In May, in North -Europe, in Greenland, and in the north of North America, pressure attains the maximum of the year. Pressure continues to increase in the south temperate zone, and the isobar of 30.1 now nearly encircles the globe. At this time the highest pressure in the southern hemisphere occurs in the S. E. of Australia, where, at Deniliquin, it is 30.185 inches. Pressure is rapidly falling over Asia and

the United States.

"In June, July and August, pressure falls in the central regions of Asia to about 29.5. In this season this diminution of pressure, which may be regarded as entirely

determining the summer climate of Asia, reaches its lowest point. Pressure falls also in the interior of North America, where, at Salt Lake City, it is only 29.7 inches. The annual maximum of the south temperate zone is attained in these months. The isobar of 30.1 goes entirely round the globe, and a still higher pressure prevails over South Africa, and the portions of the ocean immediately to the west and east of it. In these months the arrangement of the isobars may be regarded as being, generally speaking, reversed from that of December, January and February, and in this respect a comparison of these two groups of months is very instructive.

"From this period, pressures increase over the continents of the northern hemisphere, and diminish over the south temperate zone, till the distribution of is regained which has been shown to prevail during the winter months.

pressure

"In September and October an interesting feature of these lines is a very rapid. diminution of pressure, indicated as taking place in the north of the Atlantic and surrounding regions. This is the season of the year when the first great decrease of temperature takes place, which is accompanied by heavy rains and furious storms. The increase of pressure in Sweden in October, taken in connection with the simultaneous decrease in Greenland, Iceland, the north of Norway, and the British Islands, is interesting as bearing on the transport of masses of the atmosphere from one region into another.

"In November, pressure rises considerably over the continents of the northern hemisphere, and falls in the south temperate zone. And the belt of low pressure in the equatorial regions may be regarded as passing completely around the globe. This belt, towards which the trades on each side of the equator blow, does not occur in the summer months in the Indian Ocean; but, on the contrary, there is a continuous diminution of pressure northward, from Australia and Mauritius to the interior of Asia. It will be seen that in November, as compared with October, the isobars have advanced a little northward from the British Islands to Iceland, and eastward from Baffin's Bay to Iceland, thus indicating a general increase of pressure over the north of the Atlantic and regions adjoining. Coincident with this increase of pressure, there occurs a diminution of pressure to the southeast of it, including Austria, Italy, and countries adjoining the Mediterranean; and in the Atlantic to the south of it, from about latitude 15°-45° N. Probably these extensive oscillations of pressure are part of a general movement of the atmosphere, which, in one of its manifestations, has been generally known to meteorologists as the great November wave, but of which no very satisfactory account has yet been given." (Buchan, p. 577-579.)

WINDS within, or near, a space of Low Pressure." Of this class, the best example is the low pressure which prevails in the north of the Atlantic and adjoining regions in the winter months. This region of low pressure is bounded to the S. W. by the high pressure of North America, to the S. by the high pressure in the Atlantic, about 30° lat. N., to the S. E. by the high pressure in the interior of Asia. In January, the difference between the average pressure of Iceland and the interior of Asia is fully an inch."

"It is seen from the charts that in Baffin's Bay and east of the Rocky Mountains,

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