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is not from the tropical part of the ocean, it cannot bring much rain and produce the secondary areas of low pressure caused by condensation. Besides, the region of high pressure on the Atlantic is far from the low pressure of Central Asia, and near to that about Iceland; so that the movement in the first direction cannot be very constant. As to the air from over the lower latitudes of the Atlantic Ocean and the Western Mediterranean, it is attracted towards Africa, which is highly heated in summer, and open to the winds from the surrounding
The geographical features of the North American continent explain why pressure and winds are so different over it from what is seen in Asia.
The coldest region of America is known to be to the north of the continent, on the islands and ice-bound seas and sounds north of 70°. Ice and snow being bad conductors of heat, the streams of warmer water are thus effectually prevented from having an influence on the air, and the ice-bound seas to the north of America can cool as well as continents.
But, as the coldest space north of the American continent is not separated by mountains and plateaus from the surrounding regions, there cannot be such a constant high pressure there as on the corresponding coldest space of Asia. It will be remembered that the lowest pressure of the northern hemisphere, especially in winter, exists near Iceland, which is partly due to the warm waters of the Gulf Stream. The coldest regions of America are not separated by any natural barrier from this space, and thus air, even from the lowest, heaviest strata, should flow towards Iceland. That this is the case, is shown by the winds in Greenland and on the most northerly stations of the American continent; they are northerly to a very large extent. Probably the easy intercommunication between the coldest region of North America and the region of low pressure near Iceland, explains why the former has not a high mean pressure in winter. Having not a constantly high pressure, the polar regions of America cannot influence the winds in the temperate and tropical regions of this continent as the coldest region of Siberia, with its constantly high pressure, does influence the temperate and tropical regions of Asia. Next, we find a generally high pressure to the south of the United States, on the Gulf of Mexico, as well as on the western highlands and plateaus of the continent, in lat. from 30° to 40° N. Probably, also, pressure is high to about 60° lat. N. on the eastern slope of the Rocky Mountains, where, the winter being cold, the Rocky Mountains in the west not permitting the air in the lower strata to flow towards the Pacific, and the depression about Iceland being far away, there exist all conditions for a high pressure. But barometrical observations from this region are wanting.
Thus, the Mississippi Valley and seaboard of the United States have in winter regions of high pressure to the S. and W. of them; i. e. they are exposed to the influence of winds from different directions, of which those that come from the S. are warm and laden with vapor, and thus able to sustain the precipitations necessary to the progress of storm-centres, while the air from the W. and N. W. is cold and dry.
A country generally level, subjected to such different influences, must have a
very variable climate, and this is known to be the case in the United States. Nowhere in the same latitudes are the variations of temperature and pressure so great and sudden as in the Mississippi Valley and in Texas. On the Atlantic sca-board the variation is somewhat less, owing to the slight protection afforded by the Appalachian Chain.
In summer again, there are no parts of North America which are as strongly heated as the interior parts of Asia, none also where pressure is as low, and thus there are no monsoons comparable in strength and constancy to the summer monsoons of Asia. Especially is this the case with the eastern part of the United States, where the land is so much pervaded by the influence of the sea that there is scarcely a summer depression of the barometer. The Gulf of Mexico is situated just in the latitudes where pressure would be lowest on a great continent, and, owing to the relative coolness of the air over great bodies of water, pressure is nearly as high over the Gulf in summer as in winter. Yet, as there is a rarefaction of the air in the interior and western part of North America, there is a monsoon wind drawn in from the Gulf of Mexico to supply the deficiency. The mean direction of the wind is southerly in summer over a great part of the United States east of the Rocky Mountains. It is more S. E. in Texas, and S. and even S. W. in the States north and northeast of it, partly due to the earth’s rotation, and partly also to the influence of the lower pressure in the lake regions on the air over the Gulf of Mexico. On the Atlantic coast the winds have some monsoon features (as was shown by Prof. Coffin in 1848) but still the flow of air is much more from the southwest than would be the case in a real monsoon region, the ocean being to the east.
If, aside from disturbing influences, we consider only the mean direction of the wind, the influence of the Gulf of Mexico is seen to be paramount over a large and important region of the United States, extending from the Mississippi to the Appalachian Chain and from 31° to 42° N. L. The mean direction of the wind is about W. S. W. at all seasons, with a ratio of resultant of about 30. The cause of this is, that pressure is highest at all seasons to the S. and lowest to the N. and N. E.
Having now considered the influence of the pressure of the air on the direction of the winds, the influence on force remains to be shown.
It is easy to conceive, that, the influence of pressure once acknowledged, this influence would be the greater, the nearer areas of high pressure approach areas of low pressure, or, in other words, the nearer any given difference of pressure was found to exist. It was to be supposed, that the more this was the case, the greater would be the velocity of the winds. This has been found to be really the case.
This difference of pressure relative to distance was called by Stevenson barometric gradient. This term of barometric gradient may be applied to the mean direction of the wind, and the rate of progress, as well as to any given single observation. The more the isobars are crowded together, the steeper is the gradient, and the greater will be the velocity of the wind, all other conditions being the same.
There are conditions well known to science in a general way, although not
measured with accuracy, which prevent all winds from reaching the same velocity even if the relative distance of the isobars be the same.
These conditions must be considered in brief.
In the lowest stratum the velocity is lessened on account of friction on the surface of the earth, while the higher are also more or less affected by the friction of the different strata on each other.
The winds on the ocean will be less affected in this way, because of the smooth surface of the water. The greater velocity of the wind on the sea is well known. The figures published in the “ Quarterly Weather Reports” of the Meteorological Office, of London, very clearly show the decrease of velocity in the interior of Great Britain even in level parts of the country.
The following table shows this for the United States. I give the mean velocity of the wind in'a group of inland stations (Eastern New York) compared with that of the sea-coast (Cape Cod and adjacent islands) and also with the summit of Mount Washington, the highest peak of the New England States.
Mount Washington having the freest position, the strength of the winds there must be considered as more nearly normal than at the other places. The N. W. winds are the strongest, both summer and winter. But in the vicinity of Cape Cod, the N. E. winds coming over the smoother surface of the sea, are the strongest.
It is safe to present the following rules for the velocity of the wind. It is greater: 1. On high isolated peaks, than at low stations.
2. On the seashore, and especially on isolated islands, than in the interior of continents.
3. In level countries than in countries surrounded by mountains.
These rules apply to the local positions only. But we may remark that it is possible to mention some regions where the velocity of the winds is greater, others where it is less, than the average over the whole earth.
To the latter belong the equatorial calm-belt, and the calm-belts at the polar limits of the trade-winds. It would be wrong to imagine that any point on the
1 One summer and two winters, 1870–71, and January, February, and December, 1872.
surface of the earth has perpetual calms.
The calm-belts themselves are not constant, but move in the different seasons, and besides, the calms are more or less frequently disturbed.
In the trade-winds belts also, notwithstanding calms are very rare, the velocity of the wind is probably less than the average of the globe.
Probably the part of the earth where the winds have the greatest velocity, is found between 40° and 60° Lat. S., where very strong westerly winds are prevailing the whole year. The cause of this is the great difference in the pressure of the air at a small distance, or in other words the steep barometric gradient.
The great difference of the mean velocity of the winds blowing over a region, and of the progress of the air in a certain direction, should be borne in mind. Where the winds are weak, but always from one direction, as in the trade-wind region, the total rate of progress measured in miles will be considerable, frequently greater than in regions where strong winds blow from different directions. It is even possible that the winds may be so counterbalanced by one another, that there will be no resultant direction, so that the definite result, as far as progress of the air is concerned, would be the same as if absolute calms had prevailed all the time.
So far as regions are considered, where the mean direction of the wind does not vary, or varies but slightly in the different seasons, the mean annual direction with rate of resultant, gives a tolerably fair idea of the character of winds in such regions.
It is quite different where regions with very great variations in the yearly direction of the wind are considered. Here the annual direction will give but a very imperfect idea of the character of the winds. This is the reason why, as far as possible, I have always placed at least two contrasting seasons, summer and winter, in giving the percentages of the winds and the mean directions in the small tables which follow, and serve to illustrate the winds of different regions of the world. This is also the reason for constructing the two maps, Pl. 5 and 6. The same attention has been given to this subject by Prof. Coffin in his extensive tables arranged in Zones, in Series B of this work, the number of observations being given generally for the four seasons, sometimes even for each month. How far the consideration of the annual result alone would mislead, the following table will show:
It is seen from the foregoing that in Western Europe, on the Atlantic Ocean and in a certain region of the United States, the mean direction of the wind in the year, summer and winter, is between S. W. and W. S. W., and the difference between the two seasons very small. If the rate of annual resultant is not greater, it is because at all seasons there are many winds coming from other directions than the prevailing one.
In Pekin and Hakodade the mean annual direction is nearly the same as at the above named places, but the ratio of resultant is small for another reason: the winds of summer and winter being nearly opposite to one another, the resulting annual movement is small. Yet at each of the seasons the winds are very steady. The angle between the mean direction of the wind in winter and summer is 142° at Pekin, and 133o at Hakodade, or more than of a circle, and only from 30 to 20° at the above cited places of Europe and America. Again, the mean annual direction of the wind and ratio of resultant, in Southern India and Ceylon, are very similar to those observed in Europe, but the mean direction of winter and summer nearly opposite to one another, with an extremely great ratio at both seasons, there are conditions as dissimilar as possible to those of Western Europe.
In the pages which follow, the results to be drawn from the observations on the winds are considered by geographical divisions.