Chapter XV: Permanent and Periodic Winds

The winds of the world are commonly classified as the permanent, the periodic and the nonperiodic, according to their genesis and character. Their chief features may be briefly outlined.

The most conspicuous and important aërial current on the globe is the permanent double vortex playing between the equator and the poles. The heated air of the equatorial belt, uplifted by expansion, overflows beneath the isothermal layer toward the north and south, thereby increasing the pressure in the higher latitudes sufficiently to generate a surface inflow along the earth, and thus maintaining a perpetual closed circulation which is felt all over the globe. The main features of this motion have been determined mathematically by Ferrel,[62] and summarized as follows:

“In the preceding part of this chapter it has been shown that,
if all parts of the atmosphere had the same temperature, there
would be a complete calm over all parts of the earth’s surface.
But that, in consequence of the difference of temperature
between the equatorial and polar regions of the globe, and
the consequent temperature gradient, there arise pressure
gradients and forces which give rise to and maintain a vertical
circulation of the atmosphere, with a motion of the air of
the upper strata of the atmosphere from the equator toward
the poles, and a counter current in the lower part from the
poles toward the equator, as represented by the arrows in the
following figure, and that this of course requires a gradual
settling down of the air from the higher to the lower strata in
the middle and higher latitudes and the reverse in the lower
latitudes. It has also been shown that in case the earth had
no rotation on its axis, this would be exclusively a vertical
circulation in the planes of the meridians without any east or
west components of motion in any part; but that, in consequence
of the deflecting forces arising from the earth’s rotation, the
atmosphere at the earth’s surface has also an east component
of motion in the middle and higher latitudes, and the reverse
in the lower latitudes, and that the velocities of the east
components increase with increase of elevation, so that at
great altitudes they become very much greater than those
at the earth’s surface; while those of the west components
decrease with increase of altitude up to a certain altitude,
where they vanish and change signs and become east velocities,
now increasing with increase of altitude to the top of the
atmosphere.

“It has been further shown that the deflecting forces arising
from the east components of motion of each hemisphere from the
earth’s surface to the top of the atmosphere, in the middle
and higher latitudes and of the upper part of the atmosphere
in the lower latitudes, drives the atmosphere from the polar
regions toward the equator, while those arising from the west
components of motion in the lower part of the atmosphere in
the lower latitudes, having a contrary effect, but small in
comparison with the other on account of the weakness of these
forces near the equator, tend to drive the air a little from
the equator toward the poles. There is, therefore, a depression
of the isobaric surfaces at all altitudes in the polar
regions, especially in the southern hemisphere, a much smaller
depression in the equatorial regions, and a bulging up of the
isobaric surfaces in the vicinity of the parallel of 30° in
the lower part of the atmosphere, the maximum being nearer the
equator as the altitude increases, as represented in Fig. 45,
but at high altitudes there is a minimum of barometric pressure
at the poles and a maximum at the equator.

“In the accompanying figure the solid arrows in the interior
part represent the resultant motions of the winds (longer
arrows indicating greater velocities), in case of an earth
with a homogeneous surface over both hemispheres, in which
the motions would be symmetrical in both and the same at all
longitudes, and the equatorial and tropical calm belts would
be situated at equal distances from each pole. The dotted
arrows indicate the strong, almost eastern motion of the air
at all latitudes at some high altitude, as that of the cirrus
clouds.

“The outline of the outer part of the figure represents an
isobaric surface high up where the bulging up near the parallel
of 30° disappears and the maximum pressure at the same altitude
is transferred to the equator. For lower altitudes the isobaric
surfaces have a bulging up at the parallel of 30°, and a slight
depression at and near the equator. The arrows in this part
represent the polar and equatorial components of motion, the
former above and the latter below, except near the earth’s
surface on the polar sides of the tropical calm-belts, where
there is a polar component of motion arising from the air’s
being pressed out from under the belt of high pressure. This,
perhaps, does not extend beyond the polar circles, beyond
which there can be little motion in any direction, except from
abnormal disturbances.

“For reasons given in §103, the actual mean position of the
equatorial and tropical calm-belts are not precisely as here
represented, but are all a little displaced toward the north
pole, and the polar depression of the isobaric surfaces is
greater in the southern than in the northern hemisphere.”

The conclusions from this approximate analysis are in the main supported by observation, except as modified by the heterogeneity of the earth’s surface. The sea-level distribution of barometric pressure between the equator and poles, as found by Ross’ long series of measurements, manifests a variation of about one inch of mercury, with maxima at about 30° of latitude, north and south, as required by Ferrel’s theory. As a further cause of the depression toward the poles, may be mentioned the greater speed of the permanent east wind with the consequent centrifugal lift in the atmosphere.

As to the general easterly direction of the winds at middle and higher latitudes, that is well known from observation of the motion of clouds and of the air near the earth. At the cirrus level the velocity in those latitudes is almost exactly eastward. But the flow in longitude, illustrated by the outer arrows in Fig. 45, has not been fully determined by observation. Moreover, as Ferrel himself showed, the unequal heating of continents and oceans sets up gradients in longitude, especially in the northern hemisphere, thus adding considerable disturbance to the general circulation. To this agency must be added also the latitudinal shifting of insolation, due to the annual march of the sun across the equator, entailing an oscillatory seasonal shift of the hot belt, and therefore of the twin-hemispheric cycle of the atmosphere.

Some currents of the general and permanent circulation are sufficiently prominent to have special names, such as the _trade-winds_, the _antitrade-winds_, the _prevailing westerlies_, and, in the lower latitudes, the _calm belts_, where the flow is exceptionally feeble. All these currents have been known to sailors since early times, and have been of considerable importance in marine navigation. Eventually, perhaps, they may be of like importance in aërial navigation.

The trade-winds are mild tropical surface currents of remarkably steady speed and direction. Springing from the high-pressure belts in either hemisphere, at about latitude 30°, they blow toward the equator with increasing westerly trend. As shown in charts 46 and 47 for midwinter and midsummer, the trade winds cover a large portion of the tropical zones in both oceans, and shift slightly in latitude with the sun. They are separated at the heat equator by the equatorial calm belts, or doldrums, and are bounded north and south respectively by the calms of Cancer and of Capricorn. Particularly interesting are the trade-winds blowing from Spain to the West Indies, which favored Columbus on his westward voyage, and which certain adventurous Germans have proposed using to duplicate that memorable voyage, in air ships.

The antitrade-winds, or counter trades, are lofty winds blowing over and contrary to the trade winds. As some doubt regarding the direction of these counter trades had existed, an expedition was sent in 1905, by two distinguished meteorologists, Teisserenc de Bort of France, and A. Lawrence Rotch of America, to explore the atmosphere above the tropical Atlantic. Mr. Rotch has summarized their measurements and conclusions as follows:[63]

“Pilot balloons, dispatched from the island of Teneriffe and
St. Vincent, were observed with theodolites at the ends of a
base-line, and in this way the heights at which the balloons
changed direction could be ascertained. Later the balloons
were sent up from the yacht itself, which steamed after them,
measurements being made of their angular elevation. The
observations which are plotted in Fig. 46 prove conclusively
the existence of the upper counter-trade. The courses of
the balloons are represented as if projected upon the
surface of the sea and show that the northeast trade-wind
extended only to the height of 3,200 or 4,000 meters, and
then gradually turned into a southerly current which, higher
up, came from the southwest. The width of the dotted band
represents approximately the varying velocity of the trade and
counter-trade. Similar proofs of the northwest trade-wind,
south of the equator were obtained by the same expedition
during the following year, but the above suffices to show that
it would be possible for an aëronaut in the ordinary balloon
to start from the African coast, or from some of the islands
in the trade-wind region, and, after drifting towards the
southwest, to rise a few miles into the current, which would
carry the balloon north and eventually northeast back to land.
Nevertheless, it does happen in certain atmospheric situations
over the tropical north Atlantic that the winds from the
general northwesterly direction prevail up to great heights
without any evidence of the return-trade. Near the equator the
winds are easterly up to the greatest heights which have been
attained.”

The prevailing westerlies are high-latitude surface winds of the permanent circulation. In the southern hemisphere they are particularly strong and steady owing to the comparatively unbroken stretch of ocean. In the north also they are strong and persistent, but variable in direction because of disturbances by local winds due to unequal heating of tracts of land and sea. These features are well illustrated in charts 47 and 48. Of particular interest in aëronautics is the prevailing wind blowing from the United States to Europe, which has been considered a suitable current for transoceanic balloon voyages.[64]

The periodic winds are those whose gradient alternates annually or daily, due to annual or daily fluctuations of temperature on sloping or on heterogeneous parts of the globe. The annually fluctuating winds due to alternate heating and cooling of continents, or large land areas, bear the general name of monsoon. Among diurnal winds the most prominent are the land-and-sea breezes, and the mountain-and-valley breezes. Both kinds are practically available in aëronautics; the monsoons for long-distance travel, the diurnal winds for local use.

The general motive cause is the same for all periodic winds. When any portion of the earth’s surface is periodically more heated above its normal temperature, or average for the year, than the neighboring region, the resulting abnormal temperature gradient causes a periodic surface wind tending toward the excessively heated place, and a counter wind above. That is, the cooler and heavier column of air sinking and uplifting the lighter, results in a lowering of the common center of gravity of the two columns of air, and thus furnishes the driving power of the wind. For example, an island or a peninsula may be considerably hotter by day and cooler by night than the surrounding water; a continent may be much hotter in summer and much colder in winter than the bordering ocean. Thus during the hot period a moist wind blows landward; during the cold period a dry wind blows seaward. If the land has vast and lofty slopes the uprush of air during the hot period and the downrush during the cool period may be very powerful. The currents so produced by the aggregate of local agencies, including the deviation caused by the earth’s rotation, combine with the general circulation of the atmosphere to form the actual wind of the place. Thus the periodic current may conspire with the general circulation, or oppose it; may intensify, weaken or obliterate it; may overmaster, reverse or mask it completely.

Of the various continental monsoons of the globe the most powerful spring from the annual flux and reflux of the atmosphere over the vast declivities and table-lands of Asia. Here the conditions are especially favorable. As the sun approaches Cancer, the burning deserts and high plateaus, combining their force with the draft on the mountain sides, generate a continental uprush that sucks in all the aërial currents of the surrounding seas, hurling them aloft to the isothermal layer whence they radiate as the four winds of heaven; for here at this season the planetary circulation is disrupted, obliterated or reversed, appearing merely as a perturbation of the monsoon at its height. In India the force is particularly effective. Along the north the Himalayas stretch 1,300 miles in latitude, with an average height of 18,000 feet and with sunburned areas on either side. North of this range are the lofty plateaus of Thibet and Cashmere, south of it the desert of Gobi and the borders of the Indian Ocean. Over this watery tract from beyond the equatorial line, from the isles of Oceanica and from the wintry plains of Australia, the air flows in with accumulated strength, sweeping the Bay of Bengal and the Arabian Sea in a continuous gale bearing up the mountain slopes incredible floods of water. Over the Arabian Sea in summer the gale is so steady and swift that no ordinary ship can force a passage from Bombay to the Gulf of Aden. Above the Bay of Bengal the moist south winds, converging between the coast and headlands, pour cloud laden up the Himalayan slopes, precipitating their whole vapor in prodigious torrents seldom seen elsewhere. Khasia at this season sustains a Noachian deluge, the rain at times falling nearly a yard deep in one day and night.[65] Quite appropriately, therefore, the summer monsoon over India, especially its component southwest wind from the Arabian Sea, and southerly wind from the Bengal Bay and farther east, is called the wet monsoon.

The winter monsoon of Asia, is the reverse of the summer one, both in direction of gradient and in physical character. It is a cold flood of air pouring from the frigid table-lands and wintry depths of the desert, down the mountains and valleys in continual overflow on all sides of the continent, and then far out over the sea, where it reascends to complete its long cycle. In its descent all moisture vanishes by heating, and no intensive temperature gradient occurs, as in summer, to accelerate its gently modulated tide. In India the winds from Cashmere and Thibet pour down the Himalayas toward the Arabian Sea a clear current of air which unites with the trade-wind, increasing its force, and forming the moderate winter monsoon of that region, or as it is commonly called, from its lack of moisture, the dry monsoon.

The kinematic character, and the extent of both summer and winter currents, are well portrayed in charts 47 and 48 for all the south and southeast of Asia. Across the islands of Japan, it will be observed, the winds blow in opposite directions summer and winter. In Siberia the monsoon winds trend along her great rivers and valleys, generally northward in the winter and the reverse in the summer, combining in both seasons with the prevailing westerlies, due to the rotation of the earth.

All the other continents have their monsoons, though less powerful than those of Asia. In the great desert of Sahara, for example, there is an ascending hot current in the summer, causing a strong indraught from the Atlantic and the Mediterranean; but this is far less intense than if its action were fortified by lofty slopes and table-lands. In winter when the Sahara cools to nearly the oceanic temperature, little monsoon effect is perceptible, and the general circulation continues unperturbed. In Australia the monsoon influence is still feebler, owing to the limited extent of the country and to the general lowness and flatness of the land. Over parts of South America, the annual ebb and flow of the atmosphere is considerable, particularly along the northeastern coast, and in the whole Amazon Valley, whose aërial currents in general conspire with the trade-winds, strengthening them materially in the southern summer, though it is less in winter when the continental temperature more nearly approximates that of the ocean. The monsoons of North America have been described in some detail by Ferrel as follows:

“On the continent of North America we have monsoon influences
similar to those of Asia, but not nearly so strong, because
the extent of the continent, and consequently the annual range
of temperature, are not so great. They are, for the most part,
not sufficiently strong to completely overcome and reverse the
current of the general circulation of the atmosphere, and so
to produce a real monsoon, but they cause great differences
between the prevailing directions of the winter and summer
winds.

“In the summer the whole interior of the continent becomes
heated up to a temperature much above that of the oceans on the
same latitudes on each side—indeed, above that of the Gulf of
Mexico and the Pacific Ocean on its southern and southwestern
borders. The consequence is that the air over the interior of
the continent becomes more rare than over the oceans, rises
up and flows out in all directions above while the barometric
pressure is diminished, and the air from all sides, from the
Atlantic on the east to Pacific Ocean on the west, the Gulf of
Mexico on the south, and the polar sea on the north, flows in
below to supply its place. On the east the tendency to flow in
is not strong enough to counteract the general easterly motion
of the air at the earth’s surface in the middle latitudes, and
to cause a westerly current, but it simply retards the general
easterly current and gives rise to a greater prevalence of
easterly winds along the Atlantic sea-coast during the summer
season....

“In winter the thermal conditions over the continent are
reversed. The interior of the continent is now the coldest
part, and it is especially colder than the surrounding oceans
at that season. It has also very high plateaus and mountain
ranges. The air, therefore, of the lower strata, and especially
those next the earth’s surface, now tends to flow out in all
directions to the warmer oceans and the Gulf of Mexico, and
especially to run down the long slope of plateau from the
Rocky Mountains into the Mississippi Valley. The effect over
the whole of the United States east of the Rocky Mountains
is to cause the winds, which otherwise would be westerly and
southwesterly, to become generally northwesterly winds, instead
of southerly and southwesterly ones, as in summer. There is not
a complete monsoon effect, but simply a great change between
summer and winter in the prevailing directions of the winds. In
Texas, however, and farther east along the northern border of
the Gulf, the effect is somewhat that of a complete monsoon. In
New England and farther south in the Eastern States the monsoon
effect is to cause the prevailing winds to be from some point
north of west, instead of south of west as in summer.

“In summer, Central America and Mexico have a much higher
temperature than that of the adjacent tropical sea on the
southwest, and having high mountain ranges and elevated
plateaus, there is consequently a strong tendency to draw in
air from the southwest at this season, which not only entirely
counteracts the regular trade-winds of these latitudes, but
even reverses them and causes southwest winds. The effect is to
cause in midsummer a large area here, extending far westward,
of calms and irregular and light winds, mostly southwesterly
ones, and an apparent widening of the equatorial calm-belt at
this season so as to make its northern limit reach up, along
the coast, nearly to the parallel of 20°. The effect is similar
to that in the Atlantic west of the Gulf of Guinea and Liberia,
except that it here appears to be some greater, and causes
a true monsoon effect, since during the winter the regular
northeasterly trade-winds prevail, but strengthened by the
reverse thermal conditions of the winter season. On the eastern
side, and over the western end of the Gulf of Mexico, there is
a somewhat regular monsoon effect, the prevailing winds being
easterly, or blowing toward the land, during the summer, and
the reverse in winter.

“Along the west coast of North America in the middle latitudes
there is a strong monsoon influence; for the interior of the
continent becomes heated in summer to a much higher temperature
than that of the southwesterly ocean, and hence a strong
current is drawn in from this direction, at right angles to
the general trend of the coast which, combining with the
general southwesterly winds of these latitudes in the general
circulation of the atmosphere, causes the strong and steady
westerly and southwesterly winds of this region during the
summer. Farther north, up toward Alaska, the summer monsoon
effect is combined with the current caused by the deflection
of the continent as well as the general easterly current of
high latitudes, so that the winds here are generally southerly,
but still have somewhat of a monsoon character, being southerly
and southwesterly in summer and easterly and southeasterly
during the winter.

“Along the northern coast of America, as along that of Siberia,
the monsoon tendency is to draw the air from the colder land
to the warmer ocean in winter, and the reverse in summer; and
these effects, combined with the general easterly motion of
the atmosphere in these latitudes, gives rise to prevailing
southwesterly winds in winter and northwesterly ones in summer.
The winter monsoon influence, however, is small here—much
more so than in Siberia, for the ocean contains so many large
islands that it has rather a continental than an oceanic winter
temperature; and besides, it has not the influence of a warm
current—such as the continuation of a part of the Gulf Stream
along the northern coast of Europe and Asia.”

Similar to the monsoons in essential nature are the diurnal winds of seacoast and mountain side. They begin with the heating of the land in the morning, attain their maximum intensity about mid afternoon, or during the hottest of the day, and finally are reversed at night. Besides being so much briefer than monsoons, they are also in general feebler and less extensive. They may be quite noticeable on calm days, especially in clear weather and in hot climates; but usually they are masked or entirely overwhelmed where other marked currents occur—currents due either to the general circulation or monsoons, or other powerful disturbing agencies.

In land-and-sea breezes, which usually extend not far inland, there is a surface inflow of sea air during the forenoon and early afternoon, balanced by an outflow of warm air above, rising from the heated soil. After sundown this is reversed, the chilled air from inland pouring out to sea, while overhead the warmer sea air is forced landward at a higher level. These currents are strongest where the diurnal range of temperature is greatest and where the local topography is of suitable configuration. Particularly favorable are steeply declining shores, narrow bays and inlets, girded by mountains or lofty hills. During the day heated air ascends such declivities with alacrity, like smoke through an inclined flue, while at night, when cooled by radiation and contact with the soil, it rushes torrentlike down the valleys and hillsides, passing out to sea, often in sudden squalls that embarrass, or endanger, small sailing craft. Circulatory currents like the above have sometimes been used by aëronauts to carry them out to sea and back again to land at a different level.

In like manner the mountain-and-valley winds may be used by the skillful aëronaut. It is well known that these flow up the courses of rivers, cañons and land slopes generally by day, but at night reverse their course and pour down again with considerable force. For this reason experienced hunters place their camp fires below tent in a sloping valley. The strength of the breeze depends, of course, upon the daily range of temperature, and the steepness and expanse of the slope. Such winds are deftly used by the masters of soaring flight, the great robber and scavenger birds, and no doubt may be used by men in motorless aëroplanes, to gain elevation, and journey great distances without expenditure of energy.