HOW CLIMATE IS MODIFIED AND CONTROLLED
If the surface of the earth were all land, and the axis of the
earth’s rotation were perpendicular to the plane of the earth’s
orbit, the day and the night would be equal everywhere, and there
would be no seasons. There would be no wind, for the friction of the
air against the rotating earth would soon cause all levels of the
atmosphere to take up the exact easterly velocity of the solid body
below. The atmosphere would be contracted by cold and drawn downward
so as to have less depth at the poles than at any place having
latitude, and it would be deepest at the equator, where the direct
rays of the sun would expand it to an altitude of probably twice
what it could have at the poles. Centrifugal force—the force that
causes mud to fly off the rim of a swiftly rotating wagon wheel—would
further lower the height of the atmosphere at the poles and cause
it still more to extend outward at the equator. The atmosphere
would soon adjust itself to these constant conditions and forces and
thereafter remain at rest relative to the earth. There would be no
life, for there would be no vaporous atmosphere if the surface all
were without water. There would be extremely little heat to disturb
the atmosphere with motion, for the dry gases of the atmosphere are
practically diathermanous, and the heat of the sun would pass out
by radiation from the burnt and parched surface of the earth during
daytime without imparting more than a minute fraction of its energy
to the atmosphere; and at night the thin surface of the top soil that
had been heated to a furnace temperature during sunshine would be
quickly locked in the fastnesses of intense cold—probably 200° below
zero.
If we now incline the axis of our imaginary earth 23½°, we shall
introduce seasons whose only change, the one from the other, will be
in the duration of sunlight, as there is no water vapor to absorb
and utilize the sun’s rays in the initiation of motion and the
creation of storms. We are assuming that there would be enough heat
absorbed to prevent the atmosphere from liquefying, which it would
do at any temperature lower than 312° below zero. If the temperature
were to fall below the liquefying point of air, we would have the
singular phenomenon of the air expanding to a gas during daylight and
condensing to a liquid during nighttime, and, of course, that would
mean motion and winds, but of such a nature that one would hardly
dare speculate as to their peculiarities.
We introduce these hypothetical cases for the purpose of conveying a
clearer idea of the overlapping of conditions and the combinations of
forces that influence and control the seasons, the climate, and the
weather of the earth.
If the surface of the earth were all water and its axis perpendicular
to the plane of its orbit, the day and the night would everywhere
be equal and there would be no seasons. With a water surface there
would be an atmosphere nearly if not quite saturated with vapor of
water, in other words, of practically one hundred per cent. relative
humidity. It is doubtful if either animal or vegetable life could
exist; the first would die of internal heat, because a saturated air
would permit of no cooling by evaporation from the pores of the skin,
or from the tongue and mouth of animals that do not perspire; and
the second could not grow without the chemical action of sunshine,
which is a necessary part of the laboratory of the leaf of every
growing plant, the sunshine acting upon the green granular matter
which constitutes the chlorophyll of the plant. There would be little
difference between the temperature of day and of night—probably not
more than one degree. As the earth would everywhere and at all times
be covered with a deep stratum of cloud there would be little loss of
heat to space by radiation and the temperature would be excessive,
rising in the tropics to near the boiling point. We will assume that
the atmosphere would reach a stable and unchanged condition of great
heat and humidity and be without motion or precipitation.
If now we incline the axis of this water-covered earth and introduce
the complication of seasons, we shall not only have variation in the
hours of sunshine, increasing as we go from the equator toward the
poles, but, the capacity of air for moisture being less and less with
falling temperature, we shall have downpours of rain as the summer
slowly merges into fall and the latter into winter. Although the air
will be saturated, there probably will be no rainfall from the time
when the temperature begins to rise after midwinter until it reaches
and passes the maximum heat of summer. It is fair to assume that
during the rainy period there will be cyclonic storms with torrential
precipitation, and that the anti-cyclones that are a necessary
concomitant of cyclones, while they may cause a temporary cessation
of precipitation in the area that they cover, by the dynamic heating
of the air in their downward motions, will be ineffective in fully
clearing away the clouds from a water-covered earth. It is doubtful
if such an earth would be suitable for life,—certainly not for man.
=The Real Earth of Land, Water, and Inclined Axis.= The different
manner in which land and water surfaces absorb, radiate, and reflect
the heat from the sun has a profound influence on climate, which also
is modified by latitude, elevation above sea level, elevation above
a valley or above a surrounding plane, direction of wind, height and
trend of direction of hills and mountains, the position of lakes and
inland seas, the relative position and magnitude of continents and
oceans, storm tracks, and ocean currents.
=Influence of Continents and Oceans on Climate.= Charts 1 and 2
(pages 99 and 100), constructed from observations taken on ships and
on land, for a long series of years, show certain Highs and Lows
of vast extent, sometimes called “Centers of Action”, because they
do not travel across continents and oceans, as do the migrating
Highs and Lows that cause weather. Rather do they slowly reverse
their relative positions between winter and summer. Continents
cool by radiation in winter more rapidly than do oceans; the air
contracts, settles down and grows denser and air flows in at the top
from the oceans and outward at the surface of the earth toward the
oceans; thus is built up the winter Highs, or centers of action, on
continents. Continents heat up by absorption in summer more rapidly
than do oceans; the air expands, rises, and flows away in the upper
levels to oceans and flows in at the bottom from the oceans; and
thus are the Lows, or centers of action, established on continents
in summer. It is apparent that these processes must be reversed for
the oceans, and that the Highs will be found there in the summer and
the Lows in the winter. Carefully follow the illustrations of these
principles by examining the whole region north of the equator on
Charts 1 and 2.
In the Southern Hemisphere there is not such a pronounced shifting
of the Highs and the Lows from oceans to continents and back again,
with change in the seasons, as there is in the Northern Hemisphere,
because of the small area of land in comparison with that of water;
but in the midst of the southern summer, which occurs in January
(Chart 2), Lows are shown over South America, Africa, and Australia.
Note how the winds blow out of all the Highs and into all the Lows.
Also observe that the winds generally blow from about latitude 30°
north and south towards the equator, due to the great heat of the
tropics, which causes the air to rise and in the high levels to flow
northward and southward, settling down to the earth again through
the belts of high pressure that irregularly encircle the earth at
latitudes 30° north and south.
In the interior of continents the temperature falls lower at night
and rises higher during the day, and falls lower in winter and
rises higher in summer than on any of their coasts. On the coast of
central California, for instance, the ocean is so cool in summer
and the winds blow so steadily from it that the thermometer ranges
between 55° and 70°, even when there are temperatures of over 100°
but a few hundred miles away in the great interior valleys, or on
the broad plateaus of the mountains. New York and Boston, in nearly
the same latitude, also have their summer temperatures modified by
ocean influence, but they are on the east side of a broad continent,
where the prevailing westerly winds give to them more the character
of a continental climate than one marine; but occasionally the east
wind, for a short time, gives to them the modifying influence of
the ocean. In the winter the influence of the oceans is to modify the
extremes of cold, the same as they do the excessive heat of summer.
[Illustration: CHART 12.—AVERAGE MAXIMUM TEMPERATURE FOR JULY
(Henry).]
Chart 8 (page 129) showing the lowest temperatures ever recorded at
Weather Bureau stations, and Chart 12, presenting the average of
the highest daily temperatures of July, graphically show, clearer
than any text can describe, the influence of continents and oceans
on climate. On the Atlantic the average maximum of day varies from
70° on the Maine coast to 85° on the coast of North Carolina; while
on the Pacific, where the marine influence is stronger, the average
is from 65° on the Washington coast to 80° on the coast of southern
California. But near the center of the United States where the
continental influence predominates, the average of the highest daily
temperatures varies from 85° to 90°. On Chart 8, showing the lowest
temperatures, the line of 20° below zero passes through Boston,
southwest to Chattanooga, west to Flagstaff, Arizona, and then
irregularly north to Seattle, showing the influence of both oceans in
carrying the line northward.
[Illustration: CHART 13.—OCEAN CURRENTS.]
Because of the vast extent of the Eurasian (Europe and Asia)
continent the difference between continental and marine climates is
more marked than in the Western Hemisphere. Huntington and Cushing,
in their splendid work on “Principles of Human Geography”,[3] make
a comparison between the southern Lofoten Islands, off the coast
of Norway, and Verkhoyansk in Siberia, which probably furnish the
greatest contrast to be found anywhere between places of the same
latitude. Although both are inside the Arctic Circle, the influence
of the Atlantic Ocean with its warm-water currents coming all the way
from the tropics (Chart 13) protects the Lofoten Islands from the
extreme cold that otherwise would come to them; vegetation remains
green and cattle are pastured every month in the year. But the ocean
retains nearly the same temperature in summer as in winter, and
as a result the Islands are too cold to grow trees or many crops.
Verkhoyansk is so different that one can scarcely believe that both
places are in the same latitude. At the Siberian town the winter
temperature falls to 70° or 80° below zero every winter, and has
been known to register 90° below zero. It is said that steel skates
often will not “take hold” but slip sideways as well as forward on
the surface of the excessively cold ice. This doubtless is due to
the fact that under ordinary winter cold the weight of the skater
melts a thin film of water under the edge of the skate, which freezes
instantly when the skate passes and relieves the pressure. But here
the cold is so intense that the weight of no skater is sufficient
to lubricate his movements with water molecules. Remarkable to
relate, the summer at Verkhoyansk is warmer than in the islands
off the Norwegian coast, due to the rapidity with which the land
surface warms up under the action of the solar rays in the midst of
a continental area remote from water, 75° to 80° frequently being
recorded during the long summer days. The ground never thaws for more
than a foot or so, but a number of crops are successfully grown.
In the interior of a continent like that of Siberia or of North
America not only the changes from season to season but from day to
night are extreme; while in mid-ocean the diurnal and the annual
range of temperature is small, day and night, winter and summer being
much the same. A place is influenced by the ocean in proportion to
its distance from the sea, the presence or the absence of hills or
mountains between the place and the water, and by the fact that the
prevailing winds come from or go to the ocean. Cities as far inland
as Baltimore and Philadelphia have their extremes of temperature
somewhat modified by the Atlantic Ocean, and if it were not for the
Coastal and the Sierra Nevada Mountains the influence of the Pacific
Ocean would be felt at least as far inland as Denver, and the great
Rocky Mountain plateau would be one of the garden plots of the world.
The influence of the Pacific would reach inland farther than now
does the Atlantic because of the prevailing westward drift of the
atmosphere in all middle latitudes.
=Exaggeration of the Forest Influence on Climate.= Chapter XIII,
on Change of Climate, shows more in detail the process whereby
the sun lifts up the water vapor from the Gulf of Mexico and the
Atlantic Ocean and how cyclonic storms draw this vaporous atmosphere
northwestward far into the interior of the continent, the Alleghany
Mountains not being high enough to offer serious obstruction.
The writer would again caution the reader not to be misled by any
pseudoscientist, no matter how worthy his purpose may be, who
would teach that the operations of men in changing forest areas
to cultivated fields, gardens, villages, and cities, has in the
slightest degree harmfully affected the climate, or augmented floods
or intensified droughts. A field of grass, of wheat, of corn; an
orchard of fruit; a highway bordered with towering, majestic oaks
and elms; or a grove of cultivated trees about a prosperous home
is just as beneficial to the climate as the thickest and most
impenetrable forest and far more pleasing to the eye and helpful to
mankind. Forests should be protected, conserved, and grown because we
need timber, not because a lot of foolish people are writing nonsense
about them.
=Influence of Lakes and Rivers.= With the exception of contributing
to the formation of occasional fogs over their surfaces and the
adjacent low lands, through the rising of warm water vapor into
the cold air that often collects at the bottom of valleys during
nighttime, rivers exercise little influence on climate. Lakes exert
a modifying influence on the temperature of places near their shores
but only for a few miles therefrom, and they are too small to exert
any appreciable influence on rainfall. If one examine charts showing
the average rainfall for the United States by seasons, he will
observe that the amount gradually shades off as the distance from
the Gulf or Ocean increases, without any relation whatever to the
five Great Lakes. Deserts exist on either side of the Caspian Sea,
although it slightly increases the rain of the Elburz Mountains to
the south. If these great bodies of water do not influence the
rainfall, how ridiculous to assume that the changing of forest areas
to other forms of vegetation possibly can affect precipitation or
influence droughts. Stress is laid on the fact that some land is left
bare and then is eroded into deep gullies. This is true, but the
fault is one that may be corrected by a proper system of plowing and
cultivation. And at most the area so eroded is so infinitesimal in
comparison to the vast regions changed from forests to growing crops
as to be negligible.
[Illustration: CHART 14.—MEAN ANNUAL ISOTHERMS (Buchan).]
=Influence of Ocean Currents on Climate.= Climates are markedly
influenced by the currents of oceans. Charts 15 and 16 show the
normal wind circulations of the globe; note that the centers of the
great swirls are coincident with the location of the High and the
Low centers of action located on Charts 1 and 2. Next observe Chart
13, showing the ocean currents, and it will be seen at once how
closely the circulation of the great ocean currents follows that of
the winds, due to the friction of the air upon the water, and to
the interposition of bodies of land that turn about or deflect the
currents.
[Illustration: CHART 15.—NORMAL WIND DIRECTION AND VELOCITY FOR
JANUARY AND FEBRUARY (Köppen).]
[Illustration: CHART 16.—NORMAL WIND DIRECTION AND VELOCITY FOR JULY
AND AUGUST (Köppen).]
Water has a greater capacity for heat than nearly any other
substance. It requires ten times the quantity of heat to raise a
pound of water one degree that it does a pound of iron. The
oceans therefore store up vast quantities of the heat of the sun
and, unlike the continents, distribute this heat northward and
southward without regard to latitude. Much of the heat of the tropics
is thus transported far northward and southward from the equator.
The extensive eddy-like circulation of the south half of the North
Atlantic Ocean sends currents northward along the coast of the United
States which set eastward at latitude 40°. A part of these reach
the coast of Spain and then turn south; the greater part spread out
in mid-ocean and move northeast, bathing the coasts of the British
Islands, Iceland, and Norway. They still retain some of the heat that
they absorbed from a tropical sun, and they therefore give to the
coasts that they reach a higher temperature than they would have if
the ocean waters were moving from the north, or than they would have
if there were no currents at all. On Chart 14 note how the isothermal
lines are carried northward by these currents as they cross the
Atlantic Ocean. The Gulf Stream mingles with these northeast currents
but adds little to their temperatures, for the general ocean
circulation would produce practically the same effects if there were
no Gulf Stream.
Follow the currents down the coast of Spain and of northeast Africa;
then note on Chart 14 the southward trend of the lines of equal
temperature, as the currents bring colder water southward to cool the
air. Next examine the currents of the Pacific and the isothermals.
The currents moving northward towards the equator along the west
coast of South America, and those moving southward, also toward the
equator, along the west coast of the United States and Mexico cause
a bulging of the isothermal lines from the positions that they would
occupy if there were no currents coming from colder regions.
=Influence of the Gulf Stream on Climate.= From either side of the
equator the surface winds (Charts 15 and 16) blow the water westward,
causing what are known as the “Equatorial Currents” (Chart 13).
The eastward projection of the coast of South America divides the
Atlantic equatorial current into two parts; one goes south along
the coast of South America and sets up the circulation in the South
Atlantic, which sweeps north along the southwest coast of Africa. The
other passes to the northwest, a part setting up the North Atlantic
circulation and the remainder sweeping through the Windward Islands
and storing itself in the Gulf of Mexico, whence it is driven out
at a velocity of some five miles per hour through the narrow channel
between Key West and Cuba. Here it has a depth of half a mile and a
width of forty miles. Its velocity is accelerated because it enters
the Gulf in a broad sweep and passes out through a constricted
channel. It retains its individuality as a warm river passing through
the ocean because of its greater velocity and higher temperature than
the waters in which it finds itself soon after it leaves the Gulf;
but it gradually merges with the great Atlantic circulation as it
passes to the middle of the ocean. It is the opinion of the writer
that its influence on climate has been exaggerated, that the warming
of Europe that is credited to the Gulf Stream is accomplished by
the mere presence of the ocean to the westward and to the general
circulation of that ocean without regard to the wonderful phenomenon
known as the Gulf Stream.
=Effect of Valleys on Day and Night Temperatures.= Valleys affect
temperatures in proportion to their depth and width. A deep, narrow
valley might have the effect illustrated by Figure 27, if the time
were summer and the sky clear. During the daytime radiation would
warm the interior so that the bottom of the valley would have a much
higher temperature than the free air at the top of the valley, and
the movement of the air would be sluggishly down the center and up
the sides of the depression. During nighttime all the conditions
would be reversed. Vegetation, losing heat by radiation much faster
than the air, would cool the latter as it came in contact with the
sides of the valley. The air would slowly descend along the sides
through gain in specific gravity and collect at the bottom with a
temperature much lower than it had when it started its descent.
[Illustration: FIGURE 27. Summer day temperature in a narrow valley.
Summer night temperature in the same valley.]
=Effect of Mountains on Climate.= The rarity of the atmosphere of
mountains readily allows the rays of the sun to pass through it and
thus the surface of mountains is quickly warmed, but the same
conditions permit a rapid radiation at night, so that there are
considerable extremes of temperature. Air cooled by contact with a
mountain may flow down its sides at night and collect in depressions
below, often causing frost on still nights where the temperature
higher up is much above freezing. Mountains may be more cloudy and
rainy than plains, for the currents of air that cross them must rise,
and in rising they cool by expansion and often reach the dew point
of the air, moisture being precipitated in the form of clouds, rain,
or snow. Often a peak is constantly capped with a crown of clouds.
Mountains may intercept vapor-bearing winds from oceans, force them
to such an elevation that their vapor is largely precipitated on the
windward side of the mountain, and receive them on the leeward side
as dry, rainless winds. Vast desert areas are often the result. A
good example is presented in the case of the Pacific coast mountains
and the desert plateau to the east.
Mountain peaks may be covered with snow, even though they be located
in the tropics, if their elevation be sufficient. This is because the
absorption of both incoming and outgoing radiation is so much greater
in the lower reaches of the atmosphere, where the water vapor is
densest. Wherever observations have been made they have shown that
the temperature of the air on high mountain peaks and crests and for
a distance of one to three hundred feet above them is cooler than
adjacent free air of the same height, due to upward deflection of air
currents and their cooling by expansion, and to radiation from the
peak.
The Himalayan Mountains exercise a profound effect on the climate of
Asia. The monsoon (any wind that alternates annually in direction
or force) of summer brings the moist air from the Bay of Bengal and
precipitates torrential rains from it as it ascends to higher and
higher elevations in passing over the great heights of the mountains.
At a place four thousand feet above the sea and not distant from
Calcutta, the annual rainfall is 466 inches, while the average for
most of the region east of the Mississippi River is only forty
inches. More than forty inches have been known to fall in one day in
the Himalayan Mountains. As in the case of all very high mountains,
the rainfall increases in these mountains up to a certain elevation
and then decreases. North of the mountains the monsoon passes into
the interior of Asia with withering dryness and vast deserts are the
result.
[Illustration: FIG. 28.—Average Monthly Temperature and Rainfall of
Typical Places in North America. (Huntington and Cushing.)]
Figure 28 graphically presents the average monthly temperature and
rainfall of typical places in North America, and Figure 29 of places
in the Old World. Here may be seen every phase of climate from
tropical to temperate and to cold, and from marine to continental.
By studying the winds on Charts 15 and 16 and the ocean currents on
Chart 13, the reader should be able to find an explanation for the
different conditions shown. For example: Mazatlan and Vera Cruz are
both on the coast of Mexico, the first on the west and the latter on
the east. Each has a rainy period in the summer, but at Vera Cruz the
rain begins earlier and lasts later and is much heavier. The reason
is that they both have north winds in winter (Charts 15 and 16), but
in summer Vera Cruz receives winds direct from the Gulf of Mexico and
at Mazatlan the winds continue to blow from the north, with but a
slight inclination landward. Again, the explanation for the fact that
Mazatlan has a monthly range of temperature from 60° in winter to 80°
in summer, while Vera Cruz has a range of only 70° to 80° is found in
the wind direction.
[Illustration: FIG. 29.—Average Monthly Temperatures and Rainfall
of Typical Places in the Old World. (From “Principles of Human
Geography.” John Wiley & Sons.)]
The City of Mexico is wonderfully favored by climate. Here a moderate
rainfall occurs from May to September. The oceans are not far distant
on either side, as distances are measured continentally, but its
great elevation on a table-land relieves it of the torrential rains
usual to the tropics; and yet it is close enough to marine influence
so that its air has not the nerve-irritating dryness of the
plateau of the Rocky Mountains, and it has a remarkable evenness of
temperature between winter and summer, with a monthly range between
50° and 60°. Its range between day and night is sufficient to be
stimulating.
Still looking at Figure 28, note the remarkable similarity between
the climate of Pittsburgh and Toronto. Each has about the same
rainfall and it is almost equally distributed throughout the months
of the year. The only difference is that Toronto is a little colder.
St. Paul and Kansas City, typical of the climate in the interior
cities, have a small amount of precipitation in winter, considerable
in summer, and a wide range of temperature; while the Pacific coast
cities have dry summers, and winters that vary from three inches of
rain at Los Angeles to fourteen inches at Astoria, with no excesses
in temperature.
=Temperatures Aloft in the Atmosphere.= Kite and balloon observations
have not been continued long enough, nor have they been made at
a sufficient number of places, to give one the data from which
the climate of any considerable altitude in the free air may be
determined, but from a large number of free balloon observations made
with self-recording instruments, in the middle latitudes of this and
foreign countries, Figure 1 (page 12) has been constructed, which
shows the manner in which the temperature decreases with elevation up
to eighteen kilometers (eleven miles). Note how rapidly it falls with
elevation up to eleven and a half kilometers (about seven miles).
This depth of air measures the thickness of the turbulent stratum in
which cyclones and anti-cyclones operate. At its top the temperature
always is about 64° below zero in winter and 70° below in summer.
And right here occurs a most wonderful phenomenon,—one of which
scientists were entirely ignorant less than two decades ago. At first
it was thought that there was something wrong with the recording
thermometers, for they failed to register falling temperature
with gaining altitude after the storm stratum was passed at seven
miles. Then it was noted that all instruments displayed the same
peculiarity, and the “Isothermal Stratum” (equally heated region)
was discovered, in which the temperature maintains the same degree
of intense cold so far as exploration had been made. From Mount
Weather, under the direction of the writer, a balloon was flown to
nineteen and one tenth miles before it exploded and sent a parachute
gently down to earth with its precious record. This flight showed
practically no change in temperature after the isothermal stratum was
reached. (See Chapter III.) One is reasonably safe in assuming that
there is no oxygen beyond an altitude of thirty miles and that at
fifty miles the nitrogen becomes inappreciable, and that, therefore,
the temperature must shade away to practically nothing when the
void of outer space is reached, notwithstanding the presence of the
newly-discovered isothermal stratum nearer the earth.