CLIMATE.
_Heat and Moisture_ are the main governing conditions of plant growth.
In a preceding chapter the relations of soils and plant growth to
water have been considered; in the present one the relations of both
moisture and heat to soils and plants will be discussed; and to do
this intelligibly to those not making a specialty of such studies, it
becomes necessary to introduce, first, a summary consideration of the
subject of climate.
Climatic conditions control, and to a great extent determine, the
industrial pursuits of every country; all the more so as the rapid
communication and transportation by means of modern appliances now
brings every part of the globe in competition with every other. The
question is not now what it may be intrinsically possible to do under
certain climatic and geographical conditions, but whether these things
can be done with a reasonable prospect of profit and commercial
success, in competition with other countries offering more or less
of similar possibilities. While it is true that the cost of labor
frequently enters most heavily into such problems, yet favorable or
unfavorable climatic or soil-conditions may in many cases turn the
scale. Thus the high price of labor and fuel on the Pacific coast of
the United States would at first blush seem to render competition
with Europe and the East in the production of beet sugar commercially
impossible; yet exceptionally favorable climatic and soil-conditions
concur to turn the scale in favor of California at least, so as to have
placed that state at the head of the sugar-producing states of the
Union. A general understanding of the climatic conditions which concern
the United States more or less directly, is therefore needful to an
appreciation of their agricultural possibilities.
_Climatic Conditions._—The factors usually mentioned as constituting
climate are temperature, rainfall and winds. Since the latter
two factors, however, are themselves merely the result of _heat_
conditions, it is proper to discuss from the outset the origin and mode
of action of heat.
TEMPERATURE.
The temperature of stellar space outside of the atmosphere is
known to be very low. The increasing cold as we ascend to greater
heights, is a fact familiar to all. Langley has calculated upon the
basis of observations made at the summit and foot of Mount Whitney
in California, that the temperature of space lies near 200° Cent.
(360° F.) below the freezing point of water; and this would be the
temperature near the Earth’s surface, were it not for the surrounding
atmosphere. The latter absorbs but a small amount of the sun’s direct
heat rays (which are of _high_ intensity), as they penetrate it to the
Earth’s surface. But as the earth’s surface is warmed, the heat rays of
_low_ intensity which it emits cannot pass back through the atmosphere
to the sun or to outer space; they are “trapped,” as it were, by the
dense air resting near the earth’s surface, which is then warmed partly
by the radiation from, partly by direct contact with, the soil. It is
to the existence of our atmosphere, then, that the possibility of our
animal and vegetable life in their present form is due; and a decrease
of the trapping effect on the sun’s heat rays makes itself quickly felt
when ascending, either in balloons or on high mountains. Moreover, it
is well known to mountain climbers that at great elevations the sun’s
heat is extremely intense at noon; even though the temperature may fall
to the freezing point at night, owing to the failure of the thin air
to prevent the radiation back into space of the heat absorbed during
the day. On the high plateaus of the Andes and of Asia, therefore, very
extreme climates prevail, on account of the great range of temperature
between day and night.
_Ascertainment and Presentation of Temperature-Conditions._—The proper
understanding of the temperature conditions of any locality or region
is by no means a simple matter. Shall we study the daily, monthly, or
annual changes of temperature, or the means deduced from either or all
of them, in order to gain a clear insight into the climatic conditions
that control crop production and health conditions?
_The Annual Mean Temperature not a Good Criterion._—Since one and
the same figure may result equally from the averaging of two widely
divergent data, and from such as are close together, it is clear that
the mean _annual_ temperature cannot be a proper criterion of the
agricultural adaptations of a country. Thus an average temperature of
60° F. might result, equally, from the averaging of 65 and 55 degrees,
or from taking the mean of 15 and 105 degrees; yet the respective
cultural adaptations would be widely different.
_Extremes of Temperature are Most Important._—It is, on the contrary,
rather the _extremes_ of temperature, more particularly of cold,
but frequently also of heat, together with the total amount of
heat available during the growing season, that determines such
adaptation so far as temperature is concerned; for no culture plant
can be successfully grown where the temperature during winter even
occasionally falls for more than a few hours below the point which
it can resist; and for each plant there is a certain aggregate
requirement of heat to carry it from germination to fruiting. Even
different varieties of one and the same plant differ materially in the
latter respect, so that it is very important that in the selection of
varieties to be grown, this factor should be taken into consideration.
It cannot be too strongly urged that the comparison of annual means of
temperature, so commonly made by promoters of colonization schemes,
must not be taken as a guide either in the estimate of cultures in
which the immigrant may desire to engage, or by those in search of a
climate adapted to their health-conditions.
_Seasonal and Monthly Means._—The statement of the mean temperatures of
the conventional four seasons—spring, summer, autumn and winter—afford
a much clearer conception of climatic adaptations; provided always
that the extreme temperatures be considered at the same time. With the
same understanding, the monthly means are still more instructive; but
here again, it is most essential that the distribution and amount of
rainfall in each be regarded at the same time, since the most desirable
temperature is of no avail without the moisture required for vegetation.
In some cases, _e. g._, that of California, it becomes necessary
for practical purposes to regard the “season,” and not the calendar
year, as the unit or reference for crop production. There the crops
depend upon what rainfall may occur from October to May, there being
no summer rains of agricultural significance, and outside of irrigated
lands, almost all vegetation save that of trees being in abeyance. In
India, there are two distinct growing seasons (“kharif” and “rabi”),
corresponding to the two “monsoon” seasons; and no matter how much rain
may fall during _one_, almost total failure may occur in other tropical
and arid sections of that country.
The _Daily Variations_ are of interest chiefly with respect to health
conditions, since most plants are more adaptable in this respect than
the average man.
RAINFALL.
_Distribution Most Important._—The summary statements of the annual
rainfall are almost equally as deceptive as are those of annual mean
temperature, since quite as much depends on the manner in which it
is distributed through the year, as upon its absolute amount; and
also upon the manner of its fall. Thus Central Montana has the same
aggregate annual rainfall as the country surrounding the Bay of San
Francisco, viz. about 24 inches; but while in the Franciscan climate
this amount of rain falls during one-half of the year, and that the
growing season, enabling crops to be grown without irrigation, in
Montana the rainfall is distributed over the entire season, so that
irrigation is absolutely essential for the successful production of
crops. This so much the more as, while the winter snowfall is very
light, the rains of summer are largely torrential, running off the
surface in muddy floods and giving little time for absorption into the
soil. Farther west, in Washington, where grain crops are largely grown
without irrigation, the sowing of winter grain is impracticable because
the dry summer is immediately followed by the very light snowfall of
winter, which falls on dry ground. Fall-sown grain would thus simply
lie dormant in the ground through the winter, with great liability
to injury from stress of weather in early spring, apart from the
depredations of birds and rodents. Hence grain is always sown there in
spring only.
These examples may suffice to show that summary statements of either
temperature or rainfall by yearly means are of little practical
interest to the farmer. What he needs to know is whether or not
sufficient rains to mature a full crop are likely to fall during the
time that the growing temperature prevails; and what are the minima and
maxima of temperature—heat and cold—that his crops will be called upon
to endure.
WINDS.
The third climatic factor mentioned, the winds, though proverbial for
their unreliability and inconstancy, are not only very incisive in
their action, but also to a considerable extent of very definite local
or regional occurrence and significance. Moreover, their occurrence,
direction, temperature and moisture-condition can, in regions whose
climatology has been reasonably well studied, be foretold with
sufficient accuracy to be of great use to the farmer.
_Heat the Cause of Winds._—As already stated, the primary cause of
all winds is heat, substantially on the principle according to which
draught is created in our domestic fires. The hot air rising creates an
indraught from all directions, especially from that which it can most
readily come; viz., from the ocean,[101] or from level lands, rather
than across mountain chains. Hence the sea-breeze in the after part
of the day, when the land has become heated; while the sea, requiring
a much larger amount of heat to change its temperature to a similar
extent, remains relatively cool. But at night the earth cools more
rapidly than the sea, by radiation; hence toward evening the sea-breeze
dies down, and toward and after sunset the land-breeze takes its place.
[101] A striking case in point is the regular wind which in summer
blows through the “Golden Gate,” a gap in the Coast Range connecting
the Pacific Ocean directly with the great interior valley of
California, along the bays of San Francisco, San Pablo, and Suisun. The
great interior valley and adjacent mountain slopes becoming intensely
heated during the rainless summer, the ascending air is replaced by
a steady indraught from the sea, which is bordered by a belt of cold
water causing fogs along the coast. The fogs are quickly dispelled on
reaching the edge of the valley near the middle of its length; whence
steady breezes blow northward and southward, up the valleys of the
Sacramento and San Joaquin respectively. These winds, popularly often,
but erroneously, called trade-winds, are really “monsoons” similar in
their origin to those of India, which, when coming from the sea cause
rains, but when from the heated land itself are hot and dry; as in the
case of the sirocco of Italy and North Africa, the terral of Spain and
the northers of California.
The principle of this local change of winds, together with the rotation
of the earth, the absorption of moisture by air, and the fact that the
latter becomes cooler when it expands on rising and warmer when it is
compressed by descending, serves to explain all the major phenomena
usually observed in connection with winds. The air of the equatorial
belt, heated throughout the year, necessarily rises and creates an
indraught from both north and south; but since the air thus flowing in
has a lower rotary velocity than the earth’s surface at the Equator,
it lags behind and so gives rise to northeast and southeast winds,
respectively, between the two tropics and the equatorial belt. These
regular winds, from the aid they give to commerce in passing from
continent to continent, are known as the _trade winds_. On the other
hand, the air that has risen from the hot equatorial belt, cooling by
expansion as it rises and flowing northward and southward from the
Equator, on descending as it mainly does into the temperate zones, has
a higher rotary velocity than the land-surface and so tends to give
rise to southwest and northwest winds in the northern and southern
hemispheres respectively. At sea, on coasts and in level inland regions
to windward of mountain chains, such winds are often quite regular
during a portion of the year.
_Cyclones._—But local disturbances arising from heated land areas or
mountain slopes, as well as wide atmospheric changes whose causes are
not fully understood, give rise to waves of alternating high and low
barometric pressure, largely converting rectilinear or slightly curved
wind-motion into whirls or “cyclones”[102] ranging from a thousand to
over two thousand miles in diameter. These in the case of low-pressure
waves or centers, _toward_ which the air flows from the _outside_,
revolve in the direction contrary to the movement of the hands of a
clock, and commonly produce rain in their east portion. A high-pressure
wave or center, _from_ which the air naturally flows toward the
_outside_, will usually bring about an “anti-cyclone” area with fair,
and in winter cold (“blizzard”) weather, the direction of the whirl
being, in this case, the reverse, or in the same direction as the hands
of a clock. Both cyclones and anti-cyclones move in North America from
west to east, mostly entering from the Pacific Ocean off the northwest
coast and traversing the continent with a slight southeast (or in the
case of cold weather almost south) trend, with a velocity of twenty
to thirty miles an hour; until upon reaching the region of the Great
Lakes they generally turn northeastward and pass into the Atlantic
Ocean from the New England and Canada coasts.—It is upon these general
facts, roughly outlined here, that the weather forecasts are in the
main based; taking into consideration, of course, the local or regional
conditions, topography, etc., which modify the application of the
general rules.
[102] This designation is popularly and incorrectly applied to the
comparatively limited, but very violent and destructive rotary storms
or whirlwinds which originate locally on the heated plains of the
Middle West of the United States, and are almost always accompanied
by violent electric phenomena. These should properly be called
_tornadoes_. At sea such whirlwinds give rise to waterspouts, in
deserts to sand storms.
In the southern hemisphere, the air-movements substantially correspond
to those observed in the northern, so far as not modified by mountain
chains; as is especially the case in South America.
INFLUENCE OF TOPOGRAPHY.
_Rains to Windward of Mountain Chains._—The surface features or
topography of the regions traversed by the air currents or winds may
materially modify both their direction and their physical condition,
especially as to moisture and temperature. Mountain chains may deflect
them, or, causing the air currents to rise on their slopes, and thus to
cool by expansion, the moisture these bring with them from the sea may
be partially, or sometimes almost wholly, deposited in the form of rain
or snow; chiefly on the windward slopes. Then, continuing across the
range, the air deprived of most of its moisture cannot readily yield
up more; hence the scarcity of rain—“arid climate”—under the lee of
mountain chains; as in the Great Basin between the Sierra Nevada and
Cascade ranges on the one hand and the Rocky Mountains on the other,
and also on the Great Plains under the lee of the latter. The abundant
rainfall between the Mississippi river and the Atlantic coast is due to
the moist winds coming from the warm waters of the Gulf of Mexico and
Caribbean sea, whose access is not interfered with by any cross-ranges
of mountains. But the Great Plains lying between the Mississippi and
the Rocky Mountains are not within the sweep of the Gulf winds, whose
trend is SW to NE; while they are equally out of reach of moisture
from the Pacific, all that having been successively deposited on the
intervening mountains; hence their deficient rainfall.
Northward of the temperate zone the rainfall generally decreases as we
approach the arctic regions; except where the influence of warm ocean
currents to windward creates comparatively local exceptions, as in the
case of Norway and Alaska.
[Illustration: FIG. 52.—Composite Curve showing distribution of
Rainfall in Europe, Africa and America projected on 100 Meridian W. L.]
_The general Distribution of Rainfall_ on the globe is well shown in
the annexed diagram, which is copied by permission of the author from
his treatise on the “Evolution of Climates,”[103] and represents the
mean deduced from data given in the Atlas of Meteorology by J. G.
Bartholomew. It is a composite curve derived from the consolidation
of four curves showing the distribution of rainfall, viz., on the
meridians of 20°E.L.; the west coasts of Europe and Africa; the 30th
meridian W.L., in the Atlantic Ocean; and the west coasts of North
and South America, projected on the plane of the 100th meridian W.L.
The latter curve corresponds with remarkable closeness to the mean
curve here given. “It is not intended that these curves should include
the rainfall upon meridians on which the distribution in belts is
interrupted by continental influences, and by the irregular oblique
belts of rain on the east coasts.” But it presents an admirable
generalization upon which, as a basis, the local disturbances may be
studied.
[103] “The Evolution of Climates”; by Marsden Manson, July, 1903; also
_Amer. Geologist_, Aug.-Oct. 1897.
It will be noted that the maximum of rainfall in the tropical rain-belt
lies several degrees to northward of the equator, owing doubtless to
the greater land area in the northern hemisphere. There is thus, on the
whole, a narrower belt of deficient rainfall or aridity between the
tropical and _northern_ temperate rain-belts, than in the _southern_
hemisphere. The southern temperate rain-belt touches only the extreme
ends of South America, Africa and New Zealand; elsewhere on the ocean
it has not been sufficiently observed as yet. The zones of rainfall and
aridity are, however, known to be subject to seasonal oscillations of
several degrees in latitude, owing to the obliquity of the plane of the
ecliptic, which shifts its position upon that of the equator.
_Ocean Currents._—Since water as a fluid is subject to the same
circulatory motions which cause winds, it is to be expected that
ocean currents should exist corresponding to those of the air, as
characterized in general above. But as water warms so much more slowly
than air, its circulation would be comparatively insensible were it
not for the effects produced by the air currents upon the surface of
the sea, combined, as in the case of the winds, with the effects of
the rotation of the earth. Without going into the details of the ocean
currents in the tropics, it may suffice to say that owing partly to the
moving and warming effects of winds, partly to the natural circulatory
motion of the water, two great warm currents flow from the tropics
northward, materially modifying what would otherwise be the climates of
the coasts they touch.
_The Gulf Stream._—The current most generally known is the Gulf Stream,
flowing partly from the Gulf of Mexico and the Caribbean Sea, partly
from outside of the same along the chain of the Lesser Antilles, along
the southeast coast of the United States (Florida, Georgia and South
Carolina); but owing to its greater rotational velocity it is soon,
like the winds of the same latitudes, deflected from a northward to a
NE. course, which carries it away from the American coast, to impart
some of its warmth, (probably mainly through the winds that blow over
it), to Great Britain and Ireland, Scandinavia, and Western Europe
generally; while the northern American coast is left to be bathed by
the icy polar current flowing from the Arctic through Baffin’s Bay,
which carries icebergs far to the south in the way of the transatlantic
traffic between the Eastern States and Europe, and causes a difference
in climate that is well exemplified in the comparison of the climate
of New York with that of Naples, both lying in the same latitude; and
similarly of the bleak coast of Labrador with that of Great Britain.
_The Japan Stream._—On the eastern Asiatic Coast, a warm current
originating in the Sunda seas, flows off the coasts of the Philippines
and of China and bathes the Japanese islands; hence it is known as
the Japanese Current, or Kuro-siro. It is partly this current which,
failing to pass into the Arctic through the shallow waters of Behring
strait, renders the coast climates of the northwest coast of America
so much milder and moister than is that in corresponding latitudes on
the east coasts of both continents. Alaska corresponds to Norway in
its moist, foggy and relatively mild coast climate; British Columbia,
Washington and Oregon participate in the benefits of the tempering
influence of the return current of the Kuro-siro. But as this return
(“Alaska”) current passes southward into the warmer seas off the
California coast, its influence is reversed; it becomes a cold current
in the warm waters of the Pacific, and the warm, moist air of the ocean
being carried by the westerly winds across this cold stream which flows
along the shore of California, in summer dense fogs are formed, which
render navigation difficult and produce a coast climate whose average
summer and winter temperatures (_e. g._ at San Francisco) may differ
by only a few degrees, viz., 15.5 and 13.0° C. (60 and 56° F.); so
that a change of clothing from season to season is hardly called for.
The Alaska Current leaves the immediate coast of California off Pt.
Conception near Santa Barbara, gradually losing itself southwestward,
but still tempering the tropical heat in the Hawaiian Islands. Hence
the coast climate is much warmer and less foggy in southern California;
but throughout the State in the interior valleys, screened from the
coast winds by the Coast ranges, the temperature in summer may rise
several degrees above 100° F. for days together; although, owing to the
dryness of the air, the heat is not oppressive.
_Contrasting Climates in N. W. America._—An even more striking
contrast, showing the effects of the warm ocean and air currents, when
intercepted by mountain chains, exists on the Pacific coast farther
northward, as already mentioned. In Oregon and Washington first the
low Coast ranges, and then the higher Cascade mountains, obstruct
the eastward progress of the westerly ocean winds. The result is a
very heavy rainfall to coastward of and within the Coast ranges, and
an almost equally heavy precipitation on the western slope of the
Cascades. Standing on the crest of the latter in summer, one may see to
westward a rolling sea of clouds, causing almost daily rains; while to
eastward the eye ranges over brown or whitish, dusty plains or rolling
lands, almost destitute of tree growth and quivering with heat, under a
deep blue sky untroubled by clouds for months.
A somewhat similar contrast is seen in the Hawaiian islands, which are
in the sweep of the subtropical northeast trade winds, and on their
windward (eastern) slopes have abundant rains; while on the leeward
slopes an almost arid climate prevails, calling for extended irrigation.
_Continental, Coast and Insular Climates._—From what has been said
above, the striking differences of climate caused by the position of
any region with reference to the sea or other large bodies of water
on the one hand, and to mountain chains on the other, can be readily
understood; provided of course that the direction of the winds and the
trend of the mountain chains be properly taken into consideration.
Western coasts in the temperate and subtropical regions will have a
relatively even, temperate and moist climate as compared with the
interior of continents, from which the tempering influence of the
sea is cut off by mountain chains. Where no such chains intervene
the coast climate may extend far inland. The latter case is that of
Europe, where the prevailing westerly winds, warmed by the Gulf Stream,
temper the climate as far east as the borders of Russia, and northward
to Norway; while to southward the warm waters of the Mediterranean and
Black seas temper both heat and cold in Spain, southern France, Italy
and the Mediterranean border generally. But to eastward, in Russia and
Siberia, the climate becomes “continental” to an extreme degree, with
very cold winters and very hot summers. The same is true of interior
North America, wherever the continental divide cuts off the tempering
influence of the westerly winds; Montana, the Dakotas and the Great
Plains states generally being examples. The climate of the Mississippi
valley, as stated before, is tempered by the winds blowing from the
Gulf of Mexico, but with occasional irruptions of the continental
climate (sometimes reaching as far east as the South Atlantic coast)
in the forms of cold “blizzards,” from which the coast climates of
the Pacific and of western Europe are practically free. The Atlantic
coast of North America (including the coast of the Gulf of Mexico),
moreover, not unfrequently suffers from the violent cyclonic storms
that originate in the Antilles and follow more or less the direction of
the Gulf Stream.
Islands, differing from continents mainly in their extent, and having
a relatively large proportion of coast, naturally have climates
controlled essentially by the surrounding ocean. The insular or oceanic
climates are therefore, as a rule, more temperate and even than are
those of the nearest mainland. It is often said that the climate of
western Europe is “insular”; and owing to its position under the lee of
the Gulf Stream, this is eminently true of Great Britain.
_Subtropic Arid Belts._—Where the surface features of the land in
relation to the ocean and prevailing winds do not interpose special
obstacles, we find to poleward of both tropics a climatic belt of
greater or less width, in which the annual, or at least the summer
rainfall is too small to maintain annual herbaceous vegetation
throughout the season, even when the temperature is favorable. These
two “arid” belts are best defined in Africa, where the northern one is
represented by the Sahara desert, lower Egypt and Arabia, while the
southern one is exemplified in the Kalahari desert, to northward of
the Cape of Good Hope. The northern belt is continued into Asia Minor,
Palestine, Syria and Persia, and is again manifest in northwestern
India; but to eastward is stopped by the influence of the great
Himalaya range. The plateau countries beyond, in Central Asia, are
extremely arid, largely by reason of their high elevation.
In Australia the southern arid belt is very strongly defined. In North
America, the arid belt is characteristically defined on the Pacific
Coast. It embraces all but the southernmost point of the peninsula of
Lower California, with about two-thirds of the State of California;
thence eastward across Sonora and Arizona to New Mexico and western
Texas. But here the influence of the mountain ranges and high plateaus
obscures the subtropical belt as such, the arid climate continuing,
east of the great Pacific ranges, through Nevada, Utah, Wyoming,
Montana, Idaho, and eastern Oregon and Washington nearly to the line of
British Columbia on the north, and with gradually decreasing aridity,
into Colorado, Kansas, Nebraska, and the Dakotas.
In South America the rainless seaward slopes of southern Peru and
northern Chile indicate the southern arid belt; but here, the great
chain of the Andes intervening, the dry pampas of Argentina, and the
Gran Chaco of southwestern Brazil, like the Nevada basin, though arid
would naturally be referred to the moisture-condensing influence of
the Andes chain, under the lee of which they lie. From this cause
the region of deficient rainfall, which on the western coast ends to
northward of Santiago de Chile, is east of the Andes continued much
farther poleward, as in North America; reaching into Patagonia.
_Utilization of the Arid Belts._—While, as already explained, the
distribution of the rainfall through the year is nearly as important
as its total amount, yet it is evident that even with the minimum
of twenty inches of total precipitation as the measure for crop
production, a very large proportion of the land of the arid region
cannot, even with the most elaborate system of water conservation,
be supplied with sufficient water for ordinary crops, and must be
otherwise utilized, mainly for pasture purposes. This is rendered
practicable to a much greater extent than might be expected, because
the rapid transition from the rainy to the permanent dry season cures
the standing herbage into hay, which affords good grazing during the
rainless season. Moreover, the use of drought-resistant, browsing
forage plants, both shrubs and trees, serves to supplement materially
any deficiency in the supply of “standing hay,” especially in case the
rains should toward the end be unduly delayed. The same is true of
the dried pods and seeds of native herbage, which in some cases (bur
clover, lupins, etc.,) afford highly nutritious additions to the leafy
forage.[104]
[104] See Rept. of the U. S. Commissioner of Agriculture for 1878, pp.
486-488; Bull. Nos. 16 and 42, Wyoming Expt. Station; Bull. No. 150
Calif. Expt. Station; Bull. No. 51, Nevada Expt. Station; South Dakota
Station Bulletins Nos. 40, 69, 70, 74; Kansas Expt. Station, Bulletin
No. 102; New Mexico Expt. Station, Bulletin No. 18; Montana Expt.
Station, Bulletin No. 30; and others.