CLIMATE
_The Elements of Climate_
North America, embracing as it does essentially a quadrant of the
earth's surface, presents a variety of climatic conditions ranging from
those characteristic of the equatorial belt to those normal to polar
regions, as well as every gradation due to variations in elevation from
sea-level and even below that horizon in Death Valley, California, to
the summits of high plateaus and lofty mountains.
[Illustration: PLATE II.--Mean Annual Rainfall & Temperature.]
The principal elements of the weather which go to make up the conditions
of the atmosphere embraced in the broader term _climate_ are
temperature, precipitation, and the winds. On the accompanying map,
Plate II, the mean annual temperature of the continent is represented by
_isotherms_, or lines drawn through localities having the same average
temperature for the year. On the same map is also shown in blue the
average depth of precipitation, including both rains and melted snow. On
Fig. 24 lines are drawn through points having the same average
barometrical pressure (_isobars_) for the months of January and July,
together with arrows indicating the general direction of the surface
winds during each of these months, which may be considered as
representative of the summer and winter seasons. The data shown on these
maps have been compiled mainly from the reports of the weather bureaus
of Canada, the United States, and Mexico, and indicate, at least in a
general way, a summary of what is known concerning the main
meteorological elements which determine the climatic conditions in North
America. An examination of these maps will suggest certain general
conclusions in reference to the leading characteristics of the climate
in various portions of the continent and the changes they undergo
from season to season.
[Illustration: FIG. 24.--Average barometric pressure and direction of
wind for January and July.]
_Distribution of Heat and Light._--The distribution over the earth's
surface of the heat and light received from the sun is not only of
fundamental importance as respects climate, but furnishes a part of the
essential conditions on which depend the presence and distribution of
living organisms. The heat and light, or more accurately, the radiant
energy of the sun, the full significance of which is probably not
thoroughly understood, we term, for convenience, _insolation_. The
intensity and seasonal distribution of insolation are prime factors on
which many important results hinge.
Owing to the inclination of the axis about which the earth rotates (23 deg.
27') to the plane in which the earth travels about the sun, or the plane
of the ecliptic, the northern end of the axis is turned towards the sun
in summer and away from it in winter--that is, the axis of rotation of
the earth at all times is parallel to the same imaginary straight line.
As a result, the sun appears to migrate northward in the heavens during
the spring-time of each year, being vertical over the equator on March
21st, and to an observer in north latitude 23 deg. 27' rises higher and
higher each moon, until on June 21st it is vertically overhead; and then
returns southward. The latitude in which the sun is in the zenith at the
time of its greatest northward migration determines the position of an
imaginary line on the earth's surface, named the Tropic of Cancer. This
line, as shown on the accompanying maps, crosses the Bahama Islands,
passes about 40 miles to the northward of Havana, divides Mexico into
two approximately equal parts, and cuts the peninsula of Lower
California near its southern end. The portion of the continent to the
south of the Tropic of Cancer lies within the torrid zone.
When the sun is vertical over the equator, as it is about March 21st and
September 23d each year, its rays, not allowing for refraction, are
tangent to the earth's surface at the poles, and the hours of light and
darkness are equal the world over. During the winter season the sun
appears to migrate southward of the equator until December 21st,
when it is vertical at noon at all points situated in south latitude 23 deg.
27', which is termed the Tropic of Capricorn. Its rays are then tangent
to the earth's surface in the northern hemisphere in latitude 66 deg. 33',
which defines the position of the arctic circle. This imaginary line on
the earth's surface, as is indicated on the accompanying maps, crosses
Canada to the north of Hudson Bay, and passes through Alaska near where
the Porcupine River joins the Yukon. To the north of the arctic circle
lies the frigid zone. Between the torrid and frigid zones is situated
the temperate zone, within which is included about seven-eighths of
North America, exclusive of Greenland. The relation of the continent to
the three great zones of climate into which the northern hemisphere is
divided is thus most fortunate so far as man's activities are concerned.
The climatic zones just referred to, while based on precise astronomical
data and representing important facts, are not separated one from
another by tangible lines, and might easily pass undiscovered by one who
studied only the surface characteristics of the earth. Each summer a
wave of heat and light sweeps northward over the continent and reaches
beyond the pole; and each winter a counteracting wave of cold and
darkness moves southward, the influence of which is marked even well
within the torrid zone. A comparison of the isothermal lines drawn on
the map forming Plate II with the parallels of latitude
shows at a glance that there is only a general relationship between the
two. In order to understand this discrepancy between what might be
expected from astronomical considerations in reference to the
distribution of solar energy and the actual conditions as learned by
observation, it is necessary to take a more critical view of the manner
in which insolation is received by the continent, and also to consider
secondary conditions which exert far-reaching influences on its
distribution.
The amount of heat, or to avoid objections, the distribution of
insolation over North America, depends on three primary conditions:
First, the angle at which the sun's rays strike the earth, the range
being from zero to 90 deg.; second, the length of time a particular
locality is exposed to sunlight; and third, variations in the distance
of the earth from the sun. Each of these conditions varies from day to
day for every locality throughout the continent. The sun is highest in
the heavens in the torrid zone, being twice vertically overhead each
year at every locality, and the hours of light and darkness each day are
approximately equal throughout the year. North of the torrid zone,
however, the rays of the sun become more and more oblique to the earth's
surface, and hence insolation becomes weaker and weaker for a given
period of sunshine as one travels from south to north. But the hours of
sunlight each day undergo marked variations, lengthening from December
21st to June 21st, and shortening as the sun makes its southward
migration. At the north pole, as all know, there are six months of light
and six months of darkness each year. The amount of insolation reaching
the northern portion of the continent each day increases with the
lengthening of the hours of light, and during midsummer is greater for a
given area in a single day (twenty-four hours) than the amount received
by a similar area in the torrid zone. The almost magical springing into
life and bloom of the vegetation over the northern portion of the
continent with the lengthening of the hours of sunshine each summer is
thus explained. In the portion of the continent within the temperate
zone, more especially within the continental basin, the large number of
hours of sunshine during a summer's day is frequently accompanied by a
temperature as great as is usually experienced in the torrid zone. It is
the high summer temperature of this region, together with the lengthened
duration of sunshine in the growing season, that makes the Mississippi
basin and the adjacent region on the east and north so favourable for
agriculture when the requisite amount of moisture is present.
The distribution of heat over the earth's surface depends not only on
the direct influence of insolation, but on its transfer from one
locality to another through the agency of the winds and ocean currents.
The movements of the waters of the ocean, it will be remembered, are
largely under the control of the winds, so that the essential
factor in the transfer of heat from place to place is atmospheric
circulation. The primary causes of movements in the air, as is a matter
of current knowledge, are the differences that arise in temperature at
various localities. In regions where the air becomes more highly heated
than over adjacent areas it expands, and in consequence becomes lighter,
volume for volume, than the air over neighbouring areas, and is forced
upward and overflows aloft. The overflow or dispersion of the warmer and
lighter air above gives origin to a reduction in barometric pressure,
the column of mercury in a barometer being counterbalanced by the
pressure of the air above it. Briefly stated, the air near the earth's
surface flows towards regions of low, and away from regions of high
barometric pressure, and winds are established. The directions taken by
the winds are influenced or controlled in various ways.
_The Planetary Winds._--The great movements in the atmosphere originate
from differences in temperature between the warm equatorial and cold
polar regions. This alone would cause the cold air from either pole to
flow towards the equator as surface winds, and the warm air in the
equatorial belt to ascend and overflow aloft towards either pole. The
earth's rotation, however, influences the direction of these winds and
causes them to be deflected from the lines of longitude which they would
otherwise follow. In the northern hemisphere the air-currents are
deflected to the right and in the southern hemisphere to the left of
their initial directions. The best known examples of these planetary
winds, as they are termed, are the trade-winds, which blow from the
northeast in the northern and from the southeast in the southern
hemisphere. Between these two belts of converging winds lies the
equatorial belt of calm, some 300 miles wide, which also encircles the
earth and is termed the doldrums.
In the quadrant of the earth's surface occupied by North America the
climatic conditions are controlled in a large measure by the planetary
winds. In the equatorial belt of calms the barometric pressure is lower
than on either side, the temperature is uniformly high, the air
is heavily charged with moisture, and torrential rains are frequent. In
the belt of the northeast trades the weight of the air for a given area
is greater than in the doldrums, the wind blows with remarkable
uniformity both of direction and force, the sky is normally clear, and
rain infrequent except when the warm moist air is forced upward either
by local storms or on coming in contact with high land. The trade-winds
blow across the West Indies, Mexico, and much of Central America. To the
north of the trade-wind belt is a belt of prevailingly high barometrical
pressure, light variable winds, narrower and less well defined than the
doldrums, which encircles the earth in the region of the Tropic of
Cancer. This belt of calms, although familiar to sailors, to whom it is
known as the "horse latitudes," is ill-defined on the land, where its
presence is masked by changes due to local conditions. To the north of
the tropical calm belt the prevailing surface winds are from the
westward, and owe their direction to the constant flow of the upper
air-currents in their poleward journey, under the influence of the
earth's rotation. This great belt of winds from the westward crosses the
portion of North America including the United States and southern
Canada, but it is subject to many disturbances. The northern portion of
the continent extends into the little known polar region of prevailingly
low barometrical pressure, where midsummer and midwinter calms normally
prevail.
The great world-encircling currents of the atmosphere, namely, the
trade-winds, blowing towards the southwest or west across the Caribbean
and Mexican region, and the prevailing westerlies, or winds blowing in
an easterly direction, over the broad temperate portion of North
America, exert the main control on the climate of the continent.
_The Seasons._--Of primary importance to the inhabitants of North
America is the fact that the climatic belts determined by the
inclination of the earth's axis to the plane of the ecliptic are subject
to annual migration towards the north and south. In the torrid zone the
equatorial belt of calms, with its humid and oppressively hot
atmosphere, prevailing cloudiness, and heavy rains, and the belt
of the northeast trades, with its prevailingly clear skies and
refreshing breezes, do not occupy the same positions throughout the
year, but migrate with the sun. The migration of these two strongly
contrasted climatic belts brings to the otherwise remarkably uniform
conditions of the atmosphere over the West Indies, Central America, and
Mexico, two, in general well-defined, periods each year, namely, a wet
and a dry season, the former occurring in the summer and the latter in
the winter. It is to be borne in mind that between the tropics there
are, with certain local exceptions, but two seasons each year, the
leading contrasts of which are determined by differences in rainfall.
To the north of the Tropic of Cancer the seasonal changes are more
varied than in the torrid zone, and contrasts in temperature become the
most marked climatic feature; while precipitation, although in general
somewhat evenly distributed throughout the year, is more abundant in
winter than in summer. On account, however, of the greater diversity in
the climatic changes experienced each year within the temperate zone,
four seasons are recognised, the most distinctive features of which
depend on changes in both temperature and humidity.
In the northern portion of the temperate zone, and extending over the
arctic zone, the seasons are again reduced to two, summer and winter,
the contrasted conditions pertaining mainly to temperature and light.
A marked variation, which has an important bearing not only on climate,
but on the distribution of life encountered in passing from equatorial
to polar regions, is found in the distribution of light. Between the
tropics the number of hours of light and darkness each day is
approximately equal; in the temperate zone there is considerable
diversity from season to season, which increases with increase in
latitude; and uniformity, of a different character than at the far
south, again becomes prominent in the frigid zone, where the number of
hours of light each day is greatly prolonged during the summer and
correspondingly decreased during the winter. The extreme contrast
occurs in the neighbourhood of the pole, where during the summer season
the sun is continuously above, and in winter continuously below the
horizon, or in familiar language, there is a six-months day (light) and
a six-months night (darkness).
In going from the equatorial to north polar regions there is a general
decrease in mean annual temperature, and in general a decrease also in
precipitation, but great variations in these gradual changes, with
increase in latitude, occur which are both continental and local in
character. In winter the interior portions of the continent, and
especially the plateaus and mountains, are colder than the lands in
corresponding latitudes near the oceans; while in summer the reverse is
true, the margin of the continent being cooler than the broad interior.
In this general view of the climatic zones and the normal changes they
undergo we may note that the torrid zone is characterized by its
simplicity and monotony of climatic conditions, although disturbed at
times, especially in the West Indies, by occasional great cyclonic
storms, termed hurricanes, which occur, however, at quite definite
seasons. The temperate belt is equally well marked by its complex and
frequently changing atmospheric conditions, the winds being subject to
numerous and great variations, and storms of diverse character being
frequent. The frigid zone, again, is without conspicuous variations
except during the change from its monotonous summer to its still more
uniform winter weather, and the reverse change six months later. The
disturbances in the balance of atmospheric conditions at the far north,
or the storms, are of a much less varied character than in the fickle
temperate zone--thunder-storms and tornadoes, for example, being
unknown.
It is the summer migration of a heated belt from the south northward
across the temperate zone, and the equally conspicuous winter advances
of cold from the north southward across the same broad region, which
gives to the United States and the southern portion of Canada a
conspicuously changeable climate. The temperate zone, so far at
least as North America is concerned, deserves its name only when the
mean of the yearly changes in temperature is considered, as much of it
is hotter in summer than equal areas between the tropics, and in winter
over all of its northern half the cold is, at times, nearly or quite as
intense as during the same season in the far north. As a whole, the
portion of the continent embraced in the temperate zone is characterized
by its pronounced seasonal changes, including wide extremes of heat and
cold over large areas, and by its frequently sudden and strongly marked
weather changes during short periods of time. It is a highly suggestive
fact that of all the great climatic zones the one having the most
changeable climate, the greatest extremes of heat and cold, and the most
frequent storms should be the one in which man has reached the highest
development both of body and mind. Evidently it is the _struggle_ for
existence, when not too severe, which insures advancement. The part of
North America most densely inhabited by descendants of Europeans, and
the portion of the continent where intellectual development has made the
greatest advance, is the east-central portion, where not only the
variation of climate from season to season, but the weather changes from
week to week and day to day are the most conspicuous.
_Secondary Conditions influencing Climate._--While the primary
conditions controlling the climate of North America in common with all
other portions of the earth's surface depend on the relation of the
earth to the sun, there are many secondary conditions to be considered.
First in importance among these, so far as the broader features of the
climate of the continent are concerned, is the unequal heating of land
and water areas. During summer, more especially in the temperate zone,
the land becomes more highly heated than the adjacent oceans, and an
inflow of the cooler and moister air from the sea over the land occurs.
In winter the land cools more quickly and to a greater degree than the
adjacent waters, and the tendency of the heavier air over the land is to
flow outward as surface winds. Continental winds are thus generated,
similar in their origin to the familiar land and sea breezes of the
ocean shore in summer, but on a large scale, which have an
important bearing on the seasonal changes. The influence of the
continental winds is sufficiently well marked to give North America two
general classes of climate. One pertains to inland regions, is
characterized by great contrasts in temperature and humidity between
summer and winter, and is termed continental. The other pertains to the
border of the land where, on account of the equalizing influence of
large water borders, the contrast between the climate of summer and
winter is less pronounced, and has received the general title of oceanic
climate. The climate of the Dakotas, for example, is of the continental
type, while that of New Jersey is of the oceanic type.
The unequal heating and cooling of adjacent portions of land areas also
produces important atmospheric movements, as, for instance, when broad,
treeless plains become more highly heated in summer than adjacent
forested areas; or on account of rapid radiation become excessively cold
in winter and lower the temperature of the air above them. In the first
instance an inflow of cooler and heavier air from adjacent regions would
be established; and in the second example the chilled air would tend to
flow outward, thus, in each instance, establishing winds which usually
acquire a more or less well-pronounced circular motion. The Prairie
plains and the Great plateaus to the east of the Rocky Mountains become
highly heated in summer, and together with several other similar regions
in North America, meet the first of the conditions just considered;
while the higher portions of the Great plateau, especially at the north,
and the still more elevated mountains of Montana, Colorado, etc., become
excessively cold in winter and illustrate the other extreme.
Mountains serve to deflect the winds blowing against them either to one
side or upward, the former frequently producing important changes in
direction of the surface air-currents, and the latter, by causing the
air to rise, permits of its expansion and consequent cooling, thus
favouring precipitation. For this and other reasons precipitation
increases with elevation, at least until an altitude of many
thousands of feet is reached, and the mountains are cooler and more
humid than the adjacent valleys. The air-currents on passing over a
mountain range and descending are warmed by compression, and having lost
a part and in many observed instances a large percentage of the moisture
they previously contained, become warm, drying winds. The chinook winds,
as they are termed in America, are marked examples of the influence
exerted by mountains on climatic conditions.
What are termed above the secondary conditions, tending to modify
climate, produce such great changes in the distribution of rainfall,
temperature, etc., and in the influence of the planetary winds, that the
subdivision of the northern hemisphere into torrid, temperate, and
frigid zones, while based on astronomical data, does not serve to
represent actual conditions, except in a general way, in reference even
to the single element of temperature expressed in these names. A
comparison of the isotherms and of the distribution of precipitation as
indicated on the preceding maps, with the parallels of latitude, shows
at once that these two most important elements of climate are
conspicuously independent of distance from the equator. A logical basis
for subdividing the continent into _climatic provinces_ must therefore
be sought in other directions.
CLIMATIC PROVINCES
In order to obtain a comprehensive idea of the climate of a continent,
it is obviously desirable to subdivide it into areas having more or less
similar and distinct atmospheric conditions. The leading difficulty in
making such subdivisions is the well-known fact that the climate of any
region which may be chosen passes by insensible gradations into that of
adjacent regions, and any boundaries that may be drawn are to a
considerable extent artificial and arbitrary. While the true basis on
which to establish climatic areas or provinces is the resultant of all
the weather elements which go to make up the atmospheric conditions
recognised under the broader term climate, so many factors have to be
considered that it is extremely difficult to arrive at a general
acceptable conclusion. The principal elements of the weather to be
considered are, approximately, in the order of their importance,
beginning with the one which exerts almost universal control--temperature,
precipitation, the winds, absolute and relative humidity of the air, and
evaporation.
[Illustration: PLATE III.--Climate & Life Provinces.]
In North America there are voluminous records in reference to each of
these elements of the weather, embracing considerable lengths of time,
and relating mainly to the United States and southeastern Canada, but
not sufficient to enable one to subdivide the entire continent into
climatic provinces. The lack of weather records embracing the entire
continent may be supplied in part by what may be termed the natural
records of atmospheric conditions as expressed by the flora and fauna,
as well as by soil conditions and topography. In recognition of this
principle, the climatic provinces of North America here adopted have
been made to conform to its life-zones.
The major climatic divisions of North America here provisionally adopted
are, in their general order from south to north, the _Tropical_, _Lower
Austral_, _Upper Austral_, _Transition_, _Boreal_, and _Arctic_ (Plate
III). These several divisions are termed climatic provinces, and are
assumed to coincide with those of the life-regions as mapped by C. Hart
Merriam. The basis for classification is mainly temperature. In the
main, the northern boundaries of the provinces and their higher limits
in mountainous regions are determined by the temperature of the season
of growth and reproduction among plants and animals; while their
southern boundaries or lower limits on the mountains are determined by
the temperature of a brief period during the hottest portion of the
year. A more definite account of the reasons for choosing these
limitations will be given later in discussing life areas. While the
principal basis for establishing climatic provinces is temperature, many
other conditions are also recognised, chief among which is
precipitation. Several of the climatic provinces have two divisions,
namely, a _humid_ and an _arid_, the dividing line being
approximately the one hundredth meridian. These are well defined and
important in the tropical, austral, and transitional, but less definite
and less well known in the boreal and arctic provinces. The regions
embraced in the several climatic provinces named above, as well as their
humid and arid divisions, so far as now understood, are shown on the
accompanying map.
_The Tropical Province_ (Plate III).--This is the most southern of the
climatic provinces which it is convenient to recognise in North America,
and includes the West Indies, Central America, and southern Mexico,
together with a narrow strip on each coast of northern Mexico and the
extreme southern ends of the peninsulas of Florida and of Lower
California. While the land areas in this widely extended province
present conspicuous differences, their climate in general is
characterized by a high mean annual temperature with but moderate
seasonal or daily variations, and by the occurrence in general of a wet
and a dry season each year. The prevailing winds are the northeast
trades. While the average yearly temperature is high, being in general
about 80 deg. F., the heat in summer is less intense than in many portions
of the austral provinces. In winter the temperature does not fall
sufficiently to produce frost, except on the higher mountains, which, in
fact, belong to one or more of the other provinces named above. On
ascending the mountains a rapid change to cooler and even to frigid
zones is experienced. Snow occurs on the higher portions of the
mountains of Central America and Mexico, and in a few instances, as on
the great volcanic cones in sight from the City of Mexico, is perennial.
On the mountains just referred to all of the several climatic provinces
are represented by well-characterized zones, arranged one above another,
and presenting in epitome the general changes one would experience in
travelling from the Gulf of Mexico to the Arctic Ocean.
In marked contrast to the prevailing uniformity of temperature at
corresponding elevations throughout the tropical provinces is the
inequality in rainfall in reference to both seasonal periods and
differences in geographical position. Of the annual changes, the
most pronounced and characteristic is the alternation of wet and dry
periods. During the summer season, or in general from May to October,
the air is usually clouded for at least a portion of each day, and heavy
downpours of rain occur. Throughout the remainder of the year the clear
skies and refreshing breezes, such as on the ocean are normally
characteristic of the trade-wind belt, prevail. The remarkable
regularity with which these changes occur each year suggests at once
that they are due to the migrations northward and southward with the sun
of the great climatic belts encircling the equatorial region of the
earth. In summer the equatorial belt of calms and heavy rainfall not
only migrates northward and occupies a portion of Central America,
bringing to Costa Rica and Nicaragua cloudy skies and abundant
precipitation, but seems to be carried bodily still farther northward,
so that the influence of the southeast trades of the southern hemisphere
makes itself felt, and four somewhat well-marked annual changes occur,
namely, two wet and two dry seasons. What is known of the climate of
this extreme southern portion of the province under review indicates
that the seasonal changes, especially on the Caribbean slope, are less
well marked than in its central portion, and rain is frequently copious
in nearly every month of the year. In the region just mentioned the mean
annual precipitation, as at Greytown, for example, at the eastern
extremity of the proposed Nicaragua Canal, is about 250 inches, and in
exceptionally wet years reaches nearly or quite 300 inches. This is, so
far as known, the heaviest annual precipitation in any portion of North
America.
In the West Indies the rainy season begins, in general, in May and lasts
until October. On the lowlands of eastern and south-central Mexico the
wet season commences in June, and the rains increase until the end of
July and end in November. This region lies to the north of the northern
limit reached by the equatorial belt of calm, but the rains in the
summer season are due to the same general influence, namely, the
lowering of temperature in the northward-moving upper air-currents, and
their effect on the trade-winds. In Cuba and along the northern
coast of the Gulf of Mexico when the trade-wind belt migrates southward
in winter the influences of the southward-moving storm from the
Mississippi Valley and Atlantic States produces what are termed
_northers_, which bring a chill and at times frost, more especially in
Florida and on the higher portions of Cuba.
While the wide-reaching seasonal changes within the tropical province
depend on the migrations of the planetary winds northward and southward
with the sun, and are due primarily to astronomical conditions, there
are equally well-marked variations in rainfall dependent on or regulated
by geographical conditions. These are of two principal classes: First,
elevation of the land; and second, the relation of broad land areas to
the direction of the prevailing winds, each of which is abundantly
illustrated in the tropical province.
The mountainous islands in the West Indies receive a heavy rainfall,
especially on their windward or easterly slopes, while the low islands,
and less markedly the southwestern border of the higher islands, are
much less humid. The eastern side of Porto Rico, for example, has an
abundant rainfall, and was originally clothed with a luxuriant native
flora, including large forest-trees, while the lowlands on its southern
and southwestern border are subject to drought, and irrigation is
desirable to insure the growth of crops. Again, while the mountainous
islands of the Lesser Antilles, with their luxuriantly forested slopes,
present ideal pictures of tropical loveliness, low-lying Sombrero,
Barbuda, St. Martin, and other similar islands are so arid that guano
has accumulated on them to such an extent as to be of commercial
importance. The reason for these striking contrasts within narrow
geographical limits is readily seen in the influence of the highlands on
the trade-winds. These air-currents blowing from the Atlantic are warm
and contain a high percentage of moisture. As they advance, however,
they invade regions that are progressively warmer and warmer, and the
capacity of the air for moisture is correspondingly increased. For this
reason the trade-winds in crossing low land become drying winds.
When the warm humid air-currents are forced upward, however, they are
cooled in part by contact with the land, but to a greater extent on
account of expansion due to decreased pressure; the dew-point is thus
lowered, and when saturation is reached precipitation follows. This
explanation applies also to the low peninsula of Yucatan, which is
within the trade-wind belt, and is exceptional among the lands of
Central America, on account of its dearth of forests, but in this
instance, and also in reference to the similar barren condition of the
Bahama Islands, in addition to the lowness of the land, the influence of
the underlying porous, limestone rocks on the vegetable growths should
be considered. In the instances just mentioned the rain that falls is
quickly absorbed by the cavernous rocks, and surface streams are rare.
In Central America the influence of mountains on the climate is much the
same as in the West India islands; in fact, the narrow rugged belt of
land connecting the two Americas may, from our present point of view, be
considered as a great island situated mainly within the trade-wind belt,
and similar to Jamaica, for example, in its influences on the winds
blowing across it. The eastern slopes of the Central American mountains,
together with the adjacent lowlands, with the exception, principally, of
Yucatan, are well watered and clothed with dense vegetation, while on
the western slopes, and especially over the narrow fringe of lowland
adjacent to the Pacific, the rainfall is less than on the Caribbean
coast, and the forests are open with many grass-covered areas which are
favourable for agriculture. In the mountainous portions of the West
Indies and of Central America, on account of the more healthy conditions
on the leeward or drier sides, as compared with the windward or humid
slopes of the mountains, the towns and the principal portion of the
white inhabitants are located on their western borders. Owing to the
great humidity and the long-continued high temperature during the hotter
portion of the year throughout the tropical province, much of the
lowland to the eastward of the high mountains is swampy and unhealthy.
This low region in Mexico and Central America is known as the
_tierra caliente_, or hot country; on the mountains and plateaus, or in
general where the elevation is between 5,000 and 7,000 feet, is the
cooler and remarkably salubrious _tierra templada_; and at still higher
elevations occur the _tierra frie_. Owing to the decrease in temperature
with elevation, and the fact that the moist warm air is forced to rise,
and in consequence expands on passing over the highland, the rainfall
probably increases with elevation through the three zones just referred
to.
While the tropical province is characterized by the uniformity with
which its atmospheric changes occur, it is nevertheless in part subject
to occasional and exceedingly violent cyclonic storms termed hurricanes,
which begin in the torrid zone, travel northward (Fig. 26, page 210),
and make their influence felt in more than one of the climatic provinces
into which North America is here divided. Thunder-storms, frequently of
great violence, also occur, especially in the Central American region in
summer, when the doldrums migrate northward.
The trade-wind belt broadens in crossing the southern portion of the
North American continent, and on the west coast and during its greatest
northward migration reaches to southern California. As we have seen, the
lowlands not adjacent to mountains in the Caribbean region receive
little or no rain from the trade-winds, for the reason that the
prevailing air-currents are moving from cooler to warmer regions, and
therefore have their capacity for moisture increased as they advance
more rapidly than their thirst can be satiated. The trade-winds are thus
normally drying winds. The same principle holds true for continents as
well as islands. The trade-winds on reaching the eastern border of the
Mexican plateau are forced upward and part with much of their moisture
in the form of rain and snow, and on descending to the lower lands
bordering the Pacific are desiccating winds. The conditions are thus
much the same as on the lowlands situated to the leeward of the
mountains of the West Indies. The narrow fringe of low-lying country on
the west border of the main body of Mexico, together with the
peninsula of Lower California and a portion of the southern extremity of
California, are arid, although in immediate proximity to the ocean. The
leading characteristics of the climate of this, the arid portion of the
tropical province, are its small rainfall, clear atmosphere, high mean
annual temperature, moderate seasonal changes, and active evaporation.
The most typical portion of the arid region bordering the Pacific from
California southward lies in northwestern Mexico, where the mean annual
precipitation on the western slope of the central table-land is from 15
to 20 inches, but gradually diminishes as one descends to the lower
lands to the westward to 10 inches, and even to 5 inches or less. A
great portion of the lowlands is practically a desert, although, as is
common on the desert-like tracts of this continent, it is sparingly
covered with detached clumps of cacti, yuccas, and other similar plants
which live with the minimum of water.
The hot, arid lands of the southwest just referred to are similar in
position, in reference to the relation of land areas to the trade-wind
belts, as other trade-wind deserts, as they are termed, such as the
Sahara, which occur on the leeward side of continents where they are
crossed by these drying winds.
_The Lower Austral Province_ (Plate III).--This province embraces the
Atlantic and Gulf coasts of the United States from Connecticut
southward, with the exception of the tropical portion of southern
Florida; the Mississippi basin south of the mouth of the Ohio; and
includes also many of the valleys amid the Pacific mountains from the
northern boundary of California to central Mexico. The larger
geographical conditions on which the boundaries of the province depend
are the warm currents in the Atlantic, which ameliorate the temperature
of the adjacent land as well as supply it with abundant moisture; the
low elevation of the central continental basin, which permits of great
extension northward of the climatic conditions originating about the
Gulf of Mexico; and the mountainous character of the western portion of
the province, where the valleys have a markedly different
temperature and rainfall from the highlands.
The most southern portions of the lower austral province within the
border of the United States has a semitropical climate and is
characterized by its high mean annual temperature, abundant rainfall,
and uniformity of weather conditions throughout the year. The summers
are hot, but tempered by winds from the Gulf of Mexico and the Atlantic,
and the winters, although normally mild and without snow, are varied at
intervals by periods of cold which bring occasional frosts. This
semitropical division of the lower austral province embraces a narrow
strip of coast-land extending from South Carolina to southeastern Texas,
with the exception of the extreme southern end of Florida, and forms a
transition between the hotter and more humid tropical province to the
southward and the cooler and less humid region to the northward.
As the saying is, a tree is judged by its fruit; in a similar way we may
judge of the climate of a region by its products. The Gulf strip of the
lower austral province is the home of a number of plants and animals not
found farther north, as, for example, the cabbage palmetto and Cuban
pine, and several species of birds and small mammals. Among the
agricultural products of this narrow coastal belt which are suggestive
of its climate are rice, sugar-cane, and sea-island cotton; of fruits it
produces, especially in Florida, oranges, mandarins, lemons, limes,
shaddocks, and pineapples.
The humid portion of the lower austral province extends northward from
the Gulf strip, and presents a transition in climatic conditions between
the semitropical Gulf coast and the more markedly temperate climate of
the upper austral. This eastern division of the lower austral,
characterized by its mild winters, general absence of snow, long hot
summers with abundant rain, extends from eastern Virginia southward
about the southern end of the Appalachian Mountains, and in the
Mississippi Valley broadens so as to reach the mouth of the Ohio River.
Its western limit is in central Texas, where the mean annual rainfall
decreases to less than 20 inches, and forests give place to
treeless, grass-covered plains and plateaus.
Through this eastern portion of the lower austral the mean annual
precipitation is in general between 50 and 60 inches, and is so
distributed throughout the year that each month receives approximately
an equal share. In general, however, the rainfall is greater in winter
than in summer. The mean annual temperature of the humid portion of the
lower austral is from 60 deg. to 68 deg. F., the mean winter temperature 40 deg. to
52 deg., and the mean summer temperature from 75 deg. to 80 deg. F. From these
records it may be inferred that the conditions are favourable for the
growth of trees. In its natural condition this entire region was clothed
with a varied and beautiful forest, consisting largely of broad-leaved
trees, but is also the home of the southern pine and the cypress. The
characteristic crops are cotton and corn (maize).
The western or arid portion of the lower austral province embraces
western Texas, a large area in northeastern Mexico, and circling about
the southern extremity of the Pacific mountains in central Mexico,
extends northward adjacent to the tropical border of the Pacific and the
Gulf of Lower California, into Arizona and southern California. A
detached area of this same province occupies the great Valley of
California.
The leading feature in the climate of the extensive and irregular region
just outlined is its aridity. The rainfall is too small to insure forest
growths; the land is treeless, except along the streams, and irrigation
is necessary for successful agriculture. With a sufficient amount of
water for irrigation, a great variety of fruits, etc., may be raised,
including many products usually considered as indicating tropical
conditions, such as mangoes, dates, figs, citrus fruits, olives,
pineapples, etc. Not only are the agricultural products numerous and
varied, but the yield per acre under the most favourable conditions is
far in excess of the best results reached in most regions where rain is
relied on to furnish the requisite moisture. Under the prevailingly
cloudless skies of the hot arid lands of the southwestern portion
of the continent insolation is intense and the growth of vegetation
phenomenal when the necessary amount of water is supplied. The land in
its present condition presents great contrasts, ranging from desolate,
sun-burned tracts which are almost absolute deserts, to the vivid green
of irrigated fields and the deep shade of heavily fruit-laden orchards.
_The Upper Austral Province_ (Plate III).--The portion of North
America embraced in this climatic province lies principally in the
central part of the United States, but includes also a narrow strip in
southern Ontario, adjacent to the north shore of Lake Erie, and a large
irregular area in the central plateau of Mexico. A marked feature of its
geography is its extreme irregularity in the portion occupied by the
Pacific mountains in the United States and Mexico. The reasons for this
lie mainly in the influence of the relief of the land on climate, the
direction of the prevailing winds, and varying distances from the ocean.
It is a familiar fact that boreal and even arctic climatic conditions
are met with on high mountains. The attention that is given to changes
in climate with increase in altitude is no doubt largely due to the fact
that the mountains present conditions which are exceptional and more or
less novel as seen from our accustomed point of view. A person living in
an elevated region, on descending into a deep valley, would be impressed
with the reverse order in which the climatic zones occur. In making such
a descent he would pass in succession from a boreal or perhaps arctic
climate, through a transitional or cold temperate, to the warm temperate
or upper austral province, and might even reach the semitropical
division of the lower austral. In the Pacific mountains within the
border of the United States the valleys are sufficiently deep to have
the climatic conditions here ascribed to the upper austral, and in the
southwestern portion of the United States descents may be made--as in
the Great Valley of California and in the arid basins of southern
Nevada, Arizona, etc.--sufficiently great to reach the lower austral.
The valleys amid the Pacific mountains, which fall in the upper austral
province, are in general low at the north in reference to
sea-level, and become higher and higher at the south. For example, the
upper austral region in central Washington is but 400 or 500 feet above
the sea, while in Mexico it lies in general at an altitude of between
4,000 and 6,000 feet.
The upper austral province may be termed warm temperate, with a marked
contrast between the heat of summer and the cold of winter. The summers
are long, with an average temperature of 70 deg. or 75 deg., while the winters
are variable, with frequent cold periods when ice forms and snow-storms
are not rare. The snow seldom remains on the ground for more than a few
days at a time, however, except in the northeast, where the warm
temperate climatic conditions of the province under review merge with
those of the colder region to the northward embraced in the transition
province.
The upper, like the lower austral, presents two well-marked divisions in
reference to humidity--an eastern or humid and a western or semihumid
portion; the dividing line is in the neighbourhood of the one hundredth
meridian. In the eastern division the mean annual precipitation in the
Piedmont region to the east of the Appalachians and on the coastal plain
adjacent to the Atlantic in Maryland and New Jersey is from 40 to 80
inches, but decreases westward, and on the border of the Great plateaus
in Kansas and Nebraska is about 20 inches. In the western division the
annual precipitation is less than 20 inches, and agriculture without
irrigation is uncertain and usually impossible. To the east of the one
hundredth meridian the rain is somewhat evenly distributed throughout
the year, although an increasing dryness of the summer is easily
detected as one travels from east to west, but in the various upper
austral valleys of the Pacific mountain region the precipitation is
mostly during the winter, and the summers are practically rainless. The
marked difference in precipitation between the humid and semihumid
division of the upper austral province is recorded on the surface of the
land by the vegetation. In the eastern division the entire region, with
the exception of the prairies in the central part of the Mississippi
basin, was originally clothed with a varied and beautiful forest,
consisting mainly of broad-leaved trees, such as the hickory, maple,
oak, etc., while the semihumid western division is treeless, except in
immediate proximity to streams.
In the southern portion of the humid division of the upper austral
province cotton is one of the staple products, but the northern limit of
the region in which it can be successfully cultivated is soon reached as
one travels northward. Tobacco is grown extensively in the southeastern
and eastern portions of the province. The principal crop of the great
central area in the Mississippi Valley is corn (maize). Successful wheat
culture begins in the northern portion of the province, but the
conditions favouring its cultivation increase to the northward and it
becomes the characteristic and most valuable crop of the transition
province.
In the western or semihumid division of the upper austral the variety of
agricultural products that can be successfully cultivated with the aid
of irrigation is greater than in its eastern portion, where irrigation
is not generally practised. The northern limit at which tobacco, fruits,
the vine, etc., may be advantageously cultivated in the west is greater
than in the east. For example, in the east the northern limit at which
tobacco is raised on a commercial scale is in Connecticut, while in the
west it reaches a large size and excellent quality in central
Washington. Various fruits, such as the peach, pear, plum, grape, etc.,
have their northern limit of successful cultivation in the east in
western New York, southern Ontario, and southern Michigan, a region
favourably influenced in this connection by the proximity of the Great
Lakes; in the west these same fruits reach a high degree of perfection,
and are produced in great abundance, with the aid of irrigation, in
north-central Washington, fully 5 degrees of latitude farther northward.
On the whole, the upper austral province may be said to have a warm
temperate climate, in which the summer season is longer and more
pronounced in its characteristics than the winter season. It
presents sufficient seasonal variations, however, to favour in a high
degree both the physical and intellectual development of man.
_The Transition Province_ (Plate III).--This, the transition region
between the austral and boreal provinces, includes the cool temperate
portions of North America. Like the austral provinces, its outlines are
irregular, and in places it occupies detached or island-like areas, on
account of the influence of mountains on climatic conditions. Its
largest continuous area is situated along the northern border of the
United States, but includes the southern portions of Assiniboia,
Ontario, and Quebec, and extends from the Atlantic to the Pacific Ocean.
From this main belt there is a marked extension southward along the
Appalachian Mountains, which carries a cool temperate climate into
northern Georgia, and another and much greater southward extension along
the Pacific mountains, which reaches central Mexico. In the northern
portion of the United States and adjacent parts of Canada, the region
under consideration has, in general, an elevation of 1,000 feet or even
less above the sea, but it rises when followed southward along the
mountains, and in the southwestern portion of the United States and on
the table-land of central Mexico attains an elevation of some 8,000 or
9,000 feet. On the higher portions of the mountains of Central America
and the West Indies the climatic conditions are similar to those of the
regions farther north here included in the transition climatic province,
but these isolated areas are not sufficiently well known to be indicated
on the accompanying map.
The mean annual temperature of the transition province is lower than
that of the upper austral, and, although a precise average is not at
present attainable, may be taken at about 45 deg. F. The winters are long
and cold, especially in the northern part of the United States and
adjacent portions of Canada, the mean temperature being approximately
20 deg. or 25 deg., but sudden and great variations are not uncommon. At times,
and frequently for continuous periods of several days, the temperature
falls to 20 deg. or more below zero of the Fahrenheit scale, then again
rises above freezing, and the frozen soil thaws and possibly
becomes entirely freed from ice.
In common with the austral provinces, the one under consideration is
divided into two portions, an eastern and a western, in reference to
precipitation, the dividing line being a little to the eastward of the
one hundredth meridian. While the western portion of the province is
characterized by its small rainfall, precipitation is greater, mainly on
account of greater elevation, than in the adjacent portions of the
austral provinces, and in Oregon and Washington there is a markedly
humid area.
Snow falls throughout the entire transition province, at least during
exceptional winters, and in general during every winter. There is great
variation in the depth of the snow from winter to winter, and also with
geographical position. It is greatest in the northeast--that is, from
the Atlantic coast westward to Wisconsin and Minnesota--and least on the
Pacific coast and the southern portion of the Pacific mountains. Over
the northeastern portion of the province the snow frequently whitens the
ground for weeks, and even for two or three months continuously. A deep
accumulation which remains for a long time unmelted is welcome, as it
protects the roots of plants from sudden changes of temperature and
prevents alternate freezing and thawing of their sap, which is injurious
to their tissues in numerous instances. Great variation in the amount of
snow that falls annually in a given locality is of common occurrence. A
large proportion of the yearly accumulation frequently occurs during one
or two great storms. For example, in January and February, 1898, there
were two severe storms, accompanied by an unusual depth of snow, the
first being most pronounced over New England, and the second in the
region of the Great Lakes. Other storms increased the amount of snow so
that at the beginning of March the average depth in Maine was 40 to 70
inches, in New Hampshire and Vermont 10 to 40 inches, and in
Massachusetts 10 to 20 inches, while in Michigan and Wisconsin the
general depth on level ground was 1 to 2 feet, with many drifts 10 to 15
feet deep.
[Illustration: FIG. 25.--Ice-palace erected in Montreal, Canada, during
the winter of 1889.]
The winter in the northeastern portion of the transition province may be
said to be the most characteristic feature of the climate, as it is the
one that is most pronounced and exceptional, when a comparison is made
with other thickly peopled portions of the continent. The period of cold
and snow each year is long, extending in general from November to March,
and the coming of the flowers and birds in spring is frequently much
delayed. The long cold winters have a decided influence on plant and
animal life, and in a marked way modify the lives of men. In the
northeastern portion of the United States and adjacent provinces of
Canada various forms of sleighs are extensively used during the winters,
and skating on the frozen lakes and streams and excursions on snow-shoes
over the fields and through the forests are a popular and healthful
exercise, while coasting and tobogganing--or to explain these terms to
people living in regions where snow does not fall, the sliding down
steep snow- or ice-covered slopes on sleds or flat-bottomed
toboggans--are highly enjoyable sports indulged in by children and grown
people alike. In certain cities, notably Montreal and Quebec, what are
termed ice-palaces (Fig. 25) are built of blocks of ice and are utilized
for winter carnivals.
The summers throughout the transition province are hot, with little rain
in the western portion, but refreshing showers and occasional
destructive storms in the humid eastern portion. Owing to the latitude
of the main transcontinental belt of the province, the number of hours
of sunlight each day in summer is increased beyond what it is in the
main portions of the austral provinces, thus favouring the growth of
vegetation. There is also a lengthening of the morning and evening
duration of twilight, and magnificent sunrises and sunsets are frequent.
The mean summer temperature is in the neighbourhood of 70 deg. F., but hot
spells, lasting for days, and even weeks, are of common occurrence.
During these trying and frequently unhealthy intervals the temperature
in the shade reaches or even exceeds 100 deg. F., and sunstrokes or
prostrations by reason of the heat, particularly in the cities, are
numerous. The four seasons of the year are better marked and have
more pronounced characteristics in this division of the continent than
in any other, and it is the region of greatest seasonal climatic changes
as well as of marked weekly and even daily variations in weather
conditions. The most delightful months to most people are May, when the
returning migratory birds are nesting, the trees unfolding their many
tinted leaves, and the air laden with the perfume of multitudes of
blossoms, and October, when the rich colours of ripened leaves give to
the forests a marvellous variety and brilliancy of colour and the
tranquil, hazy atmosphere is undisturbed by storms for days and even
weeks together. This annual period of tranquil weather, extending
frequently far into November, is known as Indian summer.
In the northern portion of the transition province the broad-leaved,
deciduous trees of the central and eastern portions of the United States
reach their northern limit, and become mingled with a southward
extension of the conifers which form the major portion of the forest of
Canada. A similar but less marked change occurs among the Pacific
mountains, where the scattered growths of oaks, pinyon pines, sycamores,
etc., of the lower mountain slopes and stream sides mingle with the
spruces and yellow and white pines of the more elevated region, where
the climate is similar to that of central Canada. As remarked by
Merriam, the province as a whole is characterized by comparatively few
distinctive animals or plants, but rather by the occurrence together of
southern species which there find their northern limit and northern
species which there reach their southern limit. It embraces the northern
portion of the truly agricultural lands of the continent. The plants of
economic importance which there reach their highest stage of perfection
are wheat, oats, and other cereals, the sugar-beet, numerous vegetables,
the white potato, apples in great variety and abundance, cherries,
plums, grapes, etc. It is the northern limit of corn, and includes
nearly the entire area in which maple-sugar is produced. In the eastern
portion of the province several varieties of native nuts, such as the
beechnut, butternut, chestnut, hazelnut, hickory-nut, walnut,
etc., grow wild and in great abundance; but nut-bearing hardwood trees
are also a characteristic feature of the forests of the humid portion of
the austral provinces.
In the western division of the province a humid area--embracing western
Washington and Oregon, part of northern California, including the Coast
Range of the same States--presents a marked contrast to the more widely
extended and excessively irregular arid portion which surrounds the
higher mountains and is for the most part remote from the ocean. Both
the humid and arid divisions of the western part of the province are
alike favourable for agriculture, as is shown by the vast and highly
productive wheat-fields of the semihumid eastern portion of the States
just named and the productive hop lands, orchards, and vineyards of
their humid western portions.
The climate of a great land area not only finds expression in its fauna
and flora, but in the industries and the intellectual development of its
people. While it is difficult to translate man's physical and
intellectual development into terms of climate, it is evident that the
transition province favours both bodily and mental activity more than
any of the other climatic provinces into which North America is here
divided. Although the boundary between the upper austral and the
transition provinces is indefinite, it is easily to be seen, from the
geographical distribution of cities, agricultural population,
manufactories, colleges, and other institutions of learning, etc., that
the climate of the province under review is on the whole the one in
which the greatest intellectual advance has been made and the one which
holds out the greatest promise for the future.
_The Boreal Province_ (Plate III).--This climatic division of North
America extends in a broad belt diagonally across the continent from the
eastern portion of Labrador nearly to the shore of Bering Sea, and is
represented by detached areas in both the Atlantic and Pacific mountains
far beyond its general southern limit. Its northern border, in the
Continental basin, is marked by the cessation of forests, and on the
mountains to the southward its upper limit coincides with the
timber-line. Its leading climatic features are its low mean
annual temperature--in general from 32 deg. to 40 deg. F.--its long, cold
winters, and short, hot summers. The differences in mean annual
precipitation in various parts of the province are less marked than in
the several provinces previously noticed, but in the far north a cold
arid division should be recognised. Although but few direct measures of
precipitation are available for comparison, our general knowledge of the
great boreal province and the character of its vegetation indicate that
there is a decrease in precipitation from both the eastern and western
borders of the continent towards the interior, and also from its central
portion both northward and southward. The heaviest precipitation is on
the Pacific coast, from California northward to southern Alaska, and the
lightest precipitation is probably in the central Continental basin,
near the northern limit of the province. Precipitation on the Pacific
coast at low elevations is almost entirely in the form of rain, but on
the mountains there is in winter deep snow which remains for a number of
months unmelted. Throughout the portion of the province included in
Canada and Alaska the snowfall is abundant, but heaviest towards the
Atlantic coast. Along the northern margin of the province, as indicated
by observations at a small number of stations, not only is the mean
annual precipitation light, probably under 20 inches, but the winter
snow is not deep, although it remains on the ground continuously for
five or six months. In the main or northern portion of the boreal
province, owing to the comparatively high latitude, the variation in the
number of hours of light and darkness each day during a year becomes
conspicuous. In summer the sun is above the horizon from eighteen to
twenty-four hours each day, and in winter the hours of darkness are
correspondingly increased. The year is divided into but two seasons,
summer and winter, the distinctive features of spring and fall, so well
marked in the upper austral and transition provinces, disappearing. On
account of the low mean annual temperature, and especially because of
the shortness of the growing season, agriculture is of small importance.
Along its southern border, more especially in southeastern Canada and
Newfoundland, such small fruits as currants, huckleberries,
raspberries, blackberries, cranberries, etc., grow wild and yield
abundant returns when cultivated. In favoured localities white potatoes,
turnips, beets, and certain varieties of the apple, as well as the more
hardy cereals, are cultivated with moderate success.
_The Arctic Province_ (Plate III) comprises the cold, treeless plains
sloping to the Arctic Ocean and the summits of the higher mountains at
the south which rise above the transition province. The one controlling
climatic feature is the low temperature, the mean for each year being
32 deg. F. or lower. The winters are longer and more severe than in the
boreal province, and the summers short and hot. Insolation, on account
of the length of the days in summer of the main area of the province and
the free exposure on the mountain summits to the southward, is intense,
but its beneficial effect on vegetation is largely counterbalanced by
the influence of the lingering snow and ice. In the mountainous regions
of North America the arctic province is the birthplace of numerous
glaciers. Although destitute of trees, the arctic, or arctic-alpine
province, as it may be termed, is rendered glorious in numberless
localities by the profusion and brilliancy of its flowering annuals.
SECONDARY DISTURBANCES OF THE ATMOSPHERE
In the broad, general movements of the atmosphere over North America
embraced in what are termed the planetary and continental winds there
are many disturbances due to more or less local changes in conditions,
the most conspicuous of which are whirlwinds, chinook winds,
thunder-storms, tornadoes, cyclones, and hurricanes. While some of these
disturbances are local, as the whirlwind and tornadoes, and may not
extend beyond the boundaries of the particular climatic provinces where
they originate, others, as the cyclones and hurricanes, may affect the
climate of several provinces.
_Whirlwind._--A conspicuous, although minor feature in the atmospheric
phenomena of the hot, dry plains and valleys, especially of the
Mexican plateau and the Great Basin, and less markedly of the Great
plateau to the east of the Rocky Mountains, is the occurrence of small
whirlwinds which carry dust and light objects into the air in spiral
columns that are not infrequently 2,000 or 3,000 feet high, and have a
diameter of perhaps 50 to 100 feet. These small whirls of the air, in
which some of the characteristic features of the intensely active
tornadoes and widely destructive tropical hurricanes can be studied on a
small scale, occur most commonly during hot summer afternoons, when from
a commanding station half a dozen or more swaying columns may be seen
moving in various directions over the parched valleys and sun-scorched
plains. These columns not only move in various directions, showing that
they are not due to the same immediate cause, but have different
internal motions, some whirling from right to left, and others in the
opposite direction.
The generally accepted explanation of these small whirlwinds is that the
air over the surface of the deserts, which are frequently almost bare of
vegetation and perhaps white with saline incrustations, becomes locally
highly heated, especially when there is little or no wind, and is forced
upward by the inflow of the surrounding cooler and heavier air. The
inflowing currents have different velocities, and on meeting the
strongest one gives a rotary or spiral motion to the ascending column,
which acts like a chimney in allowing the escape upward of the hot air
from below. A central vertical line frequently seen in the dust columns
shows that a core of comparatively still air is present, about which the
dust-charged air rises in a spiral course. If the conditions just
outlined should be greatly increased in magnitude some of the leading
features of tornadoes and even of hurricanes would be produced. In
short, all of the winds cited above, except the chinook, are concentric,
swirling movements in ascending air, due primarily to a local increase
in temperature at the lower portion of the atmosphere.
_Chinook Winds._--On the Great plateaus adjacent to the Rocky Mountains,
and in similar situations to the eastward of the Sierra Nevada
and Cascade Mountains, warm, drying winds frequently occur, especially
in winter, when they bring a balminess as of spring. The remarkable
feature of these interesting winds is that they come from the snow-clad
mountains, but are warm and dry in contrast with the preceding condition
of the air on the plains. The capacity of the air brought by these winds
for moisture is so great that evaporation is active, and the snow in the
valleys and over the broad plains disappears without visible melting.
The change in the previously winter aspect of a region within the
influence of these _chinook_ winds, as they are termed, is truly
surprising, and to their influence is due to a marked extent the value
of the Great plateaus as stock-ranges, for the reason that the snow is
removed from them so as to allow cattle to feed on the naturally dried
grasses.
The chinook winds are the counterpart of the _foehn_ winds of
Switzerland, and are explained on the principle that descending air is
made more dense by the increased pressure to which it is subjected, and
its temperature correspondingly raised, its capacity for moisture being
at the same time increased on account of its rise in temperature. The
apparent anomaly of a warm, dry wind blowing from a snow-clad mountain
range is no longer a mystery, if we consider that the air is drawn over
the mountains towards a centre of low barometrical pressure owing to the
wide-reaching influence of a cyclonic storm or other large atmospheric
movement. The air as it rises in order to cross a mountain is cooled,
largely on account of relief of pressure, and parts with a portion,
possibly a large portion, of its moisture, which condenses on the
mountain commonly as snow; on passing the mountain the air descends and
is warmed by compression, and having less moisture than before, becomes
a drying wind, which produces the sudden and surprising changes on the
plains and valleys to the leeward.
The chinook winds of the western portion of Canada and the United States
occur principally to the eastward of high mountains, for the reason that
the prevailing air-currents of that region are from the west.
_Thunder-Storms._--In the eastern portion of the United States and
adjacent parts of Canada during the summer season the heating of the
lower portion of the atmosphere, especially on still, sultry afternoons,
causes ascending currents of warm, moist air, which become cooled as
they rise, and give origin to vast masses of cumulus clouds. These
magnificent "thunder-heads," as they are sometimes termed, illuminated
by the full sunlight are most magnificent, and usually herald the coming
of heavy showers, accompanied by frequently destructive lightning and
heavy thunder. The bases of the clouds when seen from a distance are
usually horizontal and may have curtain-like festoons beneath, due to
falling rain; while aloft the white vapour boils upward in fleece-like
masses, revealing a strong convectional ascent of moist air. The
immediate cause of a thunder-storm is the rapid ascent of a column of
warm moist air, which becomes cooled as it rises and the moisture
contained in it condensed. The cause of the ascent of the air column, at
least over plains and plateaus, is the heating of the air in contact
with the earth. A layer of warm, and consequently light, air beneath a
layer of cooler and heavier air furnishes unstable conditions which
favour an overturning and an escape upward of the lighter air, which is
forced to ascend much as the hot air in a chimney is made to flow upward
by the pressure of cooler and heavier air around. The conditions
preceding a thunder-storm are a stagnant atmosphere over a broad region
where the lower layer of hot air is also charged with moisture. These
conditions are frequently fulfilled on the plains of the Atlantic slope
and southeastern portion of the continental basin in summer when warm
moist air is drawn in from the Gulf region towards the centre of an area
of low atmospheric pressure, and thunder-storms are there a
characteristic feature. The storms usually advance northeastward, the
direction being determined by the flow of upper air-currents, and move
over the country with a breadth of from 10 to perhaps 100 miles, and
send down copious supplies of refreshing rain.
Over the Great plateaus the air near the earth's surface is
highly heated during the summer season, but it is deficient in moisture,
and thunder-storms are rare, except for a brief period in late summer or
fall when the normal conditions are disturbed.
Thunder-storms are almost unknown in the great Canadian-Alaska province
and along the cool and humid northwest coast. They are also of rare
occurrence in the hot and dry atmosphere of the Great Basin and Mexican
plateau, but when they do come are of marked intensity, and pass under
the name of "cloudbursts." At the far south, in the region brought under
the influence of the equatorial belt of calm, thunder-storms are
frequent and of great intensity.
An upward ascent of warm moist air, in much the same manner as described
above, occurs about isolated mountains, particularly in the southern
portion of the Rocky Mountain chain, and summer thunder-storms are there
of frequent occurrence, especially in the afternoon, about the higher
mountain-peaks, while the adjacent valleys are flooded with sunlight.
Reference to this most striking phenomenon has already been made in
describing the Park Mountain.
_Tornadoes._--The fierce circular whirls in the air producing pendent,
spirally twisting clouds, which when they touch the earth are of such
intensity as to sweep away houses, trees, and nearly everything in their
paths, are known to meteorologists as tornadoes, although popularly, but
erroneously, termed cyclones. Storms of this character are of frequent
occurrence in the United States to the east of the Great plateau, and
are most numerous in the Mississippi Valley. Their path of destruction
is seldom over half a mile wide, and as a rule they progress towards the
northeast, in obedience to the movement of the upper air-currents, at a
rate of from 20 to 40 miles an hour, and may cut a swath from a few
miles to 20 or more miles long through forests, farms, villages, and
towns. They occur usually in the afternoon, and sometimes in the earlier
hours of the night, of warm, sultry days, especially in spring and early
summer, but are not strictly confined to that portion of the
year. The conditions which precede the coming of a tornado are, in
general, the same as those in advance of a thunder-storm--that is, an
indraft of hot, moist air beneath a cooler layer, thus establishing
unstable conditions. An upward draft is started, the intensity of which
becomes so great that the inflowing winds are given a rapid spiral
motion about a calm centre. The tornado may be considered as a fully
developed or exceptionally energetic thunder-storm, in which a spiral
movement is established as in desert whirlwinds. The conditions for the
origin of this class of dreaded and locally most destructive storms are
best fulfilled in the central portion of the Mississippi basin, where
they are somewhat frequent. They occur less commonly over the country to
the eastward, and are unknown in the more northern and western climatic
provinces, and, so far as the writer is aware, they have not been
reported from the region to the north of the United States.
_Cyclones._--This name is applied to the great atmospheric disturbances
marked by an inflowing of air towards a centre of low barometric
pressure from adjacent regions, commonly several hundred miles across,
and an escape and overflow aloft. As in whirlwinds and tornadoes, there
is a spiral movement established in the inflowing currents, but owing to
the large size of the area of low pressure, this seldom reaches
destructive violence. Cyclonic storms are of common occurrence,
especially in the temperate zone, and bring to that region its
characteristic diversity of weather. Most of the rain and snow-storms of
the continent are due to the vast swirls of the atmosphere about areas
of low atmospheric pressure, which cause air-currents from different
directions and with different components of heat and moisture to move
over the land.
The cyclonic storm of the Mississippi Valley, the Atlantic coast States,
and southeastern Canada frequently originate in the Great plateau
province, and are carried towards the Atlantic owing to the influence of
the eastward-blowing winds of the upper atmosphere. At times these
storms are of such magnitude and intensity that they cross the Atlantic
and are observed in England and Scandinavia. The courses they
follow may be traced from day to day on the weather-maps issued by the
United States Weather Bureau, and from the directions they are likely to
follow and the atmospheric conditions pertaining to their various parts
predictions of surprising accuracy as to the changes which the weather
in a given locality will experience can be made one or two days before
the changes occur.
_Hurricanes._--Cyclonic storms of the general nature of the tornadoes,
but of vastly greater extent and intensity, originate occasionally
during the latter portion of the summer season over the tropical portion
of the north Atlantic, move slowly westward to the vicinity of the
Lesser Antilles, where normally their courses bend northward, and then
skirt the Atlantic coast of the United States and drift eastward under
the influence of the eastward-flowing upper air-currents, and not
infrequently make their influence felt in the western portion of Europe.
Occasionally, on account of the presence of an area of high barometric
pressure to the north of Cuba, the course of one of these tropical
hurricanes, as they are termed, is rendered irregular, and it passes
over the Atlantic States or is deflected still more and crosses the Gulf
of Mexico before reaching the border of the continent, as was the case
in September, 1900, when a large part of Galveston was destroyed. The
normal paths of the tropical hurricanes as they approach the coast of
the United States and the exceptional course of the one which passed
over Galveston, are indicated on the map forming Fig. 26.
[Illustration: FIG. 26.--Tracks of West Indian hurricanes. The circles
on the tracks indicate the position of the storm centres at Greenwich
mean noon on successive days. The date of the several storms is as
follows:
1. Aug. 27-Sept. 1, 1890.
2. Aug. 19-Aug. 25, 1890.
3. Aug. 19-Aug. 31, 1891.
4. Sept. 4-Sept. 9, 1891.
5. Sept. 16-Sept. 25, 1891.
6. Sept. 28-Oct. 7, 1891.
7. Aug. 17-Aug. 22, 1892.
8. Aug. 15-Aug. 22, 1893.
9. Aug. 23-Aug. 28, 1893.
10. Sept. 6-Sept. 9, 1894.
11. Sept. 20-Oct. 4, 1894.
12. Oct. 5-Oct. 10, 1894.
13. Oct. 12-Oct. 18, 1894.
14. Oct. 24-Oct. 27, 1894.
15. Oct. 18-Oct. 25, 1895.
16. Sept. 5-Sept. 10, 1896.
17. Sept. 9-Sept. 25, 1896.
18. Sept. 26-Sept. 29, 1896.
19. Oct. 9-Oct. 14, 1896.
20. Oct. 23-Oct. 26, 1897.
21. Oct. 20-Oct. 23, 1897.
22. Sept. 11-Sept. 20, 1898.
23. Aug. 3-Aug. 25, 1899.
24. Aug. 30-Sept. 7, 1899.
25. Sept. 8-Sept. 14, 1899.
26. Sept. 1-Sept. 11, 1900.]
The analogy of a tropical hurricane to a tornado has already been
referred to, but while a tornado may lay waste a tract of country
perhaps half a mile wide, and in exceptional cases 20 to 30 miles in
length, a hurricane is from 200 to 300 miles in diameter, and may
continue to be destructive, on account of the rapid inflow of air from
the periphery towards the centre, for 2,000 or 3,000 miles. The velocity
of the spirally blowing winds which are the characteristic feature of
these great storms is frequently 100 miles or more per hour. In spite of
their magnitude, however, the conditions leading to their origin and
growth are essentially the same as in the case of tornadoes, and
even of the much smaller whirlwinds. They have their birth where the
moist, still air above the ocean in the region of the doldrums at the
season when the equatorial belt of calms is farthest north, becomes
highly heated and rises on account of the pressing in of cooler and
heavier air from adjacent regions. The ascending column is at first
carried slowly westward, in obedience to the general flow of the
atmosphere in the intertropical belt, and at the same time the currents
coming in from opposite directions give the ascending air a rotary
motion. As the currents from the northeast are stronger than those from
other directions, this whirling motion is from right to left, or
opposite to the movement of the hands of a watch. The whirling air
column extends into the upper atmosphere, and as it moves along past the
West Indies becomes influenced by the prevalent flow of the upper
air-currents, and is carried northwestward, and later eastward in a path
which approximates to a parabolic curve. The inward-rushing spiral winds
leave a calm centre, the "eye of the storm," which corresponds to the
hollow core of a whirlwind and the calm centre sometimes noted in
tornadoes. The upward ascent of warm, humid air is accompanied by a
decrease of pressure and consequent expansion and cooling which leads to
rapid condensation and a heavy downpour of rain; the change of the
moisture from a vaporous to a liquid form liberates heat, which serves
to perpetuate the upward flow of air, and thus prolongs the life of the
storm. During the passage of the central area of low barometric pressure
over a given locality the clouds frequently part and portions of the
clear sky may be seen. Accompanying the rain are frequent lightning
flashes, as during ordinary thunder-storms.
The tropical hurricanes are the most violent and most dreaded of all the
storms that sweep over any portion of our continent, but fortunately for
dwellers on the land, are confined for the most part to the sea, since
the atmospheric conditions over the land lead to their loss of energy,
although in rare instances they may be re-enforced by uniting with a
cyclonic storm, as happened in the case of the Galveston hurricane, and
thus continued after reaching the land. The destructiveness of the
hurricanes at sea has been greatly lessened in recent years, not only on
account of the general use of steam as a motive power for vessels
instead of the wind, but because meteorologists can designate the time
when they are likely to occur and the best method of sailing away
from them if encountered. Since the establishment of the United States
stations for observing and reporting the atmospheric conditions on the
West India islands, the approach of a hurricane can be foretold and
warning given to navigators and others of the coming danger.
EVAPORATION
An important element in climate is the amount of moisture the air
contains. The absolute amount of water vapour in a given volume of air
is of interest in this connection, but what is of still greater
importance is the ratio of the amount of water vapour present to that
which the air might contain, or what is termed the _relative humidity_.
The relative humidity, providing the actual amount of vapour present
remains unchanged, depends upon the temperature of the atmosphere. For
this reason, the warming of an air-current, as the trade-winds, for
example, in which the water vapour present may, previous to the warming,
have approached saturation, causes it to have a still greater capacity,
and hence decreases the relative humidity.
The winds in passing over the land may be either cooled or warmed, and
hence their influence on evaporation is continually changing; but the
mean rate of evaporation from an open water body can be determined for a
definite time, say a year, for various localities, and thus afford a
means of comparison between one region and another. Observation of the
mean annual evaporation for various stations, mostly within the United
States, have been made, and the result shown by lines drawn through
places where the rate is the same. A map showing this data, on which the
figures indicate the depth of evaporation in inches, is here presented.
The systematic study of evaporation, and especially the part played in
it by plants, has scarcely more than been begun on this continent, and
important results concerning its influence on atmospheric conditions are
to be expected. The subject is also of great importance in reference to
agriculture, the prevention of frost, etc.
[Illustration: FIG. 27.--Depth of evaporation. After T. Russell.]
LITERATURE
The great storehouse of information pertaining to the weather and
climate of the United States is the numerous publications of the United
States Weather Bureau, Washington, D. C. Similar bureaus exist at the
capitals of Canada and Mexico, which have issued valuable reports.
Of the many elementary and popular books on meteorology, the following
will be found helpful in continuing the study of the subjects outlined
in this chapter:
DAVIS, W. M. _Whirlwinds, Cyclones, and Tornadoes_, Lee & Shepard,
Boston, 1884; _Elementary Meteorology_, Ginn & Co., Boston,
1894.
FERRIL, W. _A Popular Treatise on the Winds._ Wiley & Sons, New
York, 1889.
GREELY, A. W. _American Weather._ Dodd, Mead & Co., New York,
1888.
_Maryland Weather Service_, vol. i, Baltimore, 1899. Contains a
valuable bibliography.
RUSSELL, T. _Meteorology._ Macmillan & Co., New York, 1895.
SHALER, N. S. _Aspects of the Earth._ Scribner's Sons, New York,
1889.
WALDO, F. _Modern Meteorology_, Scribner's Sons, New York, 1893;
_Elementary Meteorology_, American Book Company, New York, 1896.