CLIMATE AND CLIMATES
Some day the meteorologists of the world will join forces to produce
a great encyclopædia of climate. No work of science is more sorely
needed, but the magnitude that it would, ideally, assume is simply
staggering.
Few people realize the multiplicity and complexity of climates. It
is a common occurrence for a prospective traveler or a business man
to write to a meteorological establishment requesting, for example,
a description of the climate of South America. Of course, no such
thing exists. A continent does not have a climate, but a multitude
of climates. Even to set forth, in general terms, the more important
types of climate that prevail between Cape Horn and Panama is no small
undertaking. Moreover, general descriptions often fail to supply the
needs of those who make inquiries about climate.
Suppose, instead of the wholesale order above mentioned, the
meteorologist receives the relatively modest request to describe
the climate of Buenos Aires or Rio de Janeiro. Is it easy to comply
with such a request? That depends. If the information is sought by
a tourist who wishes chiefly to know whether he will need light or
heavy clothing at a specified season, or whether his excursions are
likely to be hampered by frequent rains, we can enlighten him in a
few brief paragraphs. If the inquiry comes from a manufacturer who
aspires to invade the South American markets, we must know, before
replying, what kind of goods he purposes to export, and just how they
are affected by climatic conditions. Are they liable to injury by high
or low temperatures, dryness or humidity? Does the demand for them
depend, as in the case of rubber coats, upon the prevalence of rain,
or, as in the case of electric fans, upon the occurrence of hot weather
during at least a part of the year? For each branch of the export trade
certain elements of climate are important, and the more detailed and
explicit the information that can be obtained about them the better.
Suppose, again, climatic data are desired by a horticulturist who has
to solve the problem of introducing a South American plant into the
United States. In order to find the best environment for it in this
country, he should know something about the climate of its original
home. The data he requires are, however, different from those sought by
the tourist or the manufacturer. Is the plant’s habitat a region where
frosts occur? How long is the growing season? Is the rainfall rather
evenly distributed over the year, or are there definite dry and rainy
seasons? Such are some of the questions he will ask. For the purposes
of medical climatology a different set of data will be sought. The
astronomer, selecting a site for a new observatory, will ask about
freedom from clouds, and also about the pureness and steadiness of the
air that insure good “seeing.” The aviator will want information about
winds and fog. And so on.
Thus it appears that climate means very different things to different
people.
Climate has been variously defined as the sum total of weather, average
weather, typical weather, etc., but the conception is still somewhat
indefinite. We know that, while the weather of any place is subject
to incessant changes, its climate persists; but we need not assume
that it persists indefinitely. The geological record proves, on the
contrary, that vast changes of climate have occurred in the course of
long ages. In Antarctica and in Spitzbergen are found deposits of coal,
constituting the débris of ancient forests such as could not exist in
the climates now prevailing in those regions. There are plenty of other
proofs that great climatic changes have taken place from one geological
period to another; but what of changes in shorter intervals of time?
An immense amount of zeal and energy has been devoted to the study of
supposed changes of climate. Evidence of such changes is sought, on the
one hand, in a painstaking examination of weather records (a process
often involving the tabulation of hundreds of thousands of figures),
and, on the other, in the collection of geographical and historical
data bearing on the question. There have been numerous reports of the
gradual drying up of African and Asiatic lakes, of the discovery of
ancient ruins indicating that prosperous agricultural communities once
flourished in regions that are now deserts, and of various other tokens
that marked vicissitudes of climate have occurred within historic
times. A recent ingenious method of studying climatic variations is
to measure the successive annual rings seen in cross sections of old
trees. Thick rings are supposed to have been formed during periods of
abundant rainfall, and thin rings when the rainfall was deficient. This
method has been applied to the giant _Sequoias_ of California, some of
which are more than 3,000 years old.
The net result of a wide range of investigations appears to be that,
on the whole, climate has everywhere been remarkably constant since
the dawn of human history. There is much evidence that, in certain
regions, there have been alternate increases and decreases--recurrent
oscillations--of temperature, rainfall, etc., but there is little
evidence of progressive changes in one direction.
In contrast to the uncertainty that still prevails in the scientific
world on the subject of climatic changes is the confidence with which
the average layman may be heard to assert that such changes have taken
place within his own recollection. The popular idea that climate has
changed perceptibly within a single human lifetime is a world-wide
delusion, and one that has, apparently, always flourished. In the
United States we hear of the “old-fashioned winter,” with its unlimited
sleighing, and also of a marked increase or falling off in the rainfall
in certain districts. It is an interesting fact that a century and more
ago Americans were indulging in the same sort of retrospections.
In the year 1770, when Benjamin Franklin was president of the American
Philosophical Society of Philadelphia, a paper was read before that
society entitled: “An Attempt to account for the Change of Climate
which has been Observed in the Middle Colonies of North America.” It is
published in the first volume of the society’s Transactions. Barring
the long _s_’s and the use of the word “colonies,” the greater part of
it might have been addressed to the owners of automobiles and Liberty
bonds. We are told of a “very observable change of climate,” remarked
by everybody who has resided long in Pennsylvania and the neighboring
colonies. “Our winters,” says the author, “are not so intensely cold,
nor are our summers so disagreeably warm as they have been.” These
changes he ascribes to the clearing and cultivation of the country.
Another firm believer in old-fashioned winters and old-fashioned
summers was Thomas Jefferson. In his “Notes on Virginia,” written in
1781, he says:
“A change in our climate is taking place very sensibly. Both heats and
colds are become much more moderate, within the memory even of the
middle-aged. Snows are less frequent and less deep. They do not often
lie, below the mountains, more than one, two or three days, and very
rarely a week. They are remembered to have been formerly frequent,
deep, and of long continuance. The elderly inform me, the earth used to
be covered with snow about three months in every winter.”
Samuel Williams, who published a “History of Vermont” in 1794, uses
almost identical language in reference to the climate of that State.
“Snows,” he says, “are neither so frequent, deep, or of so long
continuance as they were formerly; and they are yet declining very fast
in their number, quantity, and duration.” That these changes, he adds,
“are much connected with and greatly accelerated by the cultivation of
the country cannot be doubted.”
What are the facts? When the statements above quoted were written few
regular records of the weather had been maintained for any length of
time in this country. The earliest instrumental record was begun at
Charleston, S. C., in 1730. Much information was, however, available
concerning the dates of harvest, of the formation and breaking up
of ice in rivers and harbors, and other events dependent upon the
weather, which, if anybody had taken the trouble to collect and analyze
it, would have dispelled the universal belief that marked changes of
climate had recently taken place. Nowadays it is much easier to refute
the common assertion that the climate has changed within the memory
of living men. The meteorological history of our country for more
than three-quarters of a century has been recorded from day to day by
a host of careful observers in every State of the Union. The records
show that, while the weather of one year has often differed strikingly
from that of the next, there has been no real change in climate.
“Old-fashioned” winters, for example, were neither more nor less common
half a century ago than they are to-day.
Our memories of past weather mislead us, chiefly because we remember
the exceptional weather and forget that which commonly prevailed.
Other circumstances may contribute to the illusion. Thus many people
who now live in cities, where modern appliances make them more or less
independent of the weather, passed their childhood under the more
primitive conditions of the country.
If climates were not fairly constant for long periods of years, it
would be a waste of time to compile the climatic statistics that, as
we have seen, are wanted by so many different kinds of people for so
many different purposes. Such statistics are based upon past events,
but their practical value depends upon the fact that, within certain
limits, they are a safe guide to the future.
The climatic data for any place are a sort of digest of the
meteorological observations that have been made there, special
emphasis being given to those features of the meteorological record
that bear important relations to the life and activities of mankind.
Temperature and rainfall are the leading elements of climate; others
are wind, humidity, evaporation, cloudiness, etc. We have not space
to enumerate here all the kinds of data found in elaborate climatic
tables; but in order to illustrate how the records of a meteorological
station are utilized in compiling climatic statistics and to show what
complications may arise in this process, we shall consider the question
of temperature alone.
The instruments used in measuring temperature have been described
in another chapter. From these instruments are obtained the current
temperature of the air, the wet bulb temperature (used to compute
the humidity), and the maximum and minimum temperatures of the day.
Readings are made at fixed hours, known as “term hours.” At regular
stations of the United States Weather Bureau the term hours for the
observation of all the meteorological elements are 8 a. m. and 8
p. m., Eastern Time, and an observation of temperature, humidity, and
clouds is made at noon. In most other countries tri-daily readings
have been the rule, though in Europe four or more observations a
day are now taken at many stations in order to supply the frequent
weather bulletins required by aeronauts. Important stations are
generally equipped with thermographs, which make a continuous record of
temperature.
Theoretically, the mean temperature of any day is the average of 24
hourly observations, from midnight to midnight. In practice, the mean
is generally computed from the observations at the term hours, or from
the maximum and minimum. Having obtained the mean daily temperature
for each day of a month, the average of these values gives us the
mean monthly temperature. The average of the mean temperatures for the
twelve months of the year is the mean annual temperature.
These data for each day and month, and for the year--sometimes also for
other intervals, such as five-day periods, or “pentads,”--are computed
year after year, and eventually the values for all the years of the
record are averaged to form what are called “normals.” We thus obtain,
for example, for a given station, the normal temperature for January
21, the normal temperature for the month of March, the normal annual
temperature, etc.
All this is a mere beginning toward the complete discussion of a
body of temperature observations for the purposes of climatology. We
have still to obtain from the readings of the maximum and minimum
thermometers the normal maximum and minimum temperatures and range
of temperature for each day, each month, and the year; also the
“absolute” maxima, minima, and ranges (i. e., the extreme values that
have occurred during the entire record) for corresponding intervals of
time. These data furnish answers to such questions as: What was the
lowest temperature ever recorded on January 21? What is the lowest on
an _average_ January 21? What is the average range of temperature in
March? What was the highest temperature ever recorded, on any day, at
the station?
Having thus disposed of the extremes and ranges, we may compute
what is called the “variability” of temperature, i. e., the average
difference between the means of two successive days in a given month,
and the corresponding average for the entire year. These data are of
considerable importance in medical climatology. We may also compute
the frequency of occurrence of various values among the temperature
data above enumerated. The most frequent value is often quite different
from the average value. Many climatologists compute the number of
days, in an average year, on which the temperature rises to 77 degrees
(Fahr.) or above (“summer-days”), and the number of days on which the
temperature does not rise above the freezing point (“winter-days”).
Especially valuable in agricultural regions are data of the average
and extreme dates of the last frost in spring and of the first frost
in autumn. These define the length of the “growing season.” Statistics
of the temperature of the ground at the surface and at various depths
below the surface are also of agricultural interest.
From the foregoing outline it will be seen that a bewildering variety
of climatic statistics may be computed merely from observations of
temperature, and the same is true of the other elements. Moreover, the
list set forth above is by no means exhaustive even for temperature.
In fact, there is almost no limit to the number of ways in which
the raw material of climatic data--i. e., the original records of
observation--may be grouped, averaged, or otherwise treated in order to
bring out certain features of the climate that may conceivably serve
some useful purpose. The reader will now be able to understand why a
treatise on the climate of a single locality often fills a substantial
volume.
The numerical data contained in such a work are generally supplemented
by text descriptions and by various graphic devices, such as curves
showing the normal fluctuation or “march” of a weather element during
a day, year, or other interval. Works which deal with the climates
of larger areas, such as whole countries, are usually accompanied by
climatic charts. These charts furnish a quick and easy way of getting
a general idea of the climate of a region. Among the more important
climatic charts are the following:
1. _Temperature (isothermal) charts._ These include charts showing
the distribution of normal temperatures for months, seasons, and the
year; normal range of temperature for similar periods; highest and
lowest temperatures ever recorded at the different stations; etc. Lines
known as _isotherms_ are so drawn as to pass through places having
identical values of the element in question (mean temperature, highest
temperature, etc.).
2. _Rainfall (isohyetal) charts._ These show the distribution of
rainfall (including snowfall, expressed in equivalent depth of water);
especially for each month and for the year. Other charts may show the
average number of rainy days; average snowfall (actual depth, not water
equivalent); seasonal distribution of rainfall; etc.
3. _Wind charts._ These are drawn in various forms, to show the
prevailing wind directions, the frequency of winds from different
directions, the average force of the winds, etc. Charts of the winds at
different levels above the earth’s surface will eventually be drawn for
the use of aeronauts, but such charts are still in a tentative stage.
[Illustration: A MODERN ISOTHERMAL CHART OF THE GLOBE. (_Hann, 1901_)
The isotherms show the mean annual temperature in centigrade degrees.]
As in the case of tabulated climatic data, the number of charts
that might be drawn to bring out different features of climate is
practically unlimited. Sunshine, cloudiness, humidity, barometric
pressure, and the frequency of various special phenomena, such as
thunderstorms, hail, tornadoes, droughts, etc., are all charted in
some of the more extensive works on climate. Large atlases have been
published to portray the climates of certain countries. The study of
the actual distribution of climates over the earth, as distinguished
from that of climate in general, is sometimes called _climatography_.
Climates are variously classified, usually on the basis of one or
more of the climatic elements, but sometimes with reference to their
effects. The most familiar classifications refer to temperature. We
speak of tropical, temperate, and polar climates; but in using these
terms it should not be forgotten that other things besides latitude
control the distribution of temperature. Location with respect to the
ocean or other large bodies of water is almost equally important. A
land surface grows warm by day and in summer, and grows cold by night
and in winter, much more rapidly than a water surface, and the adjacent
air varies in temperature accordingly. Hence we have a classification
of climates as _marine_ and _continental_. The former, under the
influence of oceanic winds, have a moderate range of temperature, while
the latter are subject to extremes of heat and cold. With increase
of altitude temperature is diminished, but rainfall is generally
increased. The distribution of rainfall is also determined to a great
extent by the paths of cyclonic storms. Such are a few of the many
things that control the complex distribution of climates.
People who never travel far from their own homes usually cherish quite
erroneous ideas regarding the climates of distant lands. It is hard for
most Americans to realize, for example, that the Isthmus of Panama,
in the heart of the tropics, never experiences temperatures nearly so
high as those which occur every summer in the United States. A citizen
of South Dakota, where the mercury, in the shade, frequently rises
above 100° Fahr., and has been known to reach 115°, will be inclined
to revise his definition of the term “tropical” when he learns that
at Colon, the Atlantic terminus of the Canal, a temperature as high
as 90° is decidedly exceptional, and that the maximum reading during
a period of six years was only 92°. In thirteen years Canal Zone
vital statistics showed only two deaths from sunstroke and twenty-one
non-fatal cases of heat prostration among a population of 120,000. It
will also surprise most Americans to learn that the highest natural
air temperatures that have been recorded anywhere on earth were not
observed near the equator, but in a California desert. At a place on
the edge of Death Valley, rejoicing in the ironical name of Greenland
Ranch, a temperature of 134° Fahr. was registered in July, 1913. The
thermometer which furnished this remarkable reading was a tested
instrument, installed in a standard screen over an alfalfa sod, and
not exposed to the reflected heat of the desert. At the same place the
temperature reached 100° or more on 548 days in four years. Outside
of the United States the highest temperature ever recorded at a
meteorological station was 127° Fahr. at Wargla (Ouargla), Algeria.
The lowest temperatures encountered by polar explorers are considerably
higher than those experienced each winter by the inhabitants of
northern Siberia. The “record,” so far as instrumental observations go,
is held by the town of Verkhoyansk, at which the temperature fell to
90° below zero (Fahrenheit) in February, 1892. Strange to say, this
“winter cold pole” of the earth has warm summers. At Verkhoyansk the
temperature sometimes rises to 80° above zero, or higher. At Yakutsk,
Siberia, the thermometer has been known to fall to 84° below zero in
winter and to rise 102° above zero in summer; a range of temperature
exceeding the interval between the freezing point and the boiling point
of water!
Another climatic paradox is that experienced by mountaineers who, in
scaling peaks mantled in eternal snow, often suffer with the heat,
on account of the intensity of solar radiation in the pure, dry air
of high altitudes. At the health resort of Davos, in the high Alps
(altitude 5,250 feet), invalids sit out-of-doors without wraps in
midwinter, and, indeed, are sometimes driven into the shade to escape
the too ardent rays of the sun. At the same time the temperature of the
air itself may be far below freezing, and the ground covered with snow.
Certain parts of the world are often loosely described as “rainless,”
but, as we have stated elsewhere, there is actually no spot on earth
at which rain (or snow, in the polar regions) has never been known to
fall. In the driest part of the Sahara--the Libyan Desert, between
Dakhel and Kufra--the explorer Rohlfs experienced a drenching rainstorm
of three days’ duration in 1874. Neither is the Sahara, in spite of its
proverbial heat, exempt from touches of real winter. Snow is a common
occurrence in many parts of this desert, even at moderate altitudes.
On the higher Saharan peaks snow lies on the ground all winter, and
is sometimes found, in sheltered spots, in summer. Occasional falls
of snow occur in all parts of Algeria, and several falls have been
recorded in Lower Egypt.
When all is said and done, the whole fabric of what now constitutes
the science of climatology leaves much to be desired. Climate is
of practical interest, first of all, on account of its effects
on human life and health, and secondly because of its influence
upon the crops that are the mainstay of man’s material prosperity.
Under both these heads climatic data, as now commonly presented,
ignore certain atmospheric activities of the utmost importance. For
biological purposes no description of a climate can be regarded as even
approximately complete that does not furnish, for the region under
discussion, a detailed account of the different kinds of radiation
received from the sun, their intensities and fluctuations; and there
are few places in the world at which even a beginning has been made
in the collection of such data. Again, the phenomena of atmospheric
electricity, including radioactivity, are probably of real climatic
significance, but we are still in the stage of speculation with regard
to this subject. Possibly there are still other elements of climate,
now wholly neglected, that will figure prominently in the climatology
of the future.