WEATHER MAPS AND FORECASTS
“Forecast”--with the stress on the first syllable when it is a noun,
but often on the second when it is a verb--is a word that meteorology
has made peculiarly its own. This fact is not the result of accident,
but of design.
The founder of scientific weather prediction in Great Britain was
Admiral Robert FitzRoy--the same talented officer who explored
the coasts of South America in the _Beagle_ and had Darwin for a
fellow-voyager--and his first predictions were issued in 1861 from
the Meteorological Department of the Board of Trade, which was under
his charge. The boldness of this pioneer undertaking is not easily
realized by the present generation, which is accustomed to see the
official weather forecast at the head of every daily newspaper.
Weather prognostication had previously been the undisputed province of
charlatans and quacks. For a civilized government to embark upon such
an enterprise must have seemed, to the educated public, very much like
charging the Astronomer Royal with the duty of casting horoscopes.
[Illustration: A GLASS WEATHER MAP OF THE UNITED STATES WEATHER BUREAU
(_Courtesy of U. S. Weather Bureau._)]
There is much virtue in a name. A few years ago the United States
Bureau of Fisheries persuaded the American public to eat dogfish
by changing its name to “grayfish.” Similarly, FitzRoy induced the
British public to take his weather predictions seriously by calling
them “forecasts.” The name has stuck; and nowadays, throughout the
English-speaking world, the expression “weather forecast”--except
as applied comprehensively to predictions of the “long-range”
variety--means something decidedly less chimerical than the average
weather prophecy.
[Illustration: THE SUN DRAWING WATER
(_Photograph by P. K. Budlong._)]
It is still necessary, however, to emphasize the distinction. There
are probably many people among us, well above the illiterate level,
who have no clear idea as to what constitutes a scientific weather
forecast. The distinguishing feature of such a prediction, apart from
the fact that it is made by a trained meteorologist, is that it is, in
all cases, based upon a weather map.
The forecasting machine is a big one, with its human gear spread over
a wide territory. Eventually it will be spread over the entire globe,
and then we shall have better forecasts. A little manual entitled “The
Weather Map,” published by the British Meteorological Office, says:
“The making of a single forecast in any one of the meteorological
offices of Europe, America, Australia, or the Far East requires the
organized cooperation of some hundreds of persons; about a hundred
observers who note the necessary observations simultaneously at as
many separate places and hand in their reports to the telegraphists
who transmit them to one center, where the meteorological expert
charts them on a map and draws therefrom the conclusions on which the
forecasts are based. The preparation of the map is an essential part of
the process. No meteorologist in the modern sense attempts to forecast
the weather without reference to a map prepared either by himself or
by some one with whom he is in direct communication, from observations
transmitted by telegraph for the purpose. No amount of weather wisdom
or weather lore or experience is a substitute for the map. The more
expert and accomplished the meteorologist, the more certain he is that
all he can do without the materials for constructing a map, though
he may have a barometer and other instruments at hand, is to make a
guess at what the map is like and think out from that what the weather
changes are likely to be. It is a common experience of professional
meteorologists away from their base to find themselves appealed to for
an opinion about the weather, judging from the signs of the sky alone,
because they are learned in such things. That is exactly what they are
not. Accustomed to refer everything to a map, without one they feel
themselves to be rather worse off than those who are unaccustomed to
its use. A modern meteorologist thinks in maps; his language and modes
of expression are formed thereby.”
While the weather map is prepared, first of all, for the use of the
forecaster, who makes his predictions from the map before it has passed
beyond the manuscript stage, it has other important uses, which justify
its publication and widespread distribution. The weather map is a
weather newspaper. Like other newspapers, it is founded on a system of
telegraphic dispatches and is designed to keep us in touch with what
is going on in the world. Weather news is of general interest because
weather plays a part in most of the doings of humanity. Sometimes the
news we read on the face of the map merely satisfies our curiosity; at
other times it renders us more substantial service.
By way of illustrating the manifold purposes served by weather maps,
let us set down two cases that are, perhaps, at opposite ends of the
scale of utility. Our first case is that of the traveler who scans the
map to see whether the atmospheric conditions at his distant home are
propitious, that day, for some outdoor pleasure event on the family
program. This we may describe as a sentimental use of the map. The
second case is that of an aviator embarking on a flight some time in
the fore part of the day, soon after the morning map has made its
appearance. Here is a case in which the map is of vital utility, purely
as a record of current conditions. The aviator is not concerned with
the forecast of the morrow’s weather, unless he is making an unusually
long flight, but he is immensely concerned with the winds and weather
prevailing along his route at the time he flies, and these will not, as
a rule, differ radically from the conditions shown on the map of the
same morning.
Since weather affects business in a variety of ways, people who have
business interests away from their places of residence frequently have
occasion to consult the weather map. The influence of the weather on
crops explains why the map is watched with keen interest by dealers in
agricultural products. Owners of vessels navigating the ocean or the
Great Lakes take a practical interest in the present as well as the
future location of storms. And so on. It is not necessary to prolong
this list of those who use the weather map, because the popular demand
for it speaks for itself. It is worth while to record the fact that the
demand far exceeds the supply. In this country the Weather Bureau has
been constantly harassed with urgent requests for the publication of
maps at places where, in consequence of limited appropriations, it has
not been possible to issue them.
The weather maps published in various parts of the world exhibit much
diversity in detail, though they have, of course, many features in
common. As a rule a weather map covers a wider area than that of the
country in which it is published. The aim has always been to make
these publications international, as far as practicable. The longest
continuous file of printed daily weather maps in existence, viz.,
that established in France by Le Verrier in 1863 and still published,
has been called from its beginning the “Bulletin International.” It
embraces nearly the whole of Europe, a little of Africa, Iceland,
and the Azores. The other European maps now cover the same area or a
considerable part of it. Before the war the Russian meteorological
service was issuing a map that included, in addition to Europe, a wide
zone of Asia extending all the way to the Pacific Ocean. The United
States map, as published in its most extensive form at Washington,
comprises the whole of this country and southern Canada, besides
presenting tabulated statistics for more distant parts of the world.
Manuscript maps prepared daily at Washington have a still broader
outlook; they are drawn on a base map that covers the northern
hemisphere, and the printing of a map of this sort, including a chain
of stations extending around the globe, was undertaken in 1914,
but was interrupted by the war. The map published by the Argentine
Meteorological Office, at Buenos Aires, covers more than half of South
America. Most national meteorological services issue weather maps, but
there are a few that do not. No such maps are published in South
Africa or any of the South American countries except Argentina.
[Illustration: LAND AREAS EMBRACED IN DAILY WEATHER MAPS AS PUBLISHED
IN 1921]
Thus there is still much room for the horizontal extension of the
weather map, and there is even more room for its vertical extension.
Daily weather maps for aeronauts (chiefly wind maps) are now more or
less on the programme of all the leading meteorological services,
and in a few cases their publication has already begun. Probably the
first maps of this character, showing the winds at various levels over
a whole country, were those that began to appear in Italy in 1913.
The British Meteorological Office now publishes such maps, showing
winds and clouds at different levels over the British Isles at three
hours of the day. In the United States maps of the “wind aloft” are
prepared daily, at Washington, from the reports of kite and balloon
stations, but they are not yet published. The Weather Bureau has,
moreover, invented an ingenious method of depicting the winds at
several levels on a single map; in other words, constructing a map in
three dimensions. This consists of attaching arrows to little metal
posts erected on an ordinary weather map at points corresponding to
the location of the upper-air stations. Each post bears a series of
arrows--one arrow for each level charted--and the arrows are set in
positions showing the direction of the wind at each level. Numbers on
the arrowheads indicate the force of the wind. When the map is finished
it is photographed from two different angles so as to make a pair of
pictures suitable for viewing through a stereoscope. These stereoscopic
pictures were formerly made every day and a file of them is available
for reference and study.
[Illustration: STEREOSCOPIC MAP OF THE “WINDS ALOFT”]
There are a few conspicuous points of difference between the weather
maps issued in foreign countries and those issued in this country.
Thus a majority of the foreign weather services publish two or more
charts on the same sheet; either for the sake of showing different
meteorological elements separately or, in most cases, to represent
the conditions prevailing not only at the hour of the current morning
observation, but also at certain hours of the previous day. One of the
three editions of the British map includes four charts, corresponding
to observations at four different hours. By means of such series of
charts one can observe the recent changes of weather as well as the
current conditions. Weather maps published in the United States show
primarily the conditions at 8 a. m., Eastern Standard Time, of the
morning of issue; though certain features of past weather are also
indicated, including changes of temperatures, movements of storm
centers, etc. Evening maps are drawn at Washington and at many other
places, but are not published.
In this country the publication of weather maps has been carried out on
a much more liberal scale than elsewhere. Instead of issuing maps at
only one or a few places, as is the custom in other parts of the world,
it has been the policy of the American service to publish them at
populous centers all over the country. In some cases they are printed
or manifolded at the local Weather Bureau station, and distributed
by mail and messenger; in other cases they are published in the
newspapers. The daily circulation of the maps has thus, at times, run
up into the millions. This comprehensive duplication of the chart is
made possible by special arrangements with the telegraph companies. The
reports of observations are, to a large extent, sent over circuits,
along which the telegraph offices, besides forwarding the local report,
copy the reports from other stations as they pass over the wires.
Certain stations, forming connecting links between the circuits, effect
the transfer of collected reports from one circuit to another; so
that, in a very short time, upward of 150 stations receive the reports
from a large number of other stations. The maps issued at stations or
published in newspapers are generally rather crude, though they answer
their purpose; but the large lithographed map issued every day at the
Central Office, in Washington, is much the most artistic production of
its kind published anywhere in the world.
[Illustration: ENLARGED SECTION OF THE STEREOSCOPIC WIND MAP]
Large weather maps, drawn with colored chalk on a ground-glass base,
may be seen at certain produce exchanges and railway stations, on
the “boardwalk” at Atlantic City, and in the Capitol at Washington.
Motion-picture weather maps, made from series of maps showing
conditions at successive intervals of time, have been prepared
experimentally in this country and abroad.
The reports used in the construction of weather maps are telegraphed
from the stations in cipher, in order to save expense. In Europe groups
of figures are used for this purpose, but the United States Weather
Bureau makes use of a word code, which offers the advantage over a
figure code that, as a rule, mistakes in the telegrams can easily be
detected by anybody familiar with the code. The American weather code
is something of a literary curiosity. In each of the many thousand
words it contains there are certain significant letters, and these must
fall in certain sequences in order to convey the information required.
The English language has been ransacked--and somewhat stretched--to
secure the necessary words. Observers consult the code book in
enciphering their reports, but translating is easily done without the
book by those who have mastered the relatively simple principles on
which the code is constructed.
[Illustration: SIMPLIFIED WEATHER MAP FOR JAN. 25, 1905, 8 A. M.,
EASTERN STANDARD TIME]
The language of lines, shadings and symbols used in weather maps can
be learned in a few minutes, and it is, as a rule, fully explained on
the face of the map. This is true of foreign maps as well as American.
A full-fledged weather map is hardly susceptible of reproduction
in a book of ordinary dimensions. The simplified map that we show
here, taken from a Weather Bureau bulletin, will, however, serve to
illustrate some of the features of such publications. The reader should
first fix in his mind the explanations printed at the lower left-hand
corner of the map and then study the map in the light of what has been
said in Chapter VIII about the circulation and movements of highs and
lows. On this map we have an exceptionally well-developed high over
the middle of the country and a pronounced low on the Atlantic coast.
The former, with clear skies and very cold weather, constitutes a cold
wave. The latter is attended by a widespread snow storm and, as may be
inferred from the crowded isobars, by stormy winds.
Now bear in mind the fact that charts identical with this one, except
that they contained much more detailed information, were issued in
all the more important cities and towns of the United States on the
morning of January 25, 1905, about a couple hours after the taking
of the morning observations, at 8 a. m., Eastern time. The same
morning, weather forecasts, cold-wave warnings and storm warnings,
deduced from this map, were issued to some hundreds of thousands of
addresses by telegraph, telephone, mail, and messenger. The map itself
conveys information comparable in interest to the news of public
events published on the first page of the newspapers the same day.
The forecasts and especially the warnings are, in such a case, worth
millions of dollars to the people of the United States. The following
account of the cold wave appears in the Annual Report of the Chief of
the Weather Bureau for 1905:
“A severe cold wave appeared over the Dakotas, Minnesota, Nebraska, and
Iowa on January 24, 1905, and on the 25th covered the central and upper
Mississippi valleys and extended over the northern portions of the
east Gulf States, the line of zero temperature reaching into northern
Tennessee. On the 26th the cold wave covered Florida, and temperatures
below freezing were reported as far south as Tampa and Jupiter. At the
latter place, the minimum temperature, 24 degrees, equaled the lowest
ever recorded since the establishment of the Weather Bureau station at
that point. Considerable damage was done to orange trees where groves
could not be fired or protected. Ample warnings had been given of the
expected low temperatures.”
The subject of making forecasts from a weather map is one concerning
which some big books have been written, and it cannot be dealt with
very satisfactorily in the brief space at our disposal. There are two
cardinal rules--viz., (1) the weather has a characteristic distribution
in relation to the distribution of barometric pressure, and (2)
pressure systems, with their attendant winds and weather, move, in
a general way, from west to east--but these rules require various
qualifications and are subject to various exceptions. Thus highs and
lows generally take rather circuitous routes in getting, eventually, to
the eastward, and sometimes they break up or fade out. The high shown
on the annexed map actually moved much more south than east during the
following twenty-four hours, while the low moved up the coast; i. e.,
more north than east. These were, however, the movements expected by
the experienced forecaster.
Well-developed highs are nearly always regions of clear weather and, in
winter, of cold weather. Lows are attended by clouds and precipitation;
rising temperature usually precedes them and falling temperature
follows them. The professional forecaster recognizes several types of
pressure distribution other than ordinary highs and lows, and they,
also, have their characteristic winds and weather. The seven typical
forms of isobars, as classified many years ago by R. Abercromby, are:
Cyclone; anticyclone; secondary; V-shaped depression, or trough;
wedge of high pressure; col, or saddle, between two anticyclones; and
straight isobars. These may be combined in a variety of ways on the
weather map. The forecaster learns to classify these combinations
consciously or subconsciously, and grows familiar with their habits and
mannerisms. Forecasters spend a good deal of time in studying the files
of weather maps for past years; but the results of such studies are not
easy to reduce to definite statements. Scientific forecasting is, in
its present stage, almost wholly empirical. The dependence of weather
changes upon the phenomena of atmospheric circulation is generally
easy to make out, but the vagaries of winds and pressure are still in
the main mysterious; notwithstanding such interesting developments as
(1) the much-discussed rules of M. Gabriel Guilbert for predicting the
movements of barometric depressions from abnormalities in the force and
direction of the winds; (2) the systematic charting of “isallobars,”
or lines of equal pressure change, associated especially with the name
of Dr. Nils Ekholm; and finally (3) the hypothesis of a sharp line
of demarcation between masses of equatorial and polar air along the
so-called “polar front,” forming the basis of a system of forecasting
that originated at the Geophysical Institute of Bergen, Norway, and has
had a marked influence upon the methods of forecasters in other parts
of the world.
As to the practical results of this empirical art, one point of the
utmost importance is commonly overlooked by the public when it
complains about the mistakes of the forecaster. He is required to make
forecasts of weather day after day, regardless of the kind of map
that is laid before him. Sometimes the map is so featureless (or, as
the forecasters say, “flat”) that there is little in it on which to
build a forecast. At other times there is an abundance of features,
but they are in process of rapid and disconcerting change. In either
case the ordinary day-to-day weather forecast is likely to go astray.
The brighter side of this picture is that the atmospheric phenomena
that count heavily in terms of dollars and imperiled human lives are
not found on “flat” maps, and, when they appear on the map, generally
behave in a simple, straightforward way. In other words, such events as
great storms and cold waves are far easier to forecast than everyday
weather, and it is the successful prediction of these events that
furnishes the principal _raison d’être_ of an expensive telegraphic
forecasting service.
Since weather predictions serve a variety of purposes, many different
kinds of forecasts and warnings have been developed by meteorological
services. In this country there are, first of all, district forecasts
and local forecasts; the former, covering whole States and groups of
States, being issued at a few main forecasting centers, while the
latter, applying to a single town and its vicinity, are issued at a
large number of the ordinary Weather Bureau stations. There is a long
list of forecasts and warnings intended for special classes of the
community; indeed the specialization of forecasts is carried so far
that an individual citizen or a single business firm can generally
obtain, by asking for it, a forecast of any specified predictable
feature of the weather for a particular place. The established types
of special prediction issued regularly, or when conditions warrant,
include wind and weather forecasts and storm warnings for mariners;
shippers’ forecasts, relating to temperatures injurious to perishable
goods; aviation forecasts; “fire-weather” warnings, issued when the
weather is conducive to fires in the western forests; avalanche
warnings; and several different kinds of advices for the benefit
of agriculture and horticulture. The United States Weather Bureau,
although it is not the only branch of the Government that carries
on work in hydrology, is the one charged with the duty of issuing
river-stage predictions and flood warnings. An elaborate organization
is maintained for this purpose, and the results are extremely
successful.
The period of time covered by an official weather forecast is
generally from one to two days. In this country the morning forecast
is ordinarily for 36 hours from 8 a. m., and the evening forecast for
48 hours from 8 p. m., but occasionally the period is extended for an
additional day. For some years the Weather Bureau has issued every
Saturday a forecast in quite general terms relating to the whole of
the following week. These long-range forecasts are made for extensive
areas of the country, such as the North and Middle Atlantic States, the
Ohio Valley and Tennessee, and the Great Lakes region. “From the weekly
forecast,” says an official publication, “a farmer may know whether
it is safe to cut his hay at the beginning of the week or whether
it would be better to wait till the last of the week; and a produce
dealer may know whether it is safe, at a particular time in the early
spring, to start a carload of strawberries to a northern market.” The
British Meteorological Office follows a more cautious plan. Its regular
forecasts are for twenty-four hours, but occasionally, when conditions
are fairly settled, announcements are made of what is termed the
“further outlook.” The same office sends notices to the agricultural
districts when a spell of fine weather, favorable for haying or the
like, appears to be on the programme.
There are a few official meteorological establishments that have
embarked on much more ambitious undertakings in long-range forecasting.
The classic example is furnished by the Indian Meteorological
Department, which has issued seasonal forecasts of rainfall ever since
1882. These were originally based upon reports of the snowfall in
the Himalaya, abnormalities of which, as noted in the spring, appear
to be related to the intensity of the subsequent monsoon rainfall.
Eventually the Indian meteorologists began to seek in more remote
regions for clues to the character of the Indian seasons, and they
believe they have found them; the barometric pressure in South America
and at Mauritius, the rainfall at Zanzibar and Seychelles, the Nile
flood, and summer rains in Australia all seems to bear some relation to
meteorological conditions in India.
The study of world-wide interrelations of weather, although it has
not generally, as in the case just mentioned, furnished the basis
of official forecasts, has engaged the attention of a great many
able meteorologists. We have spoken on another page of the “centers
of action” that seem to be such important indexes to changes in the
circulation of the atmosphere, with concomitant fluctuations in
weather. Telegraphic reports from some of these centers, including the
Iceland and Aleutian lows and the Siberian and Azores highs, have
helped to guide the Weather Bureau in making its weekly forecasts. Many
attempts have been made to predict the weather months in advance from
variations in the temperatures of the water in different parts of the
ocean or from the distribution of sea ice in high latitudes. Another
line of attack upon the problem of long-range forecasting is through
observations of solar activities, as indicated by fluctuations in solar
radiation, the prevalence of sun spots, etc. Lastly, an immense amount
of energy has been expended in efforts to detect definite cycles or
periodicities in the weather itself, without regard to their causes.
The thirty-five year period of rainfall and temperature variations,
announced in 1890 by Prof. E. Brückner, has found a place in all the
current textbooks on meteorology, and several other alleged weather
periods have been the subject of serious discussion.
From all of which it appears that the professional meteorologist is
not at all inclined to discountenance attempts at long-range weather
prediction, provided they are made both honestly and intelligently.
Unfortunately the vast majority of people who, in all ages, have
indulged in this sort of vaticination--and their name is legion--have
been either dishonest or ignorant, or both. The world is still well
supplied with them, and they are, undeniably, a thorn in the flesh
of the scientific forecaster, who sometimes sees his predictions
confounded with theirs by the public, and who commonly incurs the
charge of jealousy and narrow-mindedness because he declines to
acknowledge brotherhood with the cranks and impostors who hang about
the outskirts of his profession.
Quack weather predictions are nearly always made for long periods in
advance, and their popularity depends upon the fact that they give
the public something--however fallacious it may be--that science
does not attempt to give. The making of such predictions appears to
be a particularly easy way of acquiring both fame and fortune. In
this country there has hardly ever been a time when some exponent
of this industry did not enjoy a nation-wide reputation. It is a
satisfaction to record, however, that foreign countries produce the
same sort of celebrities. Dr. Gustav Hellmann, writing in Germany,
has recently published an extremely interesting account of the famous
“weather prophets” of the 19th and 20th centuries. Their geographical
distribution is given as follows: Belgium, 2; Germany, 36; England,
25; France, 14; Italy, 2; Austria-Hungary, 8; Russia, 1; Sweden, 1;
Switzerland, 5; Spain, 2; North America, 9. The list for the United
States is, to be sure, conspicuously incomplete, but we need not grieve
over the fact that the fame of the American prophets omitted from the
list has not spread to the Old World.
The almanac is, as it has always been, the chief stronghold of
long-range weather predicting. Nobody knows to what extent the almanac
prognostications are taken seriously by the public, or are meant
to be by the publishers. It is to be feared that the percentage of
the population that “swears by them” is not inconsiderable. Almanac
publishers would undoubtedly perform a public service, and perhaps
save themselves some pangs of conscience, if they would append to
their weather predictions the statement that, like the portrait of the
gentleman who displays his anatomy to the signs of the zodiac at the
front of the book, they are published merely for the sake of keeping
up an old custom, and if they would conclude every almanac with the
following candid avowal, which we find in Gabriel Frende’s “Almanack
and Prognostication” for 1589:
Thou hast my guess at daily weather
Here present in thy view.
My credit shall not lie thereon
That every word is true:
Yet some to please I thought it best
To shew my mynde among the reste.