WEATHER AND WEATHER INSTRUMENTS
The fact that a vast proportion of the conversations in which human
beings engage begin with remarks about the weather has often been
noted, but perhaps never fully explained. Meteorologists sometimes
adduce this fact as evidence that weather is a subject of overshadowing
importance. This bit of reasoning will not, however, bear critical
analysis. It carries with it the implication that people talk about
weather because weather is uppermost in their thoughts. How often is
such the case? Brown, meeting Jones, remarks that it is a fine day. Are
we to infer that Brown was meditating upon the agreeable state of the
atmosphere before he vouchsafed this not altogether novel observation?
Hardly. There is about one chance in a thousand that weather was in his
mind at all.
It is a plausible thesis that people talk so much about weather
because, at an earlier period in the history of mankind, this subject
_was_ of supreme importance. Perhaps it is a custom handed down from
our remote ancestors, whose occupations were nearly all carried on
out-of-doors and who enjoyed but a precarious shelter from the elements
in their rude habitations. In India, as the period of the monsoon rains
approaches, anxiety about the timely arrival and the abundance of
these showers eclipses all other thoughts in the mind of the peasant,
because a severe drought at this season means a famine. When our
forefathers lived by hunting, fishing, and crude systems of grazing and
agriculture, they were, no doubt, equally solicitous about atmospheric
conditions that directly affected their food supply. In those days
comments on the weather were by no means empty formulas. Men rejoiced
together that the day was fine, because it was a circumstance upon
which their dinner depended; and the prehistoric equivalent of “What
beastly weather!” was probably accompanied by a significant tightening
of the belt.
Certain it is that in very early times people gave a great deal of
attention to the weather and acquired a fund of wisdom on the subject
which, along with a certain amount of superstitious unwisdom, has come
down to us in the shape of weather proverbs. Many of these proverbs
undoubtedly originated before the dawn of history, for they are found
in substantially the same form among widely scattered races of mankind.
Various popular weather prognostics familiar at the present day are
mentioned in such ancient documents as the Vedas, the Bible, and the
cuneiform tablets from the library of Assurbanipal.
Speculations about the weather occupy much space in the writings of
the Greek philosophers, and a formal treatise on meteorology, written
by Aristotle (fourth century B. C.), remained the standard work on
this subject for two thousand years. More or less systematic weather
records were kept by the Greeks long before the Christian era, and they
produced a number of almanacs, in the shape of marble tablets, showing
the average winds and weather for particular dates throughout the
year. A copious collection of the weather indications found in both
Greek and Roman almanacs, dating back to the fifth century B. C., has
been made by Dr. Gustav Hellmann.
Some of the meteorological instruments used today have a very
respectable antiquity. Ancient statistics of the rainfall of India,
recently brought to light, show that some sort of rain gauge must
have been in use in that country in the fourth century before our
era. Measurements of rainfall were made in Palestine in the first
century A. D. The only other meteorological instrument dating back to
classical antiquity, so far as known, is the weather vane. The Tower
of the Winds, at Athens, built about a century before the Christian
era, originally bore at its summit a vane in the shape of a bronze
Triton, holding in his hand a wand, which was designed to point at
one or another of the eight symbolical figures of the principal winds
surrounding the octagonal tower, thus showing which way the wind was
blowing at the time. The Roman writer Varro has left us a description
of a vane that could be read indoors by means of a dial on the ceiling.
Instrumental weather observations did not become the rule, however,
until the end of the seventeenth century, when the use of thermometers,
hygrometers, barometers, and rain gauges began in Italy and spread
rapidly to other countries. The origin of each of these instruments is
commonly ascribed to a particular inventor--the thermometer to Galileo,
the barometer to Torricelli, etc.--but the truth is that the idea of
the instrument was, in each case, a slow growth, to which many minds
contributed. Thus a form of thermoscope--a device for showing but
not for measuring the expansion and contraction of air with changes
of temperature--was described by Philo of Byzantium in the third
century B. C. Galileo supplied such an instrument with a scale, but
without fixed points, thus converting it into a crude thermometer, but
it was not until half a century later that the Grand Duke Ferdinand
II of Tuscany introduced the idea of filling the thermometer with
alcohol, in place of air, and sealing it so that it was not affected
by changes in barometric pressure. The thermometric scale now used in
English-speaking countries, which bears the name of Fahrenheit, appears
to have been devised by the Danish astronomer Ole Römer, from whom
Fahrenheit borrowed it. In short, any _brief_ account of the invention
of the principal meteorological instruments necessarily ignores the
just claims of many inventors; to say nothing of the fact that what is
written on the subject to-day is likely to be refuted to-morrow by the
discovery of some forgotten book or manuscript.
We are on safer ground in saying that the plan of _measuring_ the
weather, instead of merely observing it, became general early in
the eighteenth century; and that about the middle of the nineteenth
century the further improvement was introduced of making meteorological
instruments trace their own records, so that the human observer was, to
a great extent, dispensed with. Self-registering instruments are now
the rule at important meteorological observatories and stations, though
they do not, even yet, record all the elements of weather, and at a
host of minor stations none of them have yet replaced the eye of the
observer.
Now let us see what things go to make up the weather, and how these
things are observed by the modern meteorologist.
The pressure of the atmosphere, if not exactly a part of the weather,
is so intimately associated with it that we cannot exclude it from our
list of weather phenomena. Atmospheric pressure is measured with the
_barometer_, and the importance of this instrument as a key to weather
changes is fully recognized--and indeed overrated--by the layman, who
sometimes calls it the “weather glass.”
[Illustration: MERCURIAL BAROMETER (Fortin type)]
Until recently all British and American barometers were read in
inches and all others in millimeters. Since atmospheric pressure is
a force, the practice of measuring it in units of length is rather
like measuring time in bushels or potatoes in hours. The inconsistency
is serious from a scientific point of view, because it divorces
barometric measurements from other physical measurements, in which
pressures are measured in units that have nothing to do with length;
viz., dynes per square centimeter. Accordingly, some of the leading
meteorological services of the world have lately adopted a new unit
of barometric pressure, known as the _bar_, which is equivalent to
1,000,000 dynes per square centimeter. It is subdivided according to
the ordinary metric notation, and its most commonly used subdivision is
the _millibar_, equivalent to 0.03 inch on the old-fashioned barometer
scale, under standard conditions.
[Illustration: ANEROID BAROMETER, GRADUATED IN MILLIBARS AND INCHES
For the benefit of sailors a curve is shown indicating the _mean annual
pressure_ in different latitudes along the meridian of 30° W. (Courtesy
of the British Meteorological Office.)]
The mercurial barometer is so delicate and cumbersome that for many
practical purposes it is replaced by the more convenient though less
accurate _aneroid barometer_. A self-recording barometer (usually an
aneroid) is called a _barograph_. In its ordinary form, this instrument
carries a pen, which traces a continuous record of the barometric
pressure on a strip of paper wound around a cylinder turned by
clockwork. Generally the instrument runs for a week before the paper
has to be changed. The barograph is a very instructive instrument,
because it shows, not only the pressure, but also the _changes_ of
pressure--i. e., just how fast the barometer is rising or falling, or,
as meteorologists say, the “barometric tendency.” The way in which
barometric changes are related to weather will appear in a later part
of this book.
The mercurial barometer consists of a glass tube, sealed at its upper
end and having at its lower end a “cistern,” which is open to the air.
The tube is filled with mercury at its open end, and then inverted over
the cistern, and the mercury descends until the weight of the portion
standing above the level of the mercury in the cistern just balances
the pressure of the air on an area equal to the cross section of the
tube. The height of the mercurial column is read from a graduated
scale attached to the tube. Certain corrections are applied to the
reading, in order to eliminate variations due to temperature, etc.,
and, if to be entered on a weather map, the reading is reduced to
sea-level value. In the aneroid barometer, a thin-walled metal box,
exhausted of air, undergoes changes of shape in response to changes
in atmospheric pressure. The movements of the box are communicated by
levers to a pointer moving around a dial (or to the recording pen, in
the barograph).
Since the pressure of the atmosphere diminishes with increasing
altitude at a fairly definite rate, the barometer is used for measuring
heights. Sometimes it is graduated directly, for this purpose, in feet
or meters, and it is then called an _altimeter_.
Among the meteorological elements that unmistakably pertain to weather
the most important is the _temperature_ of the air. The thermometer,
with which temperature is measured, is, in its common form and in its
essential features, too familiar to require description here; but we
may remark that, as in the case of the barometer, several methods of
graduating this instrument have been used. Besides numerous obsolete
systems, there are three different thermometric scales--the Fahrenheit,
the Centigrade, and the Absolute. The first is still the prevailing
one in English-speaking countries, and the second prevails in all
other countries. The Absolute scale, long familiar to physicists, has
recently come into somewhat limited use in meteorology. It starts at
the “absolute zero”--the temperature of a body totally devoid of heat.
This temperature has been nearly attained in laboratory experiments
with liquid helium. One advantage that the Absolute scale possesses
over the others is that it has no below-zero readings. Such readings
are a source of occasional errors when temperature is recorded on the
Fahrenheit or the Centigrade scale.
The freezing point of water is 32° Fahrenheit = 0° Centigrade = 273°
Absolute. The boiling point of water, at sea level, is 212° Fahrenheit
= 100° Centigrade = 373° Absolute.
While the layman is well acquainted with the thermometer, he sometimes
fails to understand certain differences between the scientific and
unscientific methods of using this instrument for weather-measuring
purposes. On a hot summer day he is, perhaps, inclined to feel
aggrieved because the official record of temperature does not
adequately express the state of his feelings, to say nothing of being
at odds with the impressive instrument displayed at the corner drug
store. Hence the following explanation is in order:
It is the function of the official thermometer to indicate the true
temperature of the _air_. A thermometer exposed to direct sunshine
records its own temperature--i. e., the temperature of the glass and
mercury--and nothing else. A thermometer “in the shade”--under a tree,
for example--comes nearer to showing the true air temperature; but it
is exposed to radiation from surrounding objects and its readings will
vary with the nature and location of these objects. The meteorological
thermometer is nearly always installed in a kind of latticed screen,
or shelter. It is thus largely protected from radiation, while the air
circulates freely around it. Only when thermometers are exposed under
such standard conditions is it possible to obtain comparable readings
of the temperature at different places, so that, for instance, maps may
be drawn showing the distribution of this element over a country. The
best location for the thermometer screen is a few feet above sod. Many
thermometers of the United States Weather Bureau are installed on the
roofs of tall buildings; not because this is an ideal location, but
because no better is available in the heart of a large city, where, for
practical reasons, the office has to be placed. In many small towns the
site of the station is such that the thermometer screen (or “instrument
shelter,” as it is called in the Weather Bureau) can be placed close
to the ground, and at the same time get ample ventilation and be free
from the radiation of buildings. In certain large cities the Bureau
maintains a branch station in a park or in the suburbs, where a
satisfactory exposure for all instruments can be secured.
The artificial temperature of a city street is too local and indefinite
a thing to be inscribed on weather maps, utilized by the forecaster,
or embodied in climatic statistics. As a concession, however, to the
demand of the “man in the street” for a record of conditions prevailing
in his own sphere, the Weather Bureau has installed in several cities
little pavilions in which working meteorological instruments are
displayed for the benefit of the public. The thermometers in these
so-called “kiosks”--which are modeled, with improvements, after the
weather pavilions found at European health resorts--always read several
degrees higher in hot weather than the thermometer at the regular
Weather Bureau station in the same vicinity. Such records are erratic,
at best, and present indications are that the kiosks will eventually be
abolished.
[Illustration: MAXIMUM AND MINIMUM THERMOMETERS]
Besides the ordinary thermometer, there are instruments that answer the
questions “How hot was it to-day?” and “How cold was it last night?”
These are known, respectively, as the _maximum_ and the _minimum
thermometer_. They hang almost horizontally in the screen. The former
has a constriction just above the bulb, which prevents the mercury
from retreating after it has reached the highest reading for the day.
It can be reset by whirling it on a pivot. The minimum thermometer
is filled with spirit instead of mercury. A little index inside the
column is carried toward the bulb by the surface of the alcohol as the
temperature falls. When the temperature rises the index remains behind,
marking the lowest point reached. The highest and lowest temperature
of the day, as well as the temperature at any moment of the day,
can be read from the _thermograph_, or self-registering thermometer.
In the commonest type of thermograph changes of temperature alter
the curvature of a flexible metal tube filled with spirit, and the
movements of the free end of the tube are communicated by levers to a
recording pen.
On an average day, in our climates, the air is coldest about sunrise.
The appearance of the sun checks the atmospheric cooling due to the
loss of heat from the earth that has been going on through the night,
and the air begins to warm up. As long as the amount of incoming heat
from the sun is greater than the amount of outgoing heat from the
earth, the temperature will continue to rise. After noon, when the
sun is highest, the supply of solar heat diminishes, but it is still
greater, for a time, than the heat loss from the earth, and for this
reason the temperature, as a rule, keeps on rising until some time
toward the middle of the afternoon, when the maximum temperature of the
day occurs.
_Humidity_ is an element of weather that is more often talked about
than understood. Atmospheric humidity is the state of the atmosphere
with respect to the amount of moisture it contains in a gaseous form,
not in the form of a liquid. This gaseous moisture is called _water
vapor_, and it is not directly perceptible to the senses, as liquid
water is. As we have explained elsewhere, the capacity of the air for
water vapor increases with the temperature. The actual amount present
at any time, per unit volume, is called the _absolute humidity_, and
the ratio of this amount to the maximum amount the air can hold at the
same temperature is called the _relative humidity_. The latter is
generally expressed in percentage. When the air is charged to its full
capacity with aqueous vapor its relative humidity is 100 per cent.
The relative humidity usually varies greatly through the day, being
generally lowest when the temperature is highest, and _vice versa_. It
is an element of much practical interest, because it is one of the main
factors in determining the drying power of the air, the other important
one being wind. The air feels dry when evaporation proceeds rapidly
from our skin, either on account of low relative humidity, brisk
air movement, or both. People are hardly conscious of high relative
humidity except when, in hot weather, it retards the evaporation of
perspiration, and the latter collects in liquid form on the skin.
[Illustration: THERMOGRAPH]
Relative humidity does not owe its importance in human affairs solely
to its physiological effects, for it plays a prominent part in
numerous industries--textile, metallurgical, chemical, leather, food,
and all those employing drying processes. In the spinning of cotton
and wool, for example, the humidity of the workroom greatly affects
the weight of the material, the size of the yarn, and the length
and flexibility of the fibers. Humidity must likewise be taken into
account in such diverse industries as manufacturing candy, bread, high
explosives and photographic films, drying macaroni and tobacco, and
operating blast furnaces. There are engineers who specialize in the
business of installing “humidifying” and “dehumidifying” systems in
workshops, and also, for hygienic purposes, in schoolhouses and other
public buildings.
The absolute humidity, the relative humidity and the _dew point_ (the
temperature to which the air must be brought to start condensation
of its moisture) are all determined by means of instruments called
_hygrometers_. The hair hygrometer depends for its action upon the
fact that a hair, freed from oil, not only absorbs moisture from
the atmosphere, but elongates when damp and contracts when dry. The
instrument, which includes a single human hair or a bundle of such
hairs, is so designed that these changes move an index over a graduated
scale. This and other types of hygrometer can be arranged to record
their own readings continuously, constituting a _hygrograph_.
The form of hygrometer most commonly met with at meteorological
stations is called a _psychrometer_. This usually consists of a pair
of mercurial thermometers, one of which, known as the “wet-bulb
thermometer,” has its bulb wrapped in thin muslin. The other, called
the “dry-bulb,” is an ordinary thermometer. The muslin is moistened,
either just before making a reading, or continuously with a wick. In
the former case the thermometer is generally whirled several times
before the reading is taken. Unless the air is saturated, the wet bulb
is cooled by evaporation, and the difference between the readings of
the two instruments enables the observer, with the aid of suitable
tables, to obtain the absolute and relative humidity and the dew
point. The most accurate results are obtained from the _aspiration
psychrometer_, of Assmann, in which air is drawn past the bulb of the
thermometer by a small fan, driven by clockwork.
Deposits of liquid and frozen water from the atmosphere, in their
various forms, are known collectively as “precipitation,” and in
the aggregate they constitute a feature of the weather hardly less
important than temperature. Indeed an average rainstorm or snowstorm is
a more obtrusive event than any other equally common manifestation of
the weather; while an excess of precipitation or a prolonged lack of
it, constituting a _drought_, may be as serious in its consequences as
a “hot wave” or a “freeze.”
Precipitation--familiarly called “rainfall”--is much more extensively
measured than any other meteorological element, for there are,
throughout the world, a vast number of places at which this is the only
feature of the weather that is regularly observed. In Europe alone
there are about 19,000 “rainfall stations.” Rainfall is measured in
depth; viz., in inches or millimeters. A moderate shower of several
hours’ duration will yield an inch or two of rain, while in extreme
cases several inches may fall in an hour. Snow is sometimes measured
as such--i. e., the actual depth that falls, or, more commonly, the
amount lying on the ground from day to day--but in order that records
of snowfall may be combined with those of rainfall for the purpose of
determining the total precipitation, the snowfall must be reduced to
its “water equivalent,” either by melting the snow before measurement
or by estimating this equivalent or by weighing the snow caught in a
receiver of known area and computing the corresponding depth of water.
There are many kinds of _rain gauge_. As a rule the gauge has a
funnel-shaped receiver with a small opening through which the water
flows into the lower part of the gauge; loss of the accumulated water
by evaporation is thus checked. There is usually some device for
magnifying the depth of rainfall in order to facilitate measurement.
In American gauges the rain flows into an inner tube having one-tenth
the horizontal area of the receiver, and its depth is thus magnified
ten times. A measurement is made by thrusting a graduated wooden stick
to the bottom of the tube and noting the height to which the stick is
wetted.
[Illustration: TIPPING-BUCKET RAIN GAUGE]
Of devices for obtaining an automatic record of rainfall, the _tipping
bucket_ (or, as the British call it, the “tilting bucket”) is probably
the most serviceable, and it is the one most widely used in this
country. This instrument is as simple as it is ingenious. The “bucket”
is a little metal trough, pivoted in the middle, so that it can tilt
back and forth, seesaw-fashion. It is divided into two compartments by
a central partition. Rain falling into the funnel-shaped receiver at
the top of the gauge flows into whichever compartment of the bucket is
uppermost, until the weight of the water causes the bucket to tip,
thus emptying one compartment and presenting the other to the incoming
stream. When the second compartment is filled, the bucket tips in the
opposite direction. The parts of the gauge are of such dimensions that
each tip of the bucket corresponds to 0.01 inch of rainfall. The gauge
is connected electrically with registering apparatus indoors, so that
every tip of the bucket is recorded. The registration sheet shows the
time of occurrence as well as the amount of rainfall.
The two most important things about the wind that are observed and
recorded by meteorologists are its direction and its force. It is
the universal custom to regard as “the direction of the wind” the
direction _from_ which, rather than toward which, it blows. Moreover,
it is only the horizontal direction of the wind that is ordinarily
observed, though many winds have a considerable upward or downward
slant, and, locally, a wind may even blow straight up or straight
down. The direction of the wind may be observed in several makeshift
ways, such as by watching the drift of smoke from chimneys, or, as
sailors do, holding up a wet finger to the breeze. Instrumentally and
scientifically it is observed with a special type of _vane_, much more
accurate in its indications than the weather vanes and weather cocks of
ornamental and symbolical architecture. The nonscientific vane, once
set in motion, is likely to be carried too far by its own momentum,
and may even spin completely around under a sudden impulse. In the
scientific vane this tendency is restrained by means of a spread tail;
the pressure of the wind on the diverging blades serving to hold the
vane in the correct position. The vane, like most other meteorological
instruments, is self-recording at all important meteorological
stations. The type used by the Weather Bureau registers the direction
of the wind every minute.
The force of the wind is obtained from an _anemometer_. Most
anemometers do not, however, show this directly, but are designed to
measure the speed or so-called “velocity” of the wind, from which its
force may be computed. The speed is observed in miles per hour or
meters per second. In considering some of the possible effects of wind
it is well to bear in mind that its force increases as the square of
the velocity. This means, for example, that a wind of 20 miles an hour
is four times as strong, and one of 30 miles an hour nine times as
strong as a wind of 10 miles an hour.
One of the external features of a weather station that invariably
attracts the attention of the passer-by is an instrument consisting
of four hemispherical cups revolving horizontally in the wind. This
scientific whirligig is the _Robinson cup anemometer_, which, in spite
of its shortcomings, is the most widely used instrument of its class
throughout the world. As generally constructed, the cups are supposed
to turn 500 times for a mile of wind movement. Actually the relation
between the speed of the cups and the speed of the wind is somewhat
variable, and at high velocities the indications of the instrument are
seriously erroneous. The Robinson anemometer has a dial from which
direct readings can be made, but at large stations it is connected
electrically with a registering device in the observer’s office, which
makes a mark for each mile of wind and shows how the speed of the wind
varies through the day.
There are many other types of anemometer, and some of them tell a much
more detailed story of the wind’s variations than does the Robinson
instrument. On the other hand, thousands of weather observers
dispense with anemometers altogether and merely estimate the strength
of the wind from its effects. This applies to nearly all observers
at sea, and, in Europe, to the vast majority of observers on land.
Such estimates are recorded on a scale ranging from zero, for a calm,
generally up to ten or twelve for the strongest winds ever experienced.
Several different scales are in use. The best known is the Beaufort
Scale, devised by Admiral Sir F. Beaufort, in 1805. The following
table of the Beaufort Scale, as adapted for use on land, is from the
“Observer’s Handbook” of the British Meteorological Office:
--------+---------------+---------------------------------+-----------
Beaufort| Explanatory | Specification of Beaufort Scale | Equivalent
number | titles | for use on land based on | speed in
| | observations made at land | miles per
| | stations | hour at
| | | 33 feet
--------+---------------+---------------------------------+-----------
0 | Calm | Calm; smoke rises vertically | 0
1 | Light air | Direction of wind shown by |
| | smoke drift, but not by wind |
| | vanes | 2
2 | Slight breeze | Wind felt on face; leaves |
| | rustle; ordinary vane moved |
| | by wind | 5
3 | Gentle breeze | Leaves and small twigs in |
| | constant motion wind extends |
| | light flag | 10
4 | Moderate | Raises dust and loose paper; |
| breeze | small branches are moved | 15
5 | Fresh breeze | Small trees in leaf begin to |
| | sway; crested wavelets form |
| | on inland waters | 21
6 | Strong breeze | Large branches in motion; |
| | whistling heard in telegraph |
| | wires; umbrellas used with |
| | difficulty | 27
7 | High wind | Whole trees in motion; |
| | inconvenience felt when |
| | walking against wind | 35
8 | Gale | Breaks twigs off trees; |
| | generally impedes progress | 42
9 | Strong gale | Slight structural damage occurs |
| | (chimney pots and slates |
| | removed) | 50
10 | Whole gale | Seldom experienced inland; |
| | trees uprooted; considerable |
| | structural damage occurs | 59
11 | Storm | Very rarely experienced; |
| | accompanied by widespread |
| | damage | 68
12 | Hurricane | | Above 75
--------+---------------+---------------------------------+-----------
The clouds receive more attention at some weather stations than at
others. A routine observation consists of noting the kinds of clouds
visible, the direction or directions from which they are moving,
and the degree of cloudiness--i. e., the extent to which the sky is
clouded, stated in tenths, from O = cloudless, to 10 = completely
overcast. At many of the more important stations the movements of
clouds are observed with a _nephoscope_. The reflecting nephoscope,
used in this country, consists of a black mirror in which the image of
the moving cloud is watched, the direction of its motion being read off
from the graduated circular frame of the mirror. There is also a device
for measuring the apparent speed of the cloud. From this the actual
speed can be calculated if the height of the cloud is known. There are
other nephoscopes, such as Besson’s in which the cloud’s movements are
watched directly, and not by reflection.
[Illustration: BESSON’S COMB NEPHOSCOPE]
The importance of sunshine among the elements of weather and climate
is evidenced by the fact that at least two States of the Union,
South Dakota and California, contend for the title of “the Sunshine
State”--which does not properly belong to either of them. Arizona is
the sunniest State of all, and the whole Southwest is sunnier than
South Dakota.
Devices for registering the duration of sunshine are called _sunshine
recorders_. One type (the Campbell-Stokes) works on the burning-glass
principle; in others the sun’s rays trace a record on photographic
paper. The instrument used by the Weather Bureau consists of an air
thermometer having a bulb at each end, one bulb being coated with
lampblack. There is a small column of mercury between the two inclosed
masses of air. The thermometer is inclosed in a sheath of glass, from
which the air is exhausted. When the sun shines on this instrument,
the air in the black bulb warms and expands, and the mercury is
forced toward the other bulb until it comes in contact with a pair of
electrodes, thus closing an electrical circuit. While the circuit is
closed, the registering apparatus connected with the instrument makes
a step-shaped mark once every minute. When the sun stops shining, the
mercury drops back, the circuit is broken, and the recording pen merely
traces a straight line.
[Illustration: ELECTRICAL SUNSHINE RECORDER]
At the larger stations of the United States Weather Bureau the
direction and speed of the wind, the rainfall and the duration of
sunshine are all recorded on a single sheet of paper, wound around a
large cylinder, which is turned by clockwork. The paper is ruled with
lines to denote the hours and minutes of the day, and a fresh sheet is
put on the cylinder every day at noon. This complex registering device,
sometimes called in book language a _meteorograph_, but colloquially
referred to by weather men as the “triple register,” is entitled to
high rank among labor-saving machines; for, with hardly any attention,
except for a few minutes at noon, it does the work of a staff of
trained meteorologists on duty day and night.
[Illustration: BRITISH TYPE OF SUNSHINE RECORDER
(_Campbell-Stokes Pattern_)]
We have now enumerated the elements of weather most commonly observed
at meteorological stations, and the principal types of meteorological
instruments, with special reference to those used in the United States.
In nearly every civilized country there are certain stations at which
regular observations are maintained of a number of phenomena not
mentioned in the foregoing paragraphs, such as the intensity of solar
radiation (measured with the _pyrheliometer_), evaporation (measured
with _atmometers_ or _evaporimeters_), and the temperature of the
soil; and the number of stations is rapidly growing at which the winds
and weather far aloft in the atmosphere are observed by means of kites
and balloons. Meteorologists of the Old World use a great many types
of apparatus that are rarely seen in this country, and some of our
instruments are but little known abroad.