CHAPTER V

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.”

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.

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.

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.

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.

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.

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.

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.

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.