CHAPTER VWEATHER AND WEATHER INSTRUMENTS

AITKEN’S DUST COUNTER

A great many different methods are in use for determining the total amount of solid matter present in a given volume of air, counting the number of particles, or gathering samples for microscopic examination. Thus a known volume of air may be drawn through a filter of cotton wool or bubbled through distilled water, and the dust detained by the cotton or deposited in the water may be weighed. In certain types of apparatus the air is drawn or forced against a plate or tube coated with glycerin, oil, varnish, gelatin, or other adhesive surface, to which the dust remains attached. Several devices depend for their operation upon the fact that when a volume of confined air is cooled by expansion a point is eventually reached at whichthe water vapor present condenses to form a fog, each droplet of which is supposed to have a single particle of dust as its “nucleus.” This is the principle involved in the well-known Aitken dust counter, which has been so extensively used in different parts of the world, and has furnished most of the impressive statistics of air dustiness found in textbooks and reference books. Thus, from indications supplied by this instrument, it is stated that a cubic inch of town air contains 50,000,000 particles of dust; that a room, near the ceiling, was found to contain 88,000,000 particles per cubic inch; and that a cigarette smoker sends 4,000,000,000 particles into the air at every puff. Recent authorities are inclined to look upon these figures as misleading, for the reason that the nuclei counted with Aitken’s instrument are probably so infinitesimal in size that they hardly deserve to be called dust; indeed there is good reasonto believe that an indefinitely large proportion of them may actually be molecules of gases.

The effects of dust, both inorganic and organic, upon the health of humanity will be considered in another chapter. Certain kinds of dust are of economic importance on account of their inflammable and explosive character when mixed with the right proportions of air. Thus the cereal dusts made in the handling and working up of grain into food products occasionally give rise to serious accidents. These occur in cereal, flour, and feed mills, grain elevators, starch and glucose factories, and on farms in connection with the use of threshing machines. During a period of ten years, 1906–1916, cereal dust explosions resulted in the loss of eighty lives and the destruction of property to a value of $2,000,000 in the United States. A study of this subject has been made by the United States Department of Agriculture, and various recommendations have been published with a view to preventing the occurrence of sparks in the neighborhood of these dangerous dusts. Coal dust in mines likewise causes numerous explosions. Preventive measures include wetting the dust, moistening the air, and powdering the walls, roof and floor of the mine with a nonexplosive rock dust, which has the effect of stifling an incipient fire or explosion.

The last species of dust that we have to consider in this chapter is one that constitutes a literal blot on civilization, since the noblest cities and monuments of mankind are defaced with it. Neither are the evils of this kind of dust wholly æsthetic, for it is extremely injurious to health and enormously expensive. After enduring coal smoke as a necessary evil for generations, civilised humanity has now embarkedupon a vigorous campaign for its elimination, and very encouraging results have already been achieved in many parts of the world. The war against smoke is carried on by numerous societies in Europe and America; a multitude of laws and ordinances (not all of them effective) have been enacted on the subject; it has been the occasion of international conferences and expositions; and its literature has grown so copious that a partial bibliography of the subject, published a few years ago by the Mellon Institute, of Pittsburgh, fills 164 pages.

The smoking of chimneys is costly, in the first place, because it is due to imperfect combustion and the waste of part of the heating value of the fuel, and, in the second place, on account of the damage wrought by the deposit of the soot. Thus a smoky atmosphere entails big laundry and dry-cleaning bills, frequent repainting of houses, injury to metal work, damage to goods in shops, and excessive artificial lighting in the daytime. Throughout the United States it is said that smoke causes an annual waste and damage amounting to five hundred million dollars. In Pittsburgh alone—before the reform produced by vigorous legislative and scientific measures, following an exhaustive investigation by the Mellon Institute of Industrial Research—the cost of the smoke nuisance was estimated at nearly ten million dollars a year. Means of mitigating this evil include the introduction of improved appliances for burning soft coal, and the use of other kinds of fuel. The electrification of the railway lines entering cities is an important measure of relief. It is estimated that more than one-third of the smoke found in certain American cities comes from locomotives.

Systematic measurements of the amount of solid matter contributed to the atmosphere by smoke have been made at various places in this country and abroad, and yield startling figures. Measures of the “sootfall” in Pittsburgh, before the evil there was mitigated, showed an annual average deposit amounting to 1,031 tons per square mile. London’s average is 248 tons per square mile for the whole city and 426 tons in the central districts. In the heart of Glasgow the annual sootfall is 820 tons per square mile.

In Great Britain measurements and analyses of soot and the study of its effects have been carried out on a large scale for a number of years by the Advisory Committee on Atmospheric Pollution, attached to the Meteorological Office. The Committee has installed “pollution gauges,” of uniform type, at about twenty-five places in England and Scotland. The soot that falls into these gauges is collected once a month, weighed and analyzed. This organization also makes direct measurements of the purity of the air, and has acquired a unique body of observations that can be used to test the success of efforts made to abate the smoke nuisance, besides providing interesting comparisons between the incidence of respiratory diseases and the amount of solid matter in the air.

Thefact 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 subjectwasof 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 eclipsesall 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 forparticular 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, anybriefaccount 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 ofmeasuringthe 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 thebarometer, 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.”

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 thebar, 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 themillibar, equivalent to 0.03 inch on the old-fashioned barometer scale, under standard conditions.

ANEROID BAROMETER, GRADUATED IN MILLIBARS AND INCHESFor the benefit of sailors a curve is shown indicating themean annual pressurein different latitudes along the meridian of 30° W. (Courtesy of the British Meteorological Office.)

ANEROID BAROMETER, GRADUATED IN MILLIBARS AND INCHES

For the benefit of sailors a curve is shown indicating themean annual pressurein 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 accurateaneroid barometer. A self-recording barometer (usually an aneroid) is called abarograph. 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 thechangesof pressure—i. e., just how fast the barometer is rising or falling, or, as meteorologists say, the “barometrictendency.” 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 analtimeter.

Among the meteorological elements that unmistakably pertain to weather the most important is thetemperatureof 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 ofgraduating 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 theair. A thermometer exposed to direct sunshine records its own temperature—i. e., the temperature of the glass andmercury—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.

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 themaximumand theminimum 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 temperatureat any moment of the day, can be read from thethermograph, 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.

Humidityis 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 calledwater 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 theabsolute humidity, and the ratio of this amount to the maximum amount the air can hold at the same temperature is called therelativehumidity. 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, andvice 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.

THERMOGRAPH

Relative humidity does not owe its importance in human affairs solely to its physiological effects, forit 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 thedew point(the temperature to which the air must be brought to start condensation of its moisture) are all determined by means of instruments calledhygrometers. 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 ahygrograph.

The form of hygrometer most commonly met with at meteorological stations is called apsychrometer. 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 anordinary 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 theaspiration 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 adrought, 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 assuch—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 ofrain 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.

TIPPING-BUCKET RAIN GAUGE

Of devices for obtaining an automatic record of rainfall, thetipping 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 totip, thus emptying one compartment and presenting the other to the incoming stream. When the second compartment is filled, the bucket tips in the oppositedirection. 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 directionfromwhich, 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 ofvane, 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 ananemometer. 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 theRobinson 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:

BeaufortnumberExplanatorytitlesSpecification of Beaufort Scale for use on land based on observations made at land stationsEquivalent speed in miles per hour at 33 feet0CalmCalm; smoke rises vertically01Light airDirection of wind shown by smoke drift, but not by wind vanes22Slight breezeWind felt on face; leaves rustle; ordinary vane moved by wind53Gentle breezeLeaves and small twigs in constant motion wind extends light flag104Moderate breezeRaises dust and loose paper; small branches are moved155Fresh breezeSmall trees in leaf begin to sway; crested wavelets form on inland waters216Strong breezeLarge branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty277High windWhole trees in motion; inconvenience felt when walking against wind358GaleBreaks twigs off trees; generally impedes progress429Strong galeSlight structural damage occurs (chimney pots and slates removed)5010Whole galeSeldom experienced inland; trees uprooted; considerable structural damage occurs5911StormVery rarely experienced; accompanied by widespread damage6812HurricaneAbove 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 themore important stations the movements of clouds are observed with anephoscope. 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.

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 calledsunshine 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 thecircuit 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.

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 largecylinder, 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 ameteorograph, 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.

BRITISH TYPE OF SUNSHINE RECORDER(Campbell-Stokes Pattern)

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 thepyrheliometer), evaporation (measured withatmometersorevaporimeters), 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.

Oneof the things a tea kettle is good for is to provide, by means of the little cloud seen at its nozzle and erroneously called “steam,” an example of what happens when the invisible gas that is truly steam, or water vapor, is cooled below its dew point in the free air. This cloud has, however, been the starting point of a vast number of halfway explanations. A generation or so ago physicists were content to say that aqueous vapor turns to drops of water in the air merely on account of being cooled. The question of how the drops get their start, or why the moisture forms drops at all, does not seem to have troubled them.

One way in which air or any other gas is cooled is by expanding against pressure. Some of the energy in the gas, originally manifesting itself as heat, is applied to the work of expansion, and thus ceases to be heat. Hence the temperature of the gas falls. Conversely, if a mass of gas is compressed, the mere process of compression raises its temperature. The heat produced in pumping up a bicycle tire is the classic example of the latter fact. Heating by compression and cooling by expansion are called, respectively, “dynamic heating” and “dynamic cooling.” The processes thus described are of the utmost importance in meteorology.

If air of average humidity is admitted to the receiver of an air pump in the usual way and suddenly expanded by partial exhaustion, a cloud of moisture is seen to form in the receiver. This moisture is condensed and made visible by the dynamic cooling of the air. If, however, after the receiver is exhausted air of the same humidity as before is admitted through a filter of cotton wool, and is then similarly cooled by expansion, no cloud will form. Evidently the filter has removed from the air something that is essential to the process of condensation.

Perhaps it will occur to the reader that, in some obscure way, the filter has prevented water vapor itself from entering the receiver. There are several methods by which we can ascertain whether such is the case. One of the simplest is to admit a little smoke to the receiver before expanding the filtered air. In this case the clouddoesform, showing that moisture is present, and also showing that smoke, though a perfectly dry substance, aids the formation of the water drops.

Such experiments have led to the conclusion, now universally admitted, that when water drops form in the atmosphere they always form around “nuclei” of something that is not water. These nuclei are often referred to as “dust particles,” but it is recognized that a vast proportion of them are very much more minute than the dust that worries housewives. They are largely beyond the power of the microscope, and some of them, indeed, appear to be of molecular size, consisting of molecules of hygroscopic gases, such as the oxides of sulphur and of nitrogen.

Another important fact about water drops in the atmosphere has come to light within the last half century. Since water is much heavier than air,meteorologists of an earlier generation were puzzled by the fact that the drops in clouds apparently float, instead of falling to the ground. In the attempt to account for this supposed floating, bygone authorities assumed that the drops were hollow “vesicles,” like little bubbles. This assumption was eventually disproved by the optical phenomena exhibited by the drops, as well as on other physical grounds. Moreover, it is now known that a cloud never really floats, though the rate at which its constituent particles fall with respect to the air is generally very small, on account of the resistance they encounter. Thus a very slight upward current usually suffices to maintain the altitude of a cloud, or even to increase it. The speed with which a drop falls increases with its size. Hence large drops may fall rapidly from great heights all the way to the ground, constituting rain; but in a great many cases such drops evaporate on falling into warmer air below the cloud level, and thus the lower surface of the visible cloud remains at about a constant height.

The drops in clouds and fog have often been measured, either by noting their optical effects or by microscopic examination. Many are found to be from 0.0006 to 0.0008 inch in diameter. The speed with which such drops fall through still air can be calculated. A drop 0.0008 inch in diameter falls at the rate of about half an inch a second, or 150 feet an hour. Even if a cloud consisting of such drops preserved its integrity for an hour or more while sinking, its descent at this slow rate would hardly be perceptible from the ground.

Some clouds consist of ice needles or tiny snowflakes. Apparently these icy particles are produced directly in solid form, without passing through theliquid stage. It is supposed that, like drops, they require nuclei on which to condense, but this matter has not been fully investigated. Another point that awaits elucidation is the fact that the clouds that form in air much below the freezing point sometimes consist of water and sometimes of ice. The common fleecy clouds of our winter skies are composed of water drops, and such clouds also occur in the polar regions. Dr. G. C. Simpson, when serving as meteorologist of Scott’s antarctic expedition, observed fog consisting of liquid water at a temperature of 29° below zero, Fahrenheit. Why such greatly “undercooled” drops should sometimes occur in the atmosphere when at other times, with higher temperatures, atmospheric moisture takes the form of ice is not at all clear.

There are several ways in which the free air may be cooled to the point at which condensation occurs. The commonest is dynamic cooling, due to the rise of a mass of moist air and its expansion under the reduced pressure that prevails at the higher levels. This process is beautifully illustrated in the formation of the roundish masses of fleecy cloud known ascumulus, on a warm summer day. Each of these clouds marks the summit of a column of air that is rising after having been heated at the surface of the earth. When the process goes on very actively, the cloud may tower up to enormous heights, forming a thundercloud. Some clouds are formed by the mixing of air of different temperatures. Fog, which is merely cloud at the surface of the earth, is often formed by the cooling of the air in contact with cold land or water. The persistent fogs of the Newfoundland Banks are due to the passage of warm moist air from the Gulf Stream region over thecold Labrador Current. On the other hand, a cold wind blowing over warm water will also often produce a fog by lowering the temperature of the moist air overlying the water. A common cause of land fog is the cooling of the air adjacent to the ground in consequence of nocturnal radiation. The moister the air, the more readily fog forms, and hence the frequent formation of fog by night along rivers and over marshes and damp valleys.

Town fogs, such as the famous “London particular” and the fogs of Lyons, usually consist partly of smoke. Dense fogs of this sort occur when the conditions of the atmosphere are such as to cause the smoke to hang low over the city, instead of being dispersed. These fogs constitute a serious economic problem. Thus it is estimated that they cost the people of London upwards of half a million dollars a year, due to extra lighting, damage to vehicles, loss of business, etc. Since marine fog is also a source of enormous loss, through causing delays and accidents, and since fog along air routes is the greatest of all obstacles to successful aerial navigation, it is no wonder that much ingenuity has been devoted to the attempt to disperse fog artificially. Electric discharges have been successfully used for this purpose on a small scale.

The depth of a fog may be anything from inches to miles. Measurements made by the United States Coast Guard during the international ice patrol of the North Atlantic show that the fogs on the Newfoundland Banks are very commonly so shallow that the mastheads of vessels rise above them, though in some cases they were found, from observations with kites, to be from 2,500 to 3,000 feet thick. Observations on the mountains of the California coast showthat the upper level of fog in that region rarely exceeds 4,000 feet. On the other hand, aviators flying between London and Paris have encountered fog more than 10,000 feet deep.

The United States Weather Bureau classifies a fog as “dense” if it hides objects at a distance of 1,000 feet; otherwise it is described as “light.” British meteorologists record fogs on a scale of five degrees.

During the ice patrol of theSenecain 1915 samples of foggy air were examined for the purpose of calculating the amount of water and the number of drops they contained per unit volume, as well as the size of the drops. A block of dense fog 3 feet wide, 6 feet high, and 100 feet long was found to contain less than one-seventh of a glassful of water, distributed in 60,000,000,000 drops. During the densest fog of the voyage the diameter of the fog particles averaged 0.0004 inch; just about the limit of visibility with the naked eye.

In spite of the extremely attenuated state of the water in fogs, as indicated by these figures, the moisture they deposit on terrestrial objects is great enough to be of considerable agricultural importance in some parts of the world. Thus along the coast of Peru, where the rainfall is negligible (though not, as often stated, nonexistent), a wet fog known as the “garúa” suffices to maintain a luxuriant vegetation during several months of each year.

There are frozen fogs as well as frozen clouds. The “frost smoke” that rises over the Norwegian fjords and over ice-free spots in the polar seas is generally composed of icy particles or snowflakes. An ice fog that sometimes forms in mountain valleys in the western United States is known as the “pogonip”—a name derived from the Shoshoneanlanguage. This fog often appears very suddenly, even in the brightest weather. The minute needles of ice of which it consists are said to be extremely injurious to the lungs. There are tales of a whole tribe of Indians perishing from its effects. Whatever truth there may be in such stories, it is greatly dreaded by both the Indians and the whites. The mountains of Nevada appear to be the favorite home of the pogonip.

What meteorologists call “dry fog” is a haze of dust or smoke, sometimes very dense. We have already described the prevalence of this turbid state of the atmosphere following volcanic eruptions, the burning of forests and moors, and desert dust storms. Under the head of dry fog many writers include a sort of heat haze, which does not necessarily involve the suspension of either solid or liquid matter in the air, but is due to the mixing of local air currents of different densities, especially when evaporation is proceeding rapidly from moist ground under strong sunshine. Thecallinaof Spain and theqobarof the upper Nile region are probably due partly to this cause, and partly to dust.

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