Handbook of meteorology; a manual for cooperative observers and students

Front matter

HANDBOOK

OF

METEOROLOGY

The Atmosphere and Its Constituents

METEOROLOGY

PART I

THE ATMOSPHERE: ITS CONSTITUENTS

Meteorology is the science of the air. The air is the outer shell, or layer, of the earth; hence it is called the atmosphere, a Greek word meaning “air-sphere”. The movements of the air (the winds) and the variable proportion of water vapor, mingled with and forming a part of it, exert a profound influence which affects the climate, habitability and civilization of the earth.

For the greater part the movements of the air and the variations in the proportion of moisture are the results of changes in temperature. The fundamental study of the physics of the air, therefore, concerns the problems of variations in temperature and the far-reaching results of those variations.

Composition. — The atmosphere consists of a mixture of gases of which oxygen, nitrogen and the argon group constitute about 98 per cent. The composition varies very slightly so far as these are concerned; but the proportion of water vapor and of the other components varies materially. The first of the two tables which follow is the average of many analyses made by Rayleigh and Ramsay; the second is on the authority of Humphreys. Other analyses show slight differences that indicate actual differences in proportion rather than errors in analyses.

The values of all except the first three are variable; that of floating dust, hydrogen, and helium is empiric and calculated.

The values determined by Humphreys, are those of air from which the water vapor has been removed—that is, of dry air.

After Humphreys (Physics of the Air).

The distribution of the constituents of the air.

The foregoing represent the proportions at the surface of the earth. The proportions change with increasing altitude. The nitrogen disappears at a calculated height of 84 miles; the oxygen, at about 60 miles. Water vapor is calculated to exist at an altitude of 60 miles, but it is not observable above 7 or 8 miles. Carbon dioxide is not observable above an altitude of 2 or 3 miles; theoretically it may extend to a calculated height of more than 15 miles. The proportion of hydrogen and helium, on the other hand, increases with altitude, and they probably form the outer layer of the atmosphere. Because of their lightness it is not unlikely that hydrogen and helium are gradually escaping from the earth.

|Constituents | |Parts in one million of air

|Nitrogen | | 771,200

|Oxygen | | 206,600

|Argon group (approximately) | | 7,900

|Water-vapor | | 13.953

|Carbon dioxide | | 336

|Ozone | | 12

|Nitric and nitrous oxides | | 8

|Ammonia | | 1

|Dust, hydrogen, helium | | 1(?)

Depth of the Atmosphere.—Observations on the twilight arch indicate that at a height of 40 miles above sea level the air has a density sufficient to refract, reflect, and diffract light. A measurement of the parallax of a meteor seen by two observers

|Constituents | |Per cent

|Nitrogen | | 78.03

|Oxygen | | 20.99

|Argon | | 0.94

|Carbon dioxide | | 0.03

|Hydrogen | | 0.01

|Neon | | 0.0012

|Helium | | 0.0004

At an altitude varying approximately from 6 to 7 miles, according to latitude and also according to the season, a plane of contact occurs which apparently separates an upper from a lower shell of air. Below this plane practically all the local movements, especially the upward and downward, or convectional movements of the air occur. The lower or convectional shell is the troposphere; the upper shell is the stratosphere.

Constituents of the Air.—Nitrogen, the constituent of greatest volume at the surface of the rock sphere, is very inert. It does not combine with the oxygen of the air, except in very minute quantities when influenced by lightning discharges. The nitrogen of the air is now used in the manufacture of ammonium nitrate, the basis of certain explosives. Nitrogen is the chemical base of nitric acid, HNO3, and of several other oxygen compounds. It is a constituent of ammonia gas, NH3, and of cyanogen, CN, all of which enter into the structure of many thousand other compounds. Many of these compounds are very unstable; hence the rapid decomposition of animal and vegetable compounds, commonly known as putrefactive decay. The instantaneous dissociation of the nitrogen constituents of such compounds as nitroglycerine and tri-nitrotoluol, or TNT, give to such compounds their value as explosives.

Atomic weight 13·93; sp. gr. .971; temperature of liquefaction −231° F (−146° C) at 35 atmospheres pressure.

Oxygen is the active chemical element of the air. It unites readily with pretty nearly every other chemical element. Its union with carbon is the ordinary process of combustion. Iron wire in free oxygen burns about as freely as a match in the open air. The oxygen of respiration oxidizes the impurities of the blood.

The percentage of oxygen is slightly greater in the air of northerly winds of the north temperate zone than in southerly winds. It is slightly below normal over cities, as compared with open spaces. In crowded auditoriums the proportion of oxygen sometimes falls to 20 per cent; in mine tunnels it is sometimes as low as 18 per cent. Candles burn with difficulty with the oxygen content at 18 per cent; and human life cannot long exist with the proportion of oxygen as low as 17 per cent.

Atomic weight 15·88; sp. gr. 1.106; temperature of liquefaction — 182° F (119° C) at 51 atmospheres.

Carbon dioxide (carbonic acid gas), CO2, is the heaviest gaseous constituent of the air. It is derived from carbon in the ordinary process of combustion:

C + 2O = CO2

It is also derived from various hydrocarbons of rotting vegetation by dissociation and combustion, as, for instance, methane (marsh gas):

CH4 +4O = CO2 +2H2O

The normal proportion of carbon dioxide in the air is about 3⋅3 parts in 10,000 of air. In manufacturing districts, where coal is used for power-fuel, the proportion is greater. In the bracing air of a cold wave it is materially less. It is less during winter, when the temperature is below freezing and the ground is snow-covered, than in summer. Over the land the proportion is slightly greater at night than in the day-time, and during foggy weather it is materially greater than in dry weather.

In theaters, churches, schoolrooms, and poorly ventilated rooms the proportion of carbon dioxide may be as high as 12 parts per 10,000 of air; occasionally it is even greater. Breathed air is harmful, not so much on account of its carbon dioxide content as on account of the presence of products of putrefaction. Although carbon dioxide exists in the air at a calculated height of 15 miles, the proportion decreases so rapidly that it may be disregarded as a component of the air above the height of 1 mile.

Sp. gr. 1.53; liquefies and solidifies with moderate pressure at ordinary temperatures.

Water vapor in varying proportions is a constituent of the air. The maximum proportion depends chiefly on temperature. Thus at 30° F there may be nearly 2 grains by weight of water vapor per cubic foot; at 70° F, there may be nearly 8 grains. There may be less in either case, but there cannot be more; any excess will be condensed. When the maximum proportion is present the air is conveniently said to be “saturated.” The table, p. 280, shows the maximum weight of water vapor at different temperatures.

Although the proportion of water vapor mingled with the air differs from time to time, the per cent of total volume decreases from the equator toward the poles. The average annual per cent at the equator is 2.63; in latitude 70° it is only 0.22. The proportional water vapor content of the air is commonly expressed as “per cent of humidity.” Thus, with half the maximum proportion of vapor, the humidity is 50 per cent.

Sp. gr. 0.62; “boils” with vapor tension equal to that of the air at sea level, at 212° F (100° C); solidifies or “freezes” at 32° F (0° C).

Argon and the related group of elements, neon, krypton and xenon, constitute practically 8 parts per thousand of air. The gases of the argon group are chemically inert; no compounds with other elements are known to exist. This is true also of the other elements of the group. If they have any specific influence not possessed by nitrogen, the influence is not known.

Atomic weight of argon 39.88; sp. gr. 1.21; liquefies at −184° F (−120° C) under pressure of 40 atmospheres. Hydrogen is the lightest of the chemical elements, and the weight of its atom is the unit of atomic weights. Ignited with oxygen it forms water:

2H+O = H2O

Hydrogen is a constituent of all chemical compounds containing water, and of the various hydrides and hydrates. It occurs in the lower air in variable but very minute proportions which may be due to the chemical dissociation of organic matter. It is a constituent of natural gases, and of certain volcanic

gases. The proportion increases as the height in the air increases. On account of its lightness meteorologists are of the opinion that the rapid movement of the earth in space is constantly throwing it off into space. At all events, there seems to be sufficient evidence that it is the chief if not the sole constituent of the outer part of the atmosphere. It is much used for the inflation of balloons and airships, being about 15 times as buoyant as air.

Atomic weight 1; sp. gr. 069; liquefies at about −375° F (−226° C) under a pressure of 15 atmospheres.

Helium is another inert element. It is a constituent of several minerals, including pitchblende, an oxide of uranium. It occurs by absorption in many deep rocks and also in the gases that escape from deep springs. Cottrell discovered it in the proportion of about 2 per cent in certain Texas gas wells. Because it is non-explosive and non-inflammable, it has been used in the inflation of balloons. Its buoyancy is about 92 per cent of that of hydrogen and it does not readily pass through balloon fabrics. Because of its lightness and also its high molecular speed, it is thought to occur chiefly in the outer shell of the atmosphere—possibly escaping from the earth altogether. If it plays any part in meteorological phenomena, its influence is not known.

Atomic weight 4; sp. gr. approximately .128; liquefies at −452° F (−269° C) at a pressure of about 3 atmospheres.

Nitric acid (HNO3) and ammonia (NH3) are present in the air in very minute proportions. Nitric acid is most readily detected at the time of thunderstorms. From time to time the proportions of these substances vary greatly from the proportions noted in the table. The presence of ammonia is due probably to the decomposition of organic matter.

Ozone (O3) is an allotropic form of oxygen, whose normal molecule is O2. Ozone possesses a pungent odor that frequently is discernible at the time of nearby lightning discharges and the passage of high-potential electric sparks. The normal proportion in the air is exceeded many times over during thunder-storms.

The proportion of ozone varies with environment. It is greater over the sea than over the land—possibly due to the lack of oxidizable matter; and this may explain its greater pro in winter than in summer. The proportion is greater on clear, dry days than during cloudy spells. The daily variations of the ozone content of the atmosphere seem to correspond to the variations of the atmospheric electric potential.

Dust particles so fine that they escape measurement even with the highest power of the microscope, must be considered a part of the normal content of the atmosphere. Their presence is indicated by the fact that they may reflect enough light to make them visible en masse when a powerful light is turned upon them in a darkened room, or when a searchlight throws its beam at night. The path of the light is shown by the light reflected from dust motes. Dust particles of the size thus revealed behave like molecular rather than like matter of molar sizes. They are floating matter, the particles of which may not settle unless they are brought to the surface by means other than their own gravity.

The floating dust motes of the air are factors of great meteorological importance. They are the nuclei upon which the water vapor of the air condenses. Dense clouds of volcanic dust act as a screen preventing much of the sun’s heat from reaching the earth. The dust particle is the normal nucleus for the cloud particle. The flying, or windblown dust, though a highly important physiographic agent, is not a factor of importance in meteorology.

Chlorine usually occurs in the air of localities bordering upon the oceans, and sodium chloride reactions may be obtained when sea winds are blowing inland. The presence of the salt is due to the action of the wind which whips a small amount of spray into the air. The chlorine content of the air apparently plays no part in meteorology. Like smoke and chimney products it may be regarded as “foreign” matter.

From the foregoing it is apparent that oxygen, nitrogen, and the argon group of gases practically constitute the “fixed” constituents of the atmosphere. Their proportions at sea level vary but little in different parts of the earth, and they constitute about 98 per cent of the atmosphere. Ozone, the nitrogen oxides, ammonia and the various radio-active emanations may be considered practically as negligible factors in meteorology; for the greater part they are accidental. Carbon dioxide is a factor chiefly in physiological meteorology.

Water vapor and the unmeasured dust content of the atmosphere are meteorological factors of the highest degree of importance. All the fresh waters of the earth are derived from the sea by a process that is clearly one of distillation; and life as it is organized on the earth depends upon this process. Even a slight change in nature’s method of distillation would be followed by profound changes in the distribution of life.

Air and Mortality.—The difference between the sun-bathed air of out-of-doors and the air of dwellings has exerted a marked effect upon modern civilization. Various diseases of the densely peopled regions of Europe and America are practically unknown among peoples who live habitually out of doors. Tuberculosis is essentially a disease of modern civilization. Even in Europe and America, where the disease thrives, the mortality is twice as great among house dwellers as among those having out-of-door employment.

The mechanical ventilation of buildings has helped matters but very slightly. Air drawn through ventilating shafts has not the same therapeutic qualities as sunlit air coming through open windows into living rooms. An explanation of the difference is yet to be found. If meteorology is the science of the air, it should discover the difference between wholesome and unwholesome air.

Forms and Properties of Matter

FORMS AND PROPERTIES OF MATTER IN ITS RELATION TO METEOROLOGY

Ether and Matter.—It is not necessary to assume that the universe and space are one and the same; nor that the universe is boundless; nor that space is without limits. So far as that part of the universe with which human knowledge comes in contact is concerned, the existence of two factors is assumed. Matter is perceptible to the human senses. It is visible, tangible, and transformable. It can be measured and compared; some, at least, of its properties are known. Its ultimate constitution, however, is not known. It is usually described in terms of atoms, molecules and masses.

In certain respects, more is known about ether than about matter: for although the existence of ether is merely assumed, the magnitudes attributed to it are real values that have been fully established. That the universe is pervaded by an invisible, intangible, but measureable something is conceded. It is assumed that the manifestations to the senses known as heat, light, magnetism, electricity, and radiant energy traverse the known part of the universe by the means of the ether. It is not improbable that these manifestations are undulations of the ether itself.

The telescope and the spectroscope have shown that the matter entering into the composition of other visible bodies in the universe does not differ from that which composes the earth. Many of the chemical elements that compose the earth have been discovered in the sun and other heavenly bodies, and no chemical element has been discovered in any heavenly body that does not occur in the earth. Air and water vapor occur on the planet Mars, and it is not unreasonable to assume that the meteorology of this planet has much in common with the meteorology of the earth. The occasional occurrence of dust storms on Mars adds weight to the reasonableness of such an assumption.

Forms of Matter.—For practical purposes it may be assumed that matter exists in three forms—solid, liquid and gaseous. Most of the metals and some of the non-metals may be changed easily from one form to another. Thus, iron is a solid at ordinary temperatures; it “melts” or liquefies at a temperature somewhat above 2100° F (1150° C); at a still higher temperature it gives off a reddish-brown vapor. Mercury is ordinarily a liquid; it “boils” or becomes a vapor at 675 F (375° C) and “freezes” or solidifies at −38° F (−39° C). Water is the most common illustration of all; it solidifies at 32° F (0° C) and gradually becomes a vapor at ordinary temperatures; but at 212° F (100° C) the vapor pressure is that of the air at sea level. Practically all the ordinary gases have been liquefied and solidified. Liquid air and carbon dioxide are articles of commerce.

The conditions which surround the liquefaction of ice and snow, the evaporation of water, and the condensation of the water vapor of the air are fundamental factors in the science of weather. The distribution of precipitation—that is, rain, snow, hail, and the floating forms of fog and cloud—affect the habitability of the earth and human activities to a very great degree.

Matter may be changed in physical form, but it cannot be annihilated. Thus, the coal in the fire-box is changed to carbon dioxide, a gas, instead of a solid; but the chemist may separate the carbon from the oxygen. Nothing, not even the energy, is lost; to nature nothing can be added, and from nature nothing can be taken away.

Properties of Matter.—All forms of matter have certain properties—volume, density, weight, etc., in common. Other properties, such as malleability and ductility, affect groups or classes of matter—chiefly the metals.

Cohesion is a somewhat archaic term denoting molecular attraction. In solids, the cohesion is usually strong, so that more or less force is required to sunder the mass—that is, to separate the molecules. The resistance of cohesion is usually expressed in such terms as tension, torsion, shearing, etc. In the liquefaction of a solid, or the vaporization of a liquid, the force employed to overcome cohesion is measured in terms of heat. For instance, in the liquefaction of ice, about 147 times as much heat is required to change ice at 32° to water at 32° as will raise the temperature of the same weight of water one degree Fahrenheit in temperature. In the case of liquids, the cohesion seems to be slight, inasmuch as the molecules possess a considerable mobility. Nevertheless, they are held together by a powerful force. Thus, the heat used in converting one pound of water at 212° F to a vapor at 212° F would raise 967 pounds of water one degree in temperature. Measured thus, in terms of heat units, great power is required to overcome molecular attraction. In the case of gases, not only is the molecular attraction negligible, but the molecules apparently repel one another. Perhaps it is more nearly correct to say that they diffuse themselves throughout the space which contains them. In other words they apparently cease to possess molecular attraction.

Crystallization is a form of molecular attraction which indicates that the molecules possess a certain kind of polarity, usually assuming regular geometric forms. Frost and snowflakes frequently exhibit marvelous forms, infinite in variety but regular and similar in construction. The study of these forms is one of increasing importance in weather science.

Expansion-contraction is a property of matter true in its ordinary forms. The volume of a substance increases when it is heated and contracts with cooling. Thus, iron will increase 0.00000648 of its length for each degree F of increase in temperature, this being its “coefficient of linear expansion.” The coefficient of expansion of air is 0.00367; of ethyl alcohol, 0.0005; of mercury, 0.0002. In meteorology the principles of this property are fundamental. The expansion of mercury and of alcohol are used to determine the intensity of heat; and to the unequal heating of the air in different localities are due the movements of the air—that is, the winds.

Magnetism is a property pertaining chiefly to iron and steel, but possessed to a lesser degree by other metals. When in the condition known as magnetized, a piece of iron or of steel attracts and holds other pieces of iron and steel. Steel retains its magnetism permanently; iron is sensibly magnetic only when within magnetic influence—that is, a “magnetic field.” Nickel, cobalt, certain manganese alloys and tungsten alloys exhibit magnetism very sensibly. A bar of magnetized steel, suspended by a thread attached at its center of gravity, comes to rest pointing nearly or quite north and south, the negative or marked end pointing in a general way to the earth’s north magnetic pole. A few substances, chiefly bismuth, similarly suspended come to rest across the magnetic field when between the poles of a horseshoe magnet. The investigations concerning the earth’s magnetic properties are carried on in the United States by the Coast and Geodetic Survey.

Properties of Gases.—Gases are perfectly elastic. A gas fills any space within which it is confined. A cubic inch of a gas whose density has been measured, if set free in a space whose dimensions are a cubic foot, or a cubic yard, will fill the space. Manifestly its density and tension will be lessened in proportion. It is the custom to say, therefore, that a gas has no specific volume of its own; its volume is that of the container.

Equal volumes of a gas, temperature and density remaining the same, contain an equal number of molecules. If hydrogen, the lightest known gas, be taken as the unit of measurement, the molecular weight of any gas may be determined by comparing its weight with that of an equal volume of hydrogen. This is known as Avogadro’s law.

If a volume of gas be heated from 32° to 459° F (0° to 273° C) its volume will be doubled. That is, equal volumes of gases expand equally with the same increase of temperature.

If a given volume of gas—say 1 cubic foot of oxygen—be introduced within a container, its pressure or tension noted, the same volume of another gas having the same tension may be introduced without an increase of tension of the mixture. Thus 1 volume of oxygen added to I volume of nitrogen will make but 1 volume of the mixture, having the same tension as each of the two gases. That is, one gas is practically a vacuum for another. This property has its limitations; when several other gases are introduced within the container a noticeable increase of the tension of the mixture takes place.

Inasmuch as the science of meteorology is chiefly a study of the air, a mixture of gases differing in their specific properties, a clear exposition of these general properties is necessary to an understanding thereof—more especially to the solving of the problems of weather, climate, and habitability.

Gravity is a property of matter that exists apparently throughout the known universe. The apparent fact that matter in masses attracts all other matter in masses is practically all that is known of the essence of it. The whirling of the sun and the planets about a common center of gravity balances the attraction that otherwise would bring them together. More exactly stated, planetary bodies tend to move in straight lines; gravity tends to draw them to a common center; the result is orbital movement. These complex movements and forces have a great and very measureable influence on the movements of the sea and the air.

It is convenient to note the density—practically the “weight” —of various kinds of matter, comparing them volume for volume, under standard conditions, with the density of a given substance. This ratio of weight is the specific gravity of the substance. Distilled water at its maximum density, 39.1° F (3.94° C) is usually taken for the comparison. Thus, a given volume of mercury weighs 13.6 times as much as an equal volume of water, and an equal volume of alcohol 0.81 times as much as an equal volume of water.

For gases, air and hydrogen are both used as units of comparison. The specific gravity of hydrogen, in terms of air, is 0.069; of air in terms of hydrogen, 14.4; of coal gas, commonly used for inflating balloons, about 0.061; of water vapor, 0.62. When metric units are employed, the weight of a cubic decimeter is the specific gravity of the given substance, a cubic decimeter of water under standard conditions, weighing in theory, but not in fact, 1 kilogram.

Heat

HEAT: ITS NATURE, PROPERTIES AND DIFFUSION

The Nature of Heat.—The phenomena of heat and light are described, the one as “molecular motion,” the other as the “radiation of solar energy.” Roughly, either definition will apply to either phenomenon. Not much is known about the real essence of either, except that they are forms of energy which have been measured, and of which certain magnitudes have been established under the name of “wave lengths.”

Radiant Heat.—It is assumed that heat and light traverse space in “waves” or vibrations of the ether. Positive knowledge is confined to the fact that heat is radiated by the sun and stars in every direction. Practically all the heat received by the earth comes from the sun; and of the whole amount radiated by the sun, the earth intercepts less than one two-billionth part. Nevertheless, this small fraction of the sun's radiant heat produces all the results upon the earth which are manifested by life and its activities.

Some of the ether waves stimulate the nerves of the eye, producing the phenomena of light and vision. Others do not affect the nerves of sight; as they fall on the body they produce the sensation of warmth, thereby stimulating the growth of living matter. Meteorology is concerned chiefly with radiant energy of this character; they are conveniently called heat waves.

Perhaps our nearest approach to actual knowledge of heat is the recognition of the fact that when heat waves fall upon matter—say, a piece of metal—they set up a motion in the molecules composing it. If the intensity of the waves increases, molecular attraction little by little is overcome; the solid becomes a liquid and, finally, a vapor. This important change is explained as being due to increasing and to wider amplitude of the oscillations of the molecules. Perhaps the theory may not be satisfactorily established, but the facts cannot be denied; the heat has increased the motion of the molecules; finally it has overcome their cohesion.

Sources of Heat.—The warmth that is concerned with life and its activities is derived from the sun. The sun warms the rock envelope of the earth; the rock envelope radiates warmth to the atmosphere; the movements of the air diffuse the warmth, bringing cool air into warm regions and sending warm air into cold regions. Weather science is concerned chiefly with these movements of the air.

The earth itself is a source of heat. The interior of the rock envelope of the earth is intensely hot. Borings into the rock envelope show an increase of temperature with depth. The rate varies with the character of the rock, a rough average being 1° F for every 70 feet. Some of the heat of the rock envelope, at such depths, is due to chemical action going on within the rocks themselves; some is due to vulcanism; but some is certainly due to the radiation of the heat of the interior of the rock envelope itself. In meteorology this source of heat practically is negligible.

The most common example of a terrestrial source of heat is the ordinary combustion of fuels. But even this is apparent rather than real terrestrial heat; in fact it is the heat of the sun stored and conserved by vital chemical processes. But all the original heat derived from within the earth and all the heat of vital chemical processes bears an infinitesimal ratio to that received from the sun. It is estimated that the heat received by the earth in one minute is sufficient to raise 42,000,000,000 tons of water from the freezing to the boiling point.

Heat as Motion.—If heat produces molecular motion, it must be assumed that the ether waves which traverse space warm nothing until they fall on matter—that is, on a substance composed of molecules which can be set in motion. But in many respects heat behaves to matter much as light does. It may pass through a substance just as light passes through glass; such a substance is said to be diathermous. Thus, glass itself is more or less diathermous, so also is clear water. Glass permits most of the heat of the sun to pass through it, but it intercepts some of it. Thereby molecular motion is set up in the glass itself and it becomes warm, radiating heat in just the same manner as a stove radiates it.

The heat “absorbed” by a substance—that is, converted into molecular motion—in time may be stored until it is again radiated and is given out as warmth. It is conveniently called “sensible heat.” It is thus distinguished from the ether waves