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.[1] 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.
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.[2] 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 different stations indicates the existence of air at a height of 200 miles. Actual measurements, however, have not extended much higher than 20 miles, the height to which sounding balloons have reached.
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.[3]
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:
It is also derived from various hydrocarbons of rotting vegetation by dissociation and combustion, as, for instance, methane (marsh gas):
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.”[4] 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.[5] 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 proportion 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.[6]
- ↑ The argon group consists of argon, krypton, xenon, and neon. Two other very rare elements, coronium and niton, are probable constituents of the air.
- ↑ The foregoing are on the authority of W. J. Humphreys.
- ↑ The temperature and pressure of liquefaction of the gases mentioned in this chapter vary slightly according to different authorities.
- ↑ According to common use air is said to “contain water vapor” or to be “saturated” under certain conditions, as though the air were a sponge, which may absorb and retain water up to a certain limit. The expression is inexact; but, in the literature of meteorology, inasmuch as the water vapor is rarely considered apart from the other constituents of the air, expressions of the sort are convenient and will be so used in this manual. In a given space, whether vacuous or filled with the other constituents of the air, there may be a certain number of molecules of water vapor at a given temperature and pressure, and no more. If additional molecules are added the excess will be “condensed” and become a liquid. The water vapor is at its maximum density, and also it is “saturated,” when the space contains all the water vapor which can exist therein up to the point of saturation. Strictly speaking, it is the vapor itself and not the space, nor the air which is “saturated.”
- ↑ Hann: Lehrbuch der Meteorologie.
- ↑ An investigation of the problem has been undertaken by a committee of the American Meteorological Society.