Similar laws apply to all light gases which do not turn into fluids or solids at low temperatures. Aqueous vapour, on the other hand, which when cooled condenses to clouds, diminishes much faster than the nearly twice as heavy oxygen, because the temperature rapidly decreases as we move upward or at a rate of about 5° C. per km. (14.5° F. per mile) up to 2.5 km. (1.5 miles) and of 8° C. per km. (23° F. per mile) at a height of 8.5 km. (5.3 miles). The quantity of water vapour shrinks to one half at 1.9 km. (slightly more than a mile) above ground. Carbon dioxide again follows the barometer-formula applicable to other gases because it occurs in such minute quantity that it never condenses to clouds. In fact, it is water vapour alone which must be treated as an exception. Carbon dioxide is nearly one and one half times heavier than the other gases of the atmosphere on an average. It should therefore diminish in the proportion 1:21.5 = 1:2.8 in a vertical distance of 5 km. (3.1 miles) while the density of the air decreases only in the ratio 1:2. Several determinations of the presence of carbon dioxide in the atmosphere as high up as 3.8 km. (2.33 miles) have been made, by S. A. Andree among others, but the percentage of this gas remains constant within the errors of observation. The same holds true to a height of 7 km. (4.35 miles) for the proportion between oxygen and nitrogen, although we might have expected a perceptible change as oxygen is 14 per cent. heavier than nitrogen. How shall we explain this fact which seemingly contradicts the theory just advanced?
The explanation is quite simple. The preceding statements hold true for a mass of air at perfect rest. But, if the air is violently agitated, the composition becomes homogeneous all through. We know that in the barometric cyclones and anticyclones strong rising and descending air currents flow. The composition of the atmosphere, therefore, becomes the same as far up as this mixing action prevails. These currents produce another effect, namely, a fall of temperature with rising height. Because when a gas is transported upward the surrounding pressure decreases, resulting in expansion and consequent cooling. It is well known that a gas is heated when (rapidly) compressed, a quality formerly made use of in the pneumatic fire-tool to ignite tinder. It is evident that conversely a gas must cool off when expanding. If now the mixing of the air were extremely rapid the thermometer would fall very close to 10° C. (18° F.) with each km. (.62 miles) rise in elevation. If, on the other hand, the air stood perfectly still in a vertical direction, the temperature would remain constant at all heights over the same point. Between these two extremes, we find the actual condition, inasmuch as the temperature of the atmosphere decreases upward 5° to 8° C. per km. (14.5° to 23° F. per mile) as observed during balloon ascensions.
This applies to the so-called “troposphere”—mixing-zone. One of the most remarkable discoveries in recent times, made by Teisserenc de Bort and Assman, is the fact that the decrease of temperature with height does not continue indefinitely but only up to a certain elevation,—in middle Europe about 11 km. (7 miles), in Lapland about 7 km. (4.5 miles), and at the equator about 15 km. (10.5 miles)—and above this point the temperature remains constant. We now meet the peculiar condition that the temperature of this upper layer, which is called stratosphere—“film-zone”—is lowest over the equator, because it commences at a great height there, and lowest over the polar regions, where it extends farther down. The stratosphere has received its name from the fact that it consists so to speak of lamellæ almost parallel to the earth’s surface and moving in a horizontal direction while vertical motions are absent. The winds in these strata have a marked westerly direction (i. e. they are east winds) and they become stronger the higher the stratum—at an altitude of 83 km. (52.5 miles) their velocity is about 100 m. (330 ft.) per second. In the troposphere on the other hand west winds predominate. The wind direction in the stratosphere was observed on the so-called luminous night-clouds which were found as high as 80 km. (50 miles) above the Earth. These strata consequently revolve slower around the earth’s axis than the solid body of the planet itself. At an elevation of 80 km. (50 miles) the rotational speed has decreased to 65 per cent. of the angular velocity of the earth’s surface. We have reason to believe that the very highest strata stand still, that is do not take part in the earth’s rotation on its axis. This would follow if outside space were not entirely devoid of vapour so that our atmosphere would merge imperceptibly into the exceedingly attenuated gas masses of interplanetary space.
As high as the mixing-zone extends, so high also is the composition of the air constant and like that at the surface of the earth. But above this limit—in Scandinavia, we might say above an elevation of 10 km. (6.2 miles)—commences a rapid tapering of the heavy gases, while the percentage of the light ones correspondingly rises. Foremost among the latter is hydrogen, with only half the weight of helium. The presence of hydrogen in the atmosphere has been shown by Boussingault, and the proportion in which it occurs has later been measured by Armand Gautier. It is about one three hundredth part of one per cent. It increases extremely rapidly with height in the stratosphere so that 80 km. (50 miles) above the earth and upward hydrogen is more abundant than all other known gases of the atmosphere at the same altitudes.
We reproduce below a somewhat revised table by Dr. Wegener of Marburg, who has made the most recent computation of the percentages of the various constituents of air at different heights. Consideration has been given to the fact that the composition of the air does not change except as regards the percentage of moisture within the troposphere, which is assumed to reach a height of 10 km. (6.2 miles). As usual in similar cases the percentages refer to volume.
| Height | Pressure | Hydrogen 2 | Helium 4 | Nitrogen 28 | Oxygen 32 | Argon 39.9 | Carbon Dioxide 44 | Water 18 | ||
| in km. | in miles | in mm. | in inches | |||||||
| 0 | 0 | 760 | 29.9 | 0.0033 | 0.0005 | 78.1 | 20.9 | 0.937 | 0.03 | 1.41 |
| 10 | 6.2 | 197 | 7.75 | 0.0033 | 0.0005 | 78.1 | 20.9 | 0.937 | 0.03 | 0.14 |
| 30 | 18.6 | 8.95 | .352 | —— | —— | 85 | 15 | 0.29 | 0.0064 | 0.5 |
| 50 | 31.0 | 0.45 | .0177 | 1 | —— | 88 | 10 | 0.10 | 0.0014 | 1.7 |
| 70 | 43.5 | 0.045 | .00177 | 13 | 1 | 80 | 6 | 0.05 | 0.0005 | —— |
| 90 | 55.8 | 0.0157 | .00062 | 68 | 5 | 26 | 1 | —— | —— | —— |
| 110 | 68.2 | 0.0116 | .00046 | 94 | 5 | 1 | 0 | —— | —— | —— |
| 130 | 80.6 | 0.0097 | .00038 | 96 | 4 | 0 | —— | —— | —— | —— |
| 210 | 130.2 | 0.0055 | .00022 | 99 | 1 | —— | —— | —— | —— | —— |
| 310 | 192.6 | 0.0032 | .00013 | 100 | —— | —— | —— | —— | —— | —— |
| 410 | 254.2 | 0.0021 | .00008 | 100 | —— | —— | —— | —— | —— | —— |
| 510 | 316.2 | 0.0016 | .00006 | 100 | —— | —— | —— | —— | —— | —— |
Under the name of each gas its molecular weight is given as a measure of corresponding specific weight. The quantity of water vapour was not included when the percentages of the other gases were calculated, because it changes considerably with locality and time. The number given in the table for water is the mean for the entire globe—it corresponds to 11.4 grammes per cubic metre (.31 oz. per cu. yd.)—or the amount present in air saturated with moisture at 16.5° C. (61.7° F.). The bulk of the water vapour forms a layer strongly concentrated toward the surface of the earth. Carbon dioxide also tapers rapidly with increasing height because its density is 1.5 times greater than that of air. This is apparent from the molecular weight 44 stated under carbon dioxide, while the average molecular weight of air is 29. Faster yet do krypton, with a molecular weight of 83, and xenon, with a molecular weight of 131, decrease as we ascend in the atmosphere. These gases, like neon, whose percentage first increases slightly with height, and argon, which decreases upward as shown in the table, do not perceptibly influence the processes of nature. The reverse is true about water vapour and carbon dioxide, which nourish the plants and also protect the Earth against a too rapid heat radiation into space. We well remember how abruptly the temperature changes in the course of the day in the dry desert climate, while corresponding variation is comparatively slight in humid climates (compare [page 86]). This is the result of the ability of water vapour to arrest the radiation from the Earth. Carbon dioxide is about evenly distributed over the globe—although somewhat sparser over highlands—and its heat-conserving and equalizing influence is, therefore, not so manifest as that of moisture. Only by the most accurate investigations has this influence been demonstrated.
In Wegener’s table a gas is included, called Geocoronium, whose existence in the air has not been directly proved. Conspicuous is, however, the green light displayed at great altitude by the Northern Light arches, a green color which does not, as far as we are aware, belong to any known constituent of the air. It is true that the corresponding spectral line (557 µµ) lies very close to a line belonging to krypton, but the latter is a heavy gas which cannot occur to any traceable extent in the high strata, more than 300 km. (186 miles) above the earth, where occasionally the Northern Light arches appear—their favoured height according to measurements by Störmer is about 120 km. (75 miles). Wegener assumes, therefore, that this green line belongs to an hitherto unknown substance, Geocoronium, which should be five times lighter than hydrogen. Recent researches present great difficulties to the acceptance of this assumption, and for this reason further discussion of the problem will be omitted. Above a height of 210 km. (130 miles), this gas, according to Wegener, would preponderate. If such postulated gas does not exist, hydrogen completely dominates in these regions and down to 85 km. (53 miles) above the Earth. Because hydrogen is so light, the density of the air in the range of a barometric pressure below 0.02 mm. (.0008 inches) increases but slowly as we descend toward the Earth. This uppermost part of the atmosphere may appropriately be designated as the hydrogen-zone. Even within this range, the “shooting stars” meet sufficient resistance to flame into light at a height of about 120 km. (75 miles) and dissolve into dust which turns dark about 85 km. (53 miles) above the Earth. E. C. Pickering recognized the spectrum of hydrogen in the light of meteors passing at great height, but decomposed water vapour might possibly be its source. Meteors crossing lower strata show the spectrum of nitrogen. Nitrogen becomes important from a height of about 85 km. (53 miles) downward and from 75 km. (46.5 miles) to the surface of the Earth it predominates. As a consequence the pressure increases rapidly as we approach the ground. In these regions or up to 80 km. (50 miles) floated the highest luminous night clouds, observed by Jesse, indicating that here commenced a new range, the nitrogen-zone. Only the heavy meteorites are able to penetrate into the nitrogen sphere, which checks their speed and causes them to explode, and thereafter the remnants fall with a velocity compatible with the air resistance they meet. To these parts descend also the lowest rays of the Northern Lights, the so-called draperies—Störmer observed them once at a height of 37 km. (23 miles). Finally, water vapour presents itself in appreciable quantities at an elevation of about 10 km. (6.2 miles), where the troposphere commences. We now meet the highest clouds, cirrus (with the exception of the “luminous night clouds” observed only in the years 1883–1892 after the eruption on Krakatoa). To these heights, reach the vertical air currents which are essential to cloud formations. Only light clouds, however, float at these altitudes; the heavy clouds (alto-cumulus) do not rise above 4 to 5 km. (2.5 to 3.1 miles) and the rain clouds proper (cumuli) occur only below a height of 2 km. (1.25 miles). This is the result of the downward concentration of water vapour within the troposphere.
If gravity decreased in intensity, the effect would be the same as if the gases were lighter. On Venus, the intensity of gravity is eight tenths of that on Earth. The difference is slight. If everything else were similar the various air-zones would reach 25 per cent. higher on Venus than they do on our globe. But one essential condition is varied by the far higher temperature on our neighbour. The proportion of moisture in the air is thereby vastly increased. The dense clouds rise to much greater heights than on Earth. If there be ten times as much water in the air on Venus as there is in the air on Earth—which might fairly represent the actual condition—the heavy rainclouds would there rise to a height of more than 10 km. (6.2 miles), and their smaller weight on Venus would also contribute to their buoyancy. The light cirrus clouds should appear as high as about 30 km. (18.5 miles) above the ground. Under such circumstances, we cannot expect but that the planet must be entirely hidden from our sight as well as from the rays of the Sun.
On Mars, the intensity of gravity is 2.68 times smaller than on Earth. In consequence barometric pressure falls 2.68 times slower with increasing height there than here. The same ratio holds for decrease in temperature and for shrinkage of proportion of moisture when comparing conditions on the two planets. The strong cold precludes anything but insignificant quantities of water vapour. The air on Mars is similar to the atmosphere on Earth in and above the cirrus-region. The clouds existing there are not only extremely thin and transparent—it is well known that cirrus clouds throw no shadows—but they are confined to small fractions of the planet’s sky. They are replaced by light mists.