It is a matter of common knowledge that the air is found to be colder the higher one ascends in the atmosphere. Thus, even in equatorial regions, the tops of high mountains are mantled in perpetual snow. The rate of this temperature decrease averages about 1 degree Fahrenheit per 300 feet. Previous to the year 1902 meteorologists supposed that the atmosphere continued to grow steadily colder in an upward direction indefinitely; but in that year a Frenchman, M. Teisserenc de Bort, who had sent aloft hundreds of small unmanned balloons carrying self-recording thermometers, announced that above a height of about six and one-half miles the temperature ceased to fall. In fact, he found that at about that level there was often a slight increase of temperature with increasing altitude for a certain distance upward, and then a nearly uniform temperature as high as the balloons ascended. This announcement was at first received with considerable skepticism, but very soon similar observations were reported from other parts of the world. A new “shell” of the atmosphere had been revealed—which, as subsequent investigations proved, differs from the lower air in other respects besides temperature—and it was at first named by its discoverer the isothermal layer. He afterward substituted the name stratosphere, now generally employed. In distinction from the stratosphere, the part of the atmosphere lying below it is called the troposphere.
The stratosphere has been explored in widely scattered parts of the earth, and information concerning it is daily accumulating. Although it extends over the whole world, the altitude at which it begins is by no means uniform. The altitude is greater in summer than in winter; it varies with the barometric pressure at the earth’s surface; and it is decidedly greater over the equator than over the poles. The last fact leads to an interesting paradox. Since over the equatorial regions the temperature keeps on falling with ascent to a greater height than in other latitudes, it is here that the lowest temperatures in the atmosphere are found. A sounding balloon sent up from Batavia, Java, in November, 1913, recorded 113° below zero Fahr., the lowest air temperature ever observed. In middle latitudes the temperature of the stratosphere averages something like 68° below zero Fahr.
The temperature of this interesting upper atmosphere varies a good deal, both vertically and horizontally, but never shows the steady vertical variation that characterizes the lower air. The stratosphere contains no clouds (except occasional dust clouds), and has a circulation quite distinct from that of the troposphere, the exact nature of which, however, has not yet been determined.
The sounding balloon, already mentioned, is one of the four principal types of aerial vehicle used in the study of the atmosphere, the others being the pilot balloon, the captive balloon, and the kite. The sounding balloon, or ballon-sonde, is a small free balloon that carries no human aeronaut, but instead a set of superhuman meteorological instruments, which register the temperature, the barometric pressure, and sometimes the humidity continuously and automatically through the whole course of their journey. The record is traced on a revolving drum or disk, usually coated with lampblack. In its commonest form the balloon is made of india-rubber, and when launched is inflated to less than its full capacity with hydrogen. As it rises to regions of diminished air pressure it gradually expands, and it finally bursts at an elevation determined approximately in advance. A sort of parachute, or sometimes an auxiliary balloon, insures a gentle fall to the ground. Attached to the apparatus there is generally a ticket offering the finder a reward for its return, and giving instructions as to packing and shipping. Sooner or later it generally comes back. In fact, the large percentage of records recovered, even in sparsely settled countries, is not the least remarkable feature of this novel method of research. Thus, of seventy-two balloons sent up by a Franco-Swedish expedition in Lapland, forty-one were eventually recovered with their instruments. One of these fell into a lake and was found after three years.
No instruments are carried by the pilot balloon, which merely serves to show, by its observed drift, the speed and direction of the air currents at different levels. The pilot balloon is sighted, while in flight, through a special form of theodolite, or, preferably, two theodolites some distance apart. Several ingenious methods have been devised for computing and plotting its actual course through the air. Such balloons, apart from their use in scientific research, have become one of the principal adjuncts of aeronautical undertakings all over the world, and are also used by artillerists to enable them to make proper allowance for the deflective effect of the wind on the flight of projectiles. Hundreds of thousands of pilot balloons were sent aloft for military purposes during the world war.
Meteorological instruments are sent up attached to kites or captive balloons whenever—as in connection with weather forecasting—the observations must be obtained more promptly than would be possible with the aid of sounding balloons, but such devices can attain only moderate altitudes. Kites have been raised to about four and one-half miles above sea level, as compared with nearly twenty-two miles reached by a sounding balloon and twenty-four miles by a pilot balloon. The average height of sounding-balloon ascents is about ten miles. As already stated, balloonists have risen to 6.7 miles. This is a little higher than the best aeroplane record.
The use of the aeroplane for making meteorological observations is still quite limited, but will inevitably increase. One other device gives promise of yielding valuable aerological information, on account of its ability to rise to extraordinary altitudes. This is a special form of rocket, recently invented by Prof. R. H. Goddard, which is propelled by several successive discharges of an explosive in the course of its upward flight, and with which the inventor thinks it will be possible to explore the whole vertical extent of the atmosphere. Meteorological apparatus for use with the Goddard rocket has been planned by Mr. S. P. Fergusson of the Weather Bureau.
The atmosphere presses down upon the earth with a weight that, at sea level, amounts to about 14.7 pounds to the square inch, on an average. This pressure is, at any point, exerted equally in all directions; it acts, for example, on the whole surface of the human body, and this means that a man of average size lives under a burden of some seventeen tons of air. He is not incommoded because the pressure from without is balanced by that of the air that permeates his body.
The pressure of the atmosphere decreases upward at nearly the same rate as its density. Thus on mountains and plateaus it is considerably less than in lowlands. At no place is the pressure invariable, nor is there a constant relation between pressure and altitude, but, knowing approximately the average atmospheric pressure over the earth’s surface, and knowing also the area of the latter, we can compute in round numbers the total weight of the atmosphere—about 5,000,000,000,000,000 tons. This is about 1/1,200,000 of the entire weight of the earth.