STRATOSPHERE, the portion of the atmosphere lying above six miles in elevation, where the temperature ceases to fall with increasing altitude, is called the stratosphere or, more de scriptively the isothermal layer. From the earth's surface up to about six miles, the temperature diminishes nearly uniformly at the rate of about 6° C. per kilometre (I° F. per 30o feet) reach ing the low level of about —55° C. (-67° F.). Beyond this height there is little change or a slight rise of temperature to an elevation of at least 25 miles. Two considerations indicate that much higher temperatures prevail in the atmosphere above 60 miles. Firstly, loud sounds are sometimes heard at great dis tances, though silent at intermediate localities. Those atmospheric layers near the earth's surface, where temperature falls with in creasing height, bend the sound path upwards, and those at high levels, where the gradient is rising, reverse the curvature and lead the path downward to the distant listener. Investigations of sound reception indicate that the atmospheric temperature above 6o miles is at least as high as that of the earth's surface at the tropics.
Secondly, recent studies of short wave radio reception have led several authors to suggest that much higher temperatures, even perhaps 1000° C. (1800° F.) must prevail at a height of 200 miles in the atmosphere.
The causes of the remarkable temperature conditions of the stratosphere and higher are not fully worked out. They depend on the absorption and emission of radiation—solar, terrestrial, and atmospheric. These depend particularly on the proportions of ozone, carbon dioxide, and water vapour which are there in minute proportions not yet exactly known.
A few samples of stratospheric gases have been collected by Russian aeronauts.* These seem to show that the atmosphere at I 2 miles elevation is of nearly the same percentage composition as regards oxygen and nitrogen as at the earth's surface. Contrary to former expectations, there was found no considerable propor tion of hydrogen or helium.
The barometric pressures prevailing at high altitudes are of course very low, as shown by the table.
The highest level yet reached by unmanned sounding balloons carrying selfrecording instruments is about 25 miles. Manned balloons require hermetically sealed chambers fed by oxygen, and specially air-conditioned, to reach heights of 10 to 15 miles. As such enclosures are necessarily very heavy, the sizes of balloons required to float them in highly rarefied air become immense, and prohibitively costly. The largest balloon thus far used, that of the National Geographic Society, had a capacity of 3,700,000 cubic feet and reached about 14 miles elevation. Should the rocket be sufficiently perfected, there is no limit to the elevation which it could reach, as its reactive force operates quite as well in a vacuum as in air, but many difficulties might be encountered in observing from rockets. Stratospheric and higher altitude ob servations of ozone and other gaseous contents, of pressure and temperature gradients, of electrical conditions, of cosmic rays, and of the ultra-violet solar spectrum are desired. (C. G. A.) *Samples were also collected in the recent National Geographic Society ascension but the results are not yet published.