THE ISOTHERMAL REGION 
91 
The Isothermal Region 
It has been found by balloon ascensions to great elevations, 
up to 20,000 meters or more, that the temperature of the atmo 
sphere diminishes at the rate of about 6.0° C. per 1,000 meters 
up to an elevation of 12,000 meters in Europe, or 15,000 meters 
in the tropics, or even to 20,000 meters over the equator, while 
above these elevations the temperature is nearly constant or 
increases a little to the highest levels explored. There have 
been many conjectures as to the cause of the permanence of the 
heat of this isothermal region, as overflow of the tropic heat 
to mid-latitudes, conductional transportation of heat from the 
lower to the higher levels, production of ozone by the incoming 
solar radiation in the upper atmosphere and absorption of the 
short waves of the solar radiation in the same region. There are 
objections to each of these hypotheses so obvious that we pro 
ceed at once to examine the thermodynamic data for at least a 
statement of the case, if not a complete explanation of the facts. 
The computations were executed for the following balloon 
ascensions, as reported in the volumes: 
Europe 
Lindenburg, April 27,1909 (52°). 
“ May 5, 1909 “ 
“ May 6, 1909 “ 
“ July 27,1908 “ 
“ Sept. 2,1909 “ 
Mailand, Sept. 7,1906 (45°). 
Atlantic Tropics 
Sept. 25,1907 (35°). 
Sept. 9,1907 (25°). 
Aug. 29,1907 (13°). 
July 29,1907 (13°). 
June 19,1906 (-2°). 
Victoria Nyanza, 1908 (0°). 
The mean values are compiled in Table 23, and illus 
trated in Figs. 5 and 6. Since the data in the isothermal region 
are not so complete as below it, these results are to be considered 
as instructive rather than definitive. It will require the work 
of many years to accumulate and compute the data necessary 
for normal conditions. The temperatures show that there 
are as wide local fluctuations in the isothermal region as below 
it. Furthermore, the temperatures are lowest over the equator, 
200°, and gradually increase to 210° in the tropics, or 215° in