THE UPPER ATMOSPHERE — DOBSON 



187 



60 



50 



40 



^30 



20 



10 



Such a complication instead of obscuring the results only gives 

 increased information, since each set of waves can be treated sep- 

 arately and the temperature can be found for the different heights 

 to which they have penetrated. Thus we get not only the tempera- 

 ture at one height but that at two or more heights and so the rate 

 of rise of temperature with height. This rise of temperature is found 

 to begin at a height of about 35 Irilometers and is at a rate of about 

 6° C. per kilometer of height and is therefore roughly the same as 

 the rate of decrease of temperature with height in the troposphere. 

 It is not yet certain how far this increase of temperature with height 

 continues, but it appears that the tem- 

 perature has risen to about 100° C. — 

 the boiling point of water — at a height 

 of 60 to 70 kilometers and that it is still 

 higher above. 



Up to the present time most of the 

 observations of the upper-air tempera- 

 tures by means of sound waves have 

 been made in Europe, but it seems now 

 to be established that the upper warm 

 region extends over the Equator at 

 much the same height as in Europe, 

 while it has been found in polar regions 

 also. When it is possible to accumu- 

 late more observations it will be inter- 

 esting to determine the diurnal and 

 annual variation of temperature at 

 these great heights, as well as to see if 

 there are day to day changes in temper- 

 ature associated with weather condi- 

 tions similar to those found at lower 

 levels. Unfortunately, the cost of spe- 

 cial explosions is high and most of the observations made in this 

 country have utilized the sound from big guns which were fired for 

 other purposes. It is a curious circumstance, probably due to wind 

 at great heights, that the sound is hardly ever heard to the west of 

 the source in winter nor to the east of it in summer. 



Cause of the warinth at great heights. — The temperature of the 

 air in the troposphere, where there is constant mixing of the air 

 between various levels, is governed largely by the supply of heat 

 constantly received by contact with the warm ground. In the 

 stratosphere, and above, the conditions are different and there is 

 little or no mixing with the air below. Here the temperature de- 

 pends on the absorption and emission of radiation. Radiation from 

 the sun — largely in the form of visible light — passes inward through 



. / 



^ 



:: 



:» 



'lOlf -50° 







50 



mc. 



FiGUKB 2. — Probable disiributioii of 

 temperature with height over 

 England. The continuous line 

 from the ground level to 20 km 

 shows the average temperature as 

 obtained from balloon ascents. 

 The dotted line indicates the prob- 

 able temperature as obtained from 

 sound waves. 



