768 The Outline of Science 



remains practically unaltered as far as registering balloons have 

 yet penetrated. In our latitudes this level is about seven miles 

 up ; over the Equator it is about ten miles, while in Polar regions 

 it is lower, being there only about five miles. Now this fact 

 introduces a rather curious paradox ; because, if the air-tempera- 

 ture keeps on decreasing until the level is reached where tem- 

 perature ceases to fall, it means that over the Equator the 

 temperature at a height above ten miles must be lower than the 

 temperature at a corresponding level over the Polar regions. 

 And this is actually the state of matters, for, notwithstanding 

 that the surface air at the Equator is much warmer than that in 

 the Polar regions, yet the temperature at the ten-mile level over 

 the Equator is about 110 degrees below zero Fahrenheit, while 

 over the Arctic Circle it is only about 60 degrees below zero. 



The existence of this level where the fall of temperature 

 ceases means that we have to consider the regions above and 

 below it as essentially different, and so, as we have seen, the two 

 regions are named the "stratosphere" and "troposphere" respec- 

 tively. Within the troposphere, where the temperature decreases 

 with height, it is possible for heated air to rise upwards, because 

 warm air is lighter than cold; but within the stratosphere this 

 cannot take place, for when a mass of air rises the decrease of 

 pressure causes it to expand, and the expansion lowers its tem- 

 perature, so that it would become colder than the surrounding 

 air; in other words, it would be prevented from rising. 



The cause of the existence of the stratosphere has not yet 

 been fully explained, but its effects are considerable. All tur- 

 bulence of the air must occur below the stratosphere, and practi- 

 cally all the clouds must be formed beneath it. All our storms 

 and changes of weather must therefore take place within the 

 limits of the troposphere. 



The Narrow Limits within which Life is Possible 



When we take into consideration the rate of fall of tem- 

 perature with height, the effect of which is forcibly demonstrated 



