The Science of the Weather 765 



different: hydrogen and helium are both believed to be present; 

 the latest theory is that helium predominates. 



So far, however, as temperature is concerned, the atmosphere 

 is divided into two parts, a lower region in which there is a fairly 

 rapid decrease of temperature with height, and an upper region 

 where, as far as the limit investigated up to the present, such a 

 condition does not prevail. 



The higher layer, in which there is no material change of 

 temperature with height, we call the "stratosphere." The lower 

 layer, in which there is considerable lapse of temperature with 

 height, we call the "troposphere." The region where troposphere 

 changes to stratosphere is generally, but not always, marked by a 

 sudden cessation of fall of temperature; sometimes the change is 

 more gradual, but in almost all cases it is possible to define the 

 level where the stratosphere commences. This level varies with 

 the seasons and with the geographical position of a place ; in our 

 latitudes it is on the average about seven miles above sea-level. 

 The temperature of either the stratosphere or the troposphere 

 is of course not constant, but shows considerable variations. The 

 point is, that in the former case temperature does not decrease 

 with height; in the latter case it does. The belief once held that 

 the higher we ascended the colder it would continue to become 

 is therefore not true. Let us see how this astounding fact was 

 revealed. 



2 



Sounding the Upper Air 



Of late a great deal of study has been given to the con- 

 ditions that exist in the air above the earth, and, whenever 

 possible, the chief stations investigate the upper air by means 

 of balloons. Two types are used, one of which is a fairly large 

 balloon called a registering balloon or "ballon sonde"; this bal- 

 loon is sent up with a small instrument called a "meteorograph" 

 attached to it. The balloon, set free, rises upward until it bursts, 



