RECENT ADVANCES IN SCIENCE 15 



very much the same up to about 10 or 11 km., but becomes 

 smaller higher up. The diurnal variation on the other hand 

 appears to decrease rapidly with height and probably ceases 

 above 2 km. 



Moisture. — Relative humidity generally increases up to 

 the level of the lowest clouds, i.e. at about i or 2 km., and 

 decreases above, but for great heights the hair hygrometer 

 will not give reliable values on account of the low tempera- 

 tures encountered. 



The Troposphere and Stratosphere. — Mention has already 

 been made of the fact that the mean temperature is the same 

 at all levels above 12 km. over Europe. In individual cases 

 the lapse-rate generally becomes zero very abruptly, the 

 height at which this occurs (H,) varying considerably on 

 different days. The region below this point is generally 

 known as the troposphere and the region above as the strato- 

 sphere. The mean value of H^ varies from 10 km. to 11 km. 

 at the different European stations, and shows a marked sea- 

 sonal variation with the minimum in February. H,, is about 

 13 km. on the average in anticyclones and about 8 km. in 

 cyclones. In the concluding pages of the Report mention is 

 made of a close connection that is found between H^ and Pg 

 (the pressure at 9 km.), and between H^ and T„ (the mean 

 temperature of the air column between i km. and 9 km.). By 

 the method of Partial Correlation, it is found that if allowance 

 is made for the effect of T„, the variations of H„ follow those 

 of P9 very closely indeed. There is no obvious reason why 

 this should be so. 



Pressure. — ^The mean pressures for the different European 

 stations differ by a few millibars at i km., and by rather more 

 (about 12 mb.) at 7 km., but above 12 km. the differences 

 rapidly diminish, and at 20 km. the pressure is everywhere 

 practically the same. Near the equator the mean at 7 km. 

 is 430 millibars, as compared with 408 mb. over Europe, but 

 the mean at 20 km. differs little from the mean for the same 

 height over Europe. 



Wind. — Some information is given by the position of the 

 falling place of the registering balloons sent up on the Conti- 

 nent, the English ascents being excluded because so many 

 balloons fall into the Channel. The mean drift is found to be 

 towards S.E. by E. Those balloons which reach great heights 

 do not seem to travel farther than those attaining a moderate 

 height, so that there is some evidence in support of the theory 

 that the wind velocity falls off rapidly with increasing height 

 in the stratosphere. 



The Connection between Pressure and Temperature. — Oyer 

 Europe, between the levels of i km. and 20 km., the following 



