GEOPOTENTIAL : DYNAMIC HEIGHTS liii 



sure at the upper station be 2.4 mb. ; at the lower station 7.3 mb. 



Let the mean temperature of the air column be ^=:5?8 C. and the 



latitude </) = 39° 25' N. 

 Table 57, with argument 448.6, gives 65 11 meters. 



Table 57, with argument 1000.3, gives 104 



Approximate value of Z 6407 meters. 



Table 61, with arguments 449 and 2.4 gives A^ = o.3 

 Table 61, with arguments 1000 and y.;^ gives A^ = o.4 

 Table 58, with 6 = 5°8 + o?7 = 6?5, and 2 = 6407 gives 



6407x0.024= 154 



Table 62 with 2 = 6561 and <^= 39° 25', gives 19 



Table 63 with 2 = 6561 and ho = o, gives 7 



Corrected value of Z =6587 meters. 



GEOPOTENTIAL : DYNAMIC HEIGHTS. 



In accordance with the " Reglement " ^ of the Commission Internationale 

 de la Haute Atmosphere adopted at the meeting held in London in April, 1925, 

 heights in all forms and publications of the International Commission are to 

 be measured as " geopotentials " in " dynamic meters " above sea level. 



The geopotential or gravity potential of a point is defined numerically 

 as the value of the potential energy relative to sea level of a unit-mass situated 

 at the point. 



The application of geopotential as a measure of height becomes more 

 evident when it is seen that surfaces of equal geopotential are identical with 

 horizontal or level surfaces, and due to the geographical variation of gravity, 

 they are not surfaces equally distant from sea level. In this regard it may be 

 emphasized that energy is involved in displacing a mass of air from one 

 position to another in which the potential energy of the mass is different, 

 whereas the displacement of air may take place along horizontal or equi- 

 geopotential surfaces without the gain or expenditure of potential energy 

 once the air is in a state of uniform motion. The latter statement, on the 

 contrary, does not hold for surfaces of equal geometric height above sea level. 



For the purposes of dynamical meteorology, in making comparisons of 

 vertical positions, certain advantages are derived by defining the height of 

 points above sea level in terms of geopotential. Heights measured in this way 



1 A fuller account of this Reglement may be found in the Avant-Propos of the Com- 

 mission Internationale de la Haute Atmosphere, Comptes Rendus des Jours Internation- 

 aux 1923, published in 1927. This may be had on application to the Secretary of this 

 Commission, c/o the Royal Meteorological Society, London. 



