MOVEMENTS OF ATMOSPHERE— GULDBERG AND MOHN I 75 



PART II 



(Christiania, 1880, revised 1885) 



Chapter IV 



on motion in general 



§18. Isobaric surfaces. — Vertical gradient 



In meteorology we speak of isobaric surfaces as surfaces of equal 

 pressure or isopiestic surfaces or simply isopiesics. If the air is in equi- 

 librium we can consider the isobaric surfaces approximately as spheres 

 concentric with the earth, and, for a small part of the surface of 

 the earth, we shall treat these surfaces as horizontal planes. If the 

 air is in motion the isobaric surfaces differ from horizontal planes. 

 In order to fix our ideas we consider a horizontal current of air 

 whose isobaric lines at the surface of the earth are concentric circles. 

 Let the values of the pressure for the different distances from the 

 center be as follows: 



r in degrees of 



a great circle = 4.5 6.5 8 9.8 11.8 14.5 17.8 



b in millimeters 



of mercury = 725 730 735 740 745 750 755 760 



The diminution of the pressure A b for a change of altitude A z 

 can be approximately calculated by the formula (see §4, eq. 2). 



A b gAz A z 



J~ ' ' ~aT ' 8200 



Giving successively to b the values 725, 730 we 



calculate the values of A z for any value of A b, and we can construct 

 a vertical section of the isobaric surfaces (fig. 15). When the air is 

 in equilibrium a vertical section of the isobaric surfaces will present- 

 a series of straight horizontal lines (fig. 16a): supposing that we 

 have a vertical current, the vertical section of the isobaric surfaces 

 would also present a series of straight horizontal lines, but different 

 from those of the series in fig. 16a. We shall call these lines of 

 intersection vertical isobars, and if we wish to introduce the term 

 vertical gradient we should be obliged to establish a definition similar 

 to that of the term horizontal gradient. Assume the vertical grad- 

 ient equal to the difference of the pressures shown by two isobars 

 divided by their distance, we shall always and even in a state of 



