200 Tabue 47 



corrections to reduce barometric readings to standard 



GRAVITY 



The observed height of a mercurial barometer, corrected for instrumental errors, is a 

 function of the temperature of the barometer and of the local acceleration of gravity, as 

 well as of the pressure. Therefore, to obtain a true relative measure of the atmospheric 

 pressure, the observed height of the mercurial column must not only be reduced to what 

 its height would be if at a standard temperature (Tables 45 and 46), but also to what it 

 would be at a standard value of gravity. 



This correction may be written in the form 



c-ElSZlPB, (1) 



go 



where 



c = correction to be applied to the barometric height B, 

 g = standard value of gravity adopted, 

 gi = value of local gravity, 



B = height of the mercury column, corrected for temperature and instrumental 

 errors. 



The correction c will be in the same units as the height of the barometer B. 



Instruction for obtaining gi are given in Table 167. For most meteorological applications 

 the standard value of gravity is 980.665 cm. sec. -2 (see Introduction, p. 3). For g t < g , the 

 correction is to be subtracted from B, for gi > go, it is to be added. 



Since c is a linear function of B and {gi —go) linear interpolation and linear combina- 

 tions of the values given in Table 47 A are valid. 

 Example : 



Barometer reading B, corrected for temperature and instrumental error = 



29.647 in. Hg. 

 Local gravity gi = 978.12 cm. sec.~ a 

 gi —go = 978.12 — 980.665 = — 2.54 cm. sec. 



From the table : 



Correction for B = 20 0.0518 



" B= 9 0233 



" 5= 0.6 0016 



" B = 0.05 0001 



Total correction 0.0768 = 0.077 



Barometer reading corrected for gravity = 29.647 — 0.077 = 29.570 in. Hg. 



For routine use at a land station, data for the appropriate gi — go can be extracted 

 from the table and conveniently expanded for the range required by the barometer in use. 

 Table 47 A can also be used with no appreciable error for reducing barometric readings 

 to other values of standard gravity go in the neighborhood of 980.665 cm. sec." a 



Ships at sea. — For reducing barometric observations made aboard ships, where it is 

 impractical to determine local gravity gi in accordance with the procedure given in 

 Table 167, it is necessary to assume that no gravity anomalies exist and that the local 

 acceleration of gravity is a function of latitude only. The correction c may then be written 



cz= g * ~ g " B (2) 



go 



where g* is the acceleration of gravity at latitude <p and sea level. 

 Table 47 B gives values of c computed from equation (2). 



{continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



