Table 67 (continued) 275 



CORRECTION OF ALTIMETER READINGS FOR MEAN TEMPERATURE OF 



THE AIR COLUMN 



Table 67 A gives values of l -£— - )> as a function of I P and H P and Table 67 B 



\ 1 mx / 



give values of T mv J -£— ? I, the actual height above the ground, as a function of 



\ T mx / 



Tmv and ( —5— I • The actual altitude above sea level, A, is then obtained by adding 



\ 1 mx / 



E, the elevation of the surface, to the value obtained from Table 67 B. It is obvious 

 from equation (3), that if H P is in error, A will be in error by approximately the same 

 amount. 



The value of H P may be related to the altimeter setting, P h at elevation E by means 

 of the approximate 3 equations 



H P = E + 925 (29.92 - Pi) ft., (4) 



where Pi is in inches of mercury, or 



H P = E + 27.3 ( 1013.2 mb. - Pi) ft., (4a) 



where Pi is in millibars. 



Correction for mean temperature, aircraft using current, local altimeter setting. — In 



the case of an aircraft flying at an indicated altitude I with the altimeter set to the 

 current, local altimeter setting, 



I P -H P = I-E (5) 

 and to a close degree of approximation 



I P + H P = I + E (6) 

 Equation (3) can now be rewritten 



A=E + T„fe£} (7) 



Table 67 A also gives values of I — ~ — J as a function of I and E and Table 67 B 



\ i mx / 



gives values of Tmv I — ), the actual height above the ground as a function of 



\ 1 mx / 



Tmv and J — J . The actual altitude above sea level, A, is then obtained by adding 



\ 1 mx / 



E, the elevation of the surface, to the value obtained from Table 67 B. 



Correction for erroneous altimeter setting. — If the altimeter is set to an erroneous 

 altimeter setting P 2 when the true current, local altimeter setting is Pi, the observed 

 indicated altitude, h, will be in error, and the appropriate value of I to be used in equa- 

 tion (7) and Table 67 A is given by 



7 = /o + 925 {Pi- P.) ft. (8) 



if Pi and Pn are in inches of mercury, and 



7 = /o + 27.3 (Pi-P,) ft. (8a) 



if Pi and P 2 are in millibars. 



From equations (8) and (8a) it is evident that errors due to use of an inappropriate 

 value for the current, local altimeter setting may be corrected (within 1.8 percent for 

 the normal range of altimeter settings 31.00-29.00 in. Hg., 1045.0-980.0 mb.) by use of 

 the following relationships : 



0.01 in. Hg. change in altimeter setting = 9.25 ft. change in indicated altitude. 

 « « « « « « « _ 2-82 m . 



0.1 mb. " " " " =2.73 ft. 



" " " " " " =0.833 m. 



If the altimeter setting used is lower than the true altimeter setting, the observed indi- 

 cated altitude should be increased in accordance with the above relationships before 



3 The last term of equations (4) and (4a) is accurate within 1.8 percent when P a lies in the range 

 31.00-29.00 in. Hg. (104S.0-980.0 mb.). 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



