274 Table 67 



CORRECTION OF ALTIMETER READINGS FOR MEAN TEMPERATURE OF 



THE AIR COLUMN 



Introduction. — The sensitive pressure altimeter is essentially a refined aneroid barometer 

 calibrated so that when the altimeter setting is 29.921 in Hg. (1013.25 mb.) the instru- 

 ment indicates the altitude in the standard atmosphere corresponding to the ambient 

 barometric pressure. The instrument is so constructed that the altimeter setting scale 

 may be adjusted to make the instrument indicate the true elevation of the ground above 

 sea level when the aircraft is on the ground. The appropriate reading on the altimeter 

 setting scale for this to be true is termed the altimeter setting (see Table 65). For 

 aircraft in flight, the altimeter indicates elevation above sea level assuming the 

 pressure to vary between the ground and flight level in accordance with a standard 

 atmosphere (see Table 63). According to current practice, aircraft flying over land 

 areas or making landings set their altimeters to the current, local altimeter setting. 

 Aircraft on transoceanic flights use a constant altimeter setting of 29.92 in Hg. 

 (1013.2 mb.). 



Sources of error. — In addition to instrumental and installation errors, which will not 

 be discussed here, the altitude indications of the altimeter may differ from the actual 

 altitude of the aircraft because of: 



1. Deviation of the mean virtual temperature of the actual air column from the 



value assumed in the standard atmosphere. 



2. Use of an altimeter setting inappropriate to the pressure and elevation at the ground 



beneath the aircraft. 



Correction for mean temperature, aircraft using constant standard altimeter setting. — 



Considering first the case of an aircraft flying at a constant altimeter setting of 29.92 

 in. Hg. (1013.2 mb.), from the hypsometric equation 



A = E + T m JJ^-^)> (1) 



where 



A = actual elevation x of the aircraft above sea level, 

 E = elevation of the surface above sea level, 

 T m v = actual mean virtual temperature (approx. absolute), 2 of the air column 

 between E and A (the corresponding value in °C. is denoted by t mv .), 

 I p = "pressure altitude" at flight level, the altitude in the standard atmosphere 

 corresponding to the pressure at flight level (= indicated altitude in this 

 case), 

 H p = "pressure altitude" at the surface, the altitude in the standard atmosphere 



corresponding to the pressure at the surface, E, 

 T mp =: mean temperature (approx. absolute) of the standard atmosphere air 



column between sea level and I P , 

 Tm> = mean temperature (approx. absolute) of the standard atmosphere air column 

 between sea level and H p 



(2) 



To evaluate equation (1), we define a temperature Tmx by the equation 



n,-H P \/ h h,\ 



\ 1 mx / \1 mp 1 m> / 



where T mx may be regarded, to a close degree of approximation, to be the mean tempera- 

 ture (approx. absolute) of the standard atmosphere air column between sea level and 

 the level (I P -f- H P ), provided (I P + H P ) is less than the height of the base of the 

 standard atmosphere stratosphere (35,332 ft. or 10,769 m. in the NACA standard 

 atmosphere). 

 Equation (1) can therefore be written 



A = E + T mv (*>j^El^ (3) 



1 Strictly speaking, A, E, Ip, and Hp should be expressed in the same units. Actually A and E are 

 customarily given in geometric units, while Ip and Hp are measured in terms of geopotential (see foot- 

 note 6, Table 63). The difference may be neglected here. 



a Approximate absolute temperature = 273 + * °C. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



