268 TABLE 64 



ICAN STANDARD ATMOSPHERE 



The standard atmosphere adopted by the International Commission for Air Navigation 

 (ICAN) in 1924 is used, with minor modifications, by many countries as a standard for 

 calibrating altimeters and in other applications. 



DEFINITION OF INTERNATIONAL STANDARD ATMOSPHERE 1 



1. It will be assumed that the air is dry and that its chemical composition is the same at 



all altitudes (it is recalled, by way of information, that this composition is in 

 volume approximately as follows : 78.03% nitrogen, 20.99% oxygen, 0.94% argon, 

 0.04% carbon dioxide) ; g will be given a uniform value of 980.62 in cgs units. 

 Nevertheless, when the accuracy of the measurements under consideration permits, 

 it will be sufficient to use for g the approximate and simpler value of 980. 



2. It will be assumed that at mean sea level the temperature is 15 °C. and the barometric 



height, reduced to °C, 760 mm. of mercury. 



3. Under these conditions, the atmospheric pressure is 10,332 kilograms-weight per square 



meter (1013.2 millibars) and the weight of a cubic meter of air is 1.226 kilogram- 

 weight. 



4. It will be assumed that, for any altitude z, measured above mean sea level and between 



and 11,000 m., the law of variation of the temperature 6 Z of the air is as follows: 



0* = 15 -0.0065s. 



5. It will be assumed that, for all altitudes above 11,000 m., the temperature of the air 



is constant and equal to — 56.5 °C. 



6. It follows that, for any altitude z measured above mean sea level and between and 



11,000 m., the barometric pressure pt, the specific weight a z , and the specific mass p« 

 of the air will vary according to the following equations : 



£. _ / 288-0.0065A 5 - 256 

 p ~ \ 288 ) 



and P L _a 1 _ / 288-0.0065A 4 - 256 

 Po oo \ 288 ) 



7. Similarly for all altitudes above 11,000 m. the foregoing equations will be replaced 



by the following: 



, />11,000 , 11,000 , all.OOO s-11,000 



lOgio t ~ = lOgio — ! = logio '■ = ,. ' 



pt Pz a, 14,600 



The principal differences between the NACA Standard Atmosphere (Table 63) widely 

 used in the United States and the ICAN Standard Atmosphere are in the value of 

 standard gravity adopted and in the definitions of the height and temperature of the tropo- 

 pause and the isothermal region above. Since the value of standard gravity adopted for 

 use in the definition of the standard atmosphere is also used for the definition of the inch 

 or millimeter of mercury, an altimeter calibrated by the NACA Standard Atmosphere 

 will read about 1.3 ft. (0.4 m.) higher at a given pressure than an altimeter calibrated by 

 the ICAN atmosphere at altitudes below the tropopause. Above the tropopause, other 

 slight differences will be introduced. 



Brombacher 2 has published a discussion of the modifications in the ICAN atmosphere 

 made by various countries as well as a summary of other standard atmospheres that have 

 been used for calibrating altimeters. 



1 International Commission for Air Navigation, Official Bulletin No. 26, Resolution No. 1053, 

 December 1938; also Official Bulletin No. 7, Resolution No. 192, December 1924. 



2 Brombacher, W. G., Journ. Washington Acad. Sci., vol. 34, p. 277, 1944. 



SMITHSONIAN METEOROLOGICAL TABLES 



