In order to provide a common denominator for comparison of the performance of various aircraft, a standard atmosphere has been adopted. The set of standard conditions presently used in the U.S. is known as the International Standard Atmosphere (ISA) and has been adopted by most of the nations and airlines of the world - the International Civil Aviation Organization.
   The standard atmosphere actually represents the mean or average properties of the atmosphere that is, it represents the year round average of the pressure/height temperature soundings observed over a period of years.

   In the standard atmosphere, sea level pressure is 29.92" Hg and 15 degrees C. (59 degrees F.); the standard lapse rate (decrease) for pressure are approximately 1" Hg per 1,000 feet increase in altitude and 2 degrees C. (3.5 degrees F.) per 1,000 feet increase (up to the tropopause). Fig. 17-62 {should be Fig. 17-69 - Ed.} illustrates the variation of the most important properties of the air throughout the standard atmosphere.

   Since all airplane performance is compared and evaluated in the environment of the standard atmosphere, all of the airplane performance instrumentation is calibrated for the standard atmosphere. Thus, certain corrections must apply to the instrumentation, as well as the airplane performance, if the actual operating conditions do not fit the standard atmosphere. In order to account properly for the nonstandard atmosphere certain related terms must be defined.

                       STANDARD ATMOSPHERE
                           Figure 17-69
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   Altitude     Pressure     Temp.     Temp.     Density -
     (ft)       (in. Hg)     (C.)      (F.)     slugs / ft^3

        0        29.92       15.0      59.0      0.002378
    1,000        28.86       13.0      55.4      0.002309
    2,000        27.82       11.0      51.9      0.002242
    3,000        26.82        9.1      48.3      0.002176
    4,000        25.84        7.1      44.7      0.002112
    5,000        24.89        5.1      41.2      0.002049
    6,000        23.98        3.1      37.6      0.001988
    7,000        23.09        1.1      34.0      0.001928
    8,000        22.22       -0.9      30.5      0.001869
    9,000        21.38       -2.8      26.9      0.001812
   10,000        20.57       -4.8      23.3      0.001756
   11,000        19.79       -6.8      19.8      0.001701
   12,000        19.02       -8.8      16.2      0.001648
   13,000        18.29      -10.8      12.6      0.001596
   14,000        17.57      -12.7       9.1      0.001545
   15,000        16.88      -14.7       5.5      0.001496
   16,000        16.21      -16.7       1.9      0.001448
   17,000        15.56      -18.7      -1.6      0.001401
   18,000        14.94      -20.7      -5.2      0.001355
   19,000        14.33      -22.6      -8.8      0.001310
   20,000        13.74      -24.6     -12.3      0.001267
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