text: The more appropriate term for correlating 
  aerodynamic performance in the nonstandard 
  atmosphere is density &it&-the 
  altitude in 
  the standard atmosphere corresponding to a 
  particular value of air density. The computa- 
  tion of density altitude must certainly involve 
  consideration of pressure (pressure altitude) 
  and temperature. Figure 1.6 illustrates 
  the 
  manner in which pressure altitude and tem- 
  perature combine to produce a certain density 
  altitude. This chart is quite standard in use 
  and is usually included in the performance 
  section of the flight handbook. Many subject 
  areas of aerodynamics and aircraft performance 
  will emphasize density altitude and temperature 
  as the most important 
  sideration. 
  
  The kinematic viscosity of air at standard sea 
  level conditions is 0.0001576 square feet per 
  second. At an altitude of 40,000 feet the 
  kinematic viscosity is increased to 0.0005059 
  square foot per second. 
  
  In order to provide a common denominator 
  for comparison of various aircraft, a standard 
  atmosphere has been adopted. The standard 
  atmosphere actually represents the mean or 
  average properties of the atmosphere. Figure 
  1.1 illustrates the variation of the most im- 
  portant properties of the air throughout 
  the 
  standard atmosphere. Notice that the lapse 
  rate is constant in the troposphere and the 
  stratosphere begins with the isothermal region. 
  Since all aircraft performance is compared 
  and,evaluated in the environment of the stand- 
  ard atmosphere, all of the aircraft instrumenta- 
  tion is calibrated for the standard atmosphere. 