text: DENSITY. The density of the air is a prop- 
  erty of greatest importance in the study of 
  aerodynamics. The density of air is simply 
  the mass of air per~cubic foot of volume and 
  is a direct measure of the quantity of matter 
  in each cubic foot of air. Air at standard sea 
  lcvcl conditions weighs 0.0765 pounds per cubic 
  foot and has a density of 0.002378 slugs per 
  cubic foot. At an altitude of 40,000 feet the 
  air density is approximately 25 percent of the 
  sea level value. 
  
  The shorthand notation used for air density 
  is p (rho) and the standard sea level air density 
  is then pO. In many parts of aerodynamics it 
  is very convenient to consider the proportion 
  of the ambient air density and standard sea 
  level air density. This density ratio is assigned 
  the shorthand notation of c (sigma). 
  
  ambient air density 
  standard sea level air density 
  
  a = PIP0 
  
  A general gas law defines the relationship of 
  pressure temperature, and density when there 
  is no change of state or heat transfer. Simply 
  stated this would be “density varies directly 
  with pressure, inversely with 
  temperature.” 
  Using the properties previously defined, 