text: Thus, certain corrections must apply to the 
  instrumentation as well as the aircraft per- 
  formance if the operating conditions do not 
  fit the standard atmosphere. In order to prop- 
  erly account for the nonstandard atmosphere 
  certain terms must be defined. Pressure .&itudc 
  the standard atmosphere 
  is the altitude 
  corresponditrg to a particular pressure. The 
  is essentially a sensitive 
  aircraft altimeter 
  barometer calibrated to 
  in 
  indicate altitude 
  If the altimeter is 
  the staotlard atmosphere. 
  set for 29.92 in. Hg the altitude indicated is 
  the pressure altitude-the altitude in the stand- 
  ard atmosphere corresponding to the sensed 
  pressure. Of course, this indicated pressure 
  altitude may not be the actual height above 
  sea level due to variations 
  in remperature, 
  lapse rate; atniospheric pressure, and possible 
  errors in the sensed pressure. 
  
  This 
  in 
  relationship has great application 
  aerodynamics and is quite fundamental and 
  necessary in certain parts of airplane perform- 
  ance. 
  
  VISCOSITY. The viscosity of the air is 
  in scale and friction effects. The 
  
  important 
  coefficient of absolute viscosity is the propor- 
  tion between the shearing stress and velocity 
  gradient for a fluid flow. The viscosity of 
  gases is unusual in that the viscosity is gen- 
  erally a function of temperature alone and an 
  increase in temperature increases the viscosity. 
  The coefficient of absolute viscosity is assigned 
  the shorthand notation I, (mu). Since many 
  parts of aerodynamics involve consideration of 
  viscosity and density, a more usual form of 
  viscosity measure is the proportion of the co- 
  efficient of absolute viscosity and density. 
  This combination 
  is termed the “kinematic 
  viscosity” and is noted by Y (nu). 