text: Altitude pressure ratio 
  
  Ambient static pressure 
  =Standard sea level static pressure 
  
  6 = PIP0 
  
  Many items of gas turbine engine perform- 
  ance are directly related to some parameter 
  the altitude pressure ratio. 
  involving 
  TEMPERATURE. The absolute tempera- 
  cure of the air is another important property. 
  The ordinary temperature measurement by the 
  Centigrade scale has a/datum at the freezing 
  point of water but absolute zero temperature 
  is obtained at a temperature of -273“ Centi- 
  grade. Thus, the standard sea level tcmpera- 
  ture of 15” C. is an absolute temperature of 
  288”. This scale of absolute temperature using 
  the Centigrade increments is the Kelvin scale, 
  for the 
  e.g., o K. The shorthand notation 
  ambient air temperature is “T” 
  and the stand- 
  ard sea level air temperature of 288’ K. is 
  signified by Ta. The more usual reference is, 
  
  2 