Abstract: An apparatus for use in an aircraft for determining and displaying a reading which is indicative of pressure density at any instant in time, comprises sensing means for determining both ambient air pressure and ambient air temperature at any instant in time, and outputting an electrical signal having a value which varies in accordance therewith. Calculating means are provided for combining those electrical signals and outputting a weighted signal for pressure density having a value based on an index value of zero for a pressure density at sea level when the ambient temperature is 59° F. and the ambient pressure is 29.92 inches of mercury. A microprocessor computer means has random access memory and read only memory, and an output signal driver for outputting a signal indicative of pressure density at any instant in time to a display.