Specific humidity can be determined from the CP-718/UM density altitude computer followinginstructions printed on the computer.The airtemperature, dew-point temperature, and pressure fromthe observation are used as arguments.Proper performance of aircraft engines and the liftproduced by the aircraft’s wings depend on the densityof the air. Air density is affected by pressure,temperature, and water content of the air. Calculatedvalues of pressure altitude, density altitude, and specifichumidity are routinely done to provide pilots withspecific quantities on which they may basecomputations for the thrust, lift, and maximum loads fortheir aircraft.Q92.Q93.REVIEW QUESTIONSWhat does a pressure altitude lower than theactual altitude mean?What happens to density altitude when the airtemperature increases?Q94. How may density altitude be calculated?Q95. How does humidity affect air density?SUMMARYIn this chapter, we have discussed the terms andprocedures used when evaluating and measuringsurface weather elements, and some of the routinelycomputed values and indicators associated with surfaceweather observations. The material has been presentedin a manner that, we hope, will guide you to a basicunderstanding of the subject of "surface weatherobservations." Additional study of the publications andmanuals referenced in the text, especiallyNAVMETOCCOMINST 3141.2, Surface METARObservations User’s Manual, and NAV-METOCCOMINST 3144.1, United States NavyManual for Ship's Surface Weather Observations, willbe necessary for a thorough understanding of thesubject.1-65
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