SUMMARY

Specific humidity can be determined from the CP- 718/UM density altitude computer following instructions printed on the computer. The air temperature, dew-point temperature, and pressure from the observation are used as arguments. Proper performance of aircraft engines and the lift produced by the aircraft’s wings depend on the density of the air. Air density is affected by pressure, temperature, and water content of the air. Calculated values of pressure altitude, density altitude, and specific humidity are routinely done to provide pilots with specific quantities on which they may base computations for the thrust, lift, and maximum loads for their aircraft. Q92. Q93. REVIEW QUESTIONS What does a pressure altitude lower than the actual altitude mean? What happens to density altitude when the air temperature increases? Q94. How may density altitude be calculated? Q95. How does humidity affect air density? SUMMARY In this chapter, we have discussed the terms and procedures used when evaluating and measuring surface weather elements, and some of the routinely computed values and indicators associated with surface weather observations. The material has been presented in a manner that, we hope, will guide you to a basic understanding of the subject of "surface weather observations." Additional study of the publications and manuals referenced in the text, especially NAVMETOCCOMINST 3141.2, Surface METAR Observations User’s Manual, and NAV- METOCCOMINST 3144.1, United States Navy Manual for Ship's Surface Weather Observations, will be necessary for a thorough understanding of the subject. 1-65