Specific humidity can be determined from the CP-
718/UM density altitude computer following
instructions printed on the computer.
temperature, dew-point temperature, and pressure from
the observation are used as arguments.
Proper performance of aircraft engines and the lift
produced by the aircrafts 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
What does a pressure altitude lower than the
actual altitude mean?
What happens to density altitude when the air
Q94. How may density altitude be calculated?
Q95. How does humidity affect air density?
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 Users 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