This practice is no longer followed. Additionally, sea-
level pressure should not be converted from
hectopascals into inches for use as an altimeter setting,
since differences in such calculations could yield false
Aircraft flying above 18,000 feet overland and on
over water flights more than 100 miles offshore
routinely use the standard pressure, 29.92 inches, as an
altimeter setting. During low-level tactical flights and
landings aboard aircraft carriers, however, accurate
altimeter settings are required. Now lets consider
The pressure tendency is the net change in the
barometric pressure during a period of time and the
trend or characteristic of the change. Normally the
pressure tendency is observed for 3-hour periods ending
at the intermediate synoptic times 0000Z, 0300Z,
0600Z, etc. Pressure tendencies for 12- and 24-hour
periods may also be observed, and routinely replace the
3-hour pressure tendencies in observations taken in the
The net change is determined by taking the
difference in the station pressure between the current
observation and the station pressure 3, 12, or 24 hours
ago. The trend or characteristic is determined from the
barograph trace, or the actual recorded station pressures
during the period.
The general trends of pressure
"higher," the "same," or "lower" than at the beginning of
the period are further described in both NAVMET-
OCCOMINST 3 141.2 and NAVMETOCCOMINST
3144.1 for reporting purposes.
We will discuss some related pressure calculations
on pressure altitude and density altitude later in this
chapter in a section on aircraft performance indicators,
but first we must cover temperature and moisture
observations, which are necessary for those
Q51. Ten millibars is equal to how many
Q52. What is meant by the term "removal correction"?
Q53. What would be the "r" factor for a shipboard
barometer located 45 feet above the water line?
Q54. What is an altimeter setting used for?
Q55. How is the overall trend or characteristic of
pressure tendency determined?
temperature. Define and describe how to
obtain dry-bulb temperature and wet-bulb
temperature readings. Define and describe
how to calculate dew-point temperature and
frost-point temperature. Define sea-surface
temperature and describe three methods used to
obtain this reading.
Temperature is defined as the amount of sensible
heat in a substance or as the measurement of molecular
motion in a substance. Molecules in motion cause heat.
As energy is added to a substance in the form of light or
as infrared radiation (heat energy), the molecules
absorb the energy, which increases molecular motion.
This increase in molecular motion is measured as an
increase in temperature. Higher temperature
substances will also give off energy by radiation.
Higher temperature substances can also transfer energy
from faster moving molecules (warmer) to slower
moving (cooler) molecules as the molecules collide.
This process is known as conduction.
In surface aviation weather observations, observers
take dry-bulb temperature and wet-bulb temperature
readings by using sling psychrometers or electric
psychrometers, or they obtain the readings from
automatic weather station equipment. From these
readings, the observer may calculate the dew-point
temperature by using the CP-165/UM psychrometric
computer, although automatic systems will calculate
this important value.
Related to the dew-point
temperature is the frost-point temperature, which may
need to be calculated. Another temperature required for
shipboard surface aviation weather observations is the
Also, once each day, the
observer must obtain a maximum and a minimum
The dry-bulb temperature (also called the ambient
air temperature, or simply the air temperature) reflects
the amount of heat present in the air. It is read directly
from a ventilated thermometer on an electric
psychrometer, sling psychrometer, rotor psychrometer,
or from automatic measuring equipment.
temperature must be obtained to the nearest 1/10 degree
and may be read in either Fahrenheit or Celsius degrees.