Figure 1-5 Temperature Deviation vs Indicated and MSL Altitude
Since these deviations due to temperature are usually relatively small, these errors are often
ignored in the early stages of flight training, and calibrated altitude is often treated directly as true
altitude. However, toward the advanced stages, tactical accuracy becomes paramount, and
temperature effects cannot be ignored. For example, when flying in cold weather over
mountainous terrain, you must take this difference between indicated and true altitude into
account by calculating a correction to the indicated altitude.
Pressure is force per unit area. Atmospheric (barometric) pressure is the pressure exerted on a
surface by the atmosphere due to the weight of the column of air directly above that surface. The
average weight of air on a square inch of the Earth's surface at sea level under standard conditions
is 14.7 pounds. Pressure, unlike temperature, always decreases with altitude. In the lower layers
of the atmosphere pressure decreases much more rapidly than it does at higher altitudes because
density decreases as altitude increases.
UNITS OF MEASUREMENT
In the U.S., two units are used to measure and report atmospheric pressure: inches of mercury
(in-Hg) and millibars (mb). Inches of mercury is a measure of the height of a column of mercury
supported by atmospheric pressure. The millibar is a direct representation of pressure, which is
defined as force per unit area. Normal sea level pressures in the atmosphere vary from as low as
28 in-Hg (about 960 mb) to as high as 31 in-Hg (about 1060 mb).
Some countries, particularly those using the metric system, use millibars for altimeter settings.
However, in the United States and Canada altimeter settings are reported in inches of mercury.
General Structure of the Atmosphere, and Atmospheric Temperature and Pressure