Atmospheric pressure is used to indicate the altitude of an aircraft. A barometer
(aneroid type), carried onboard an aircraft, is called an altimeter. The altimeter has a
scale to indicate altitude instead of pressure. As an aircraft increases altitude, there is
less air above the aircraft and therefore less pressure on the altimeter. An aircraft uses
surface pressure as a reference point, so the pilot must change altimeter setting when
flying a route below 18,000 feet (above 18,000 feet all aircraft altimeters are set at
29.92). It is critical to flying safety that an aircraft have the correct altimeter setting for
the area in which it is operating.
Effects of Changes in Atmospheric Pressure
An aircraft must have the correct altimeter setting for the area in which it is operating,
since this is what altitude and vertical separation are based on. Without having the
correct altimeter setting the indicated, altitude of the aircraft will not be correct.
For example, when an aircraft flies from a high pressure area into a low pressure area
and the altimeter setting is not corrected, the altimeter will read too high. Going from a
low pressure area to a high pressure area, the altimeter will read too low. A simple rule
to help you remember this is shown in Table 1-3.
High to low pressure
Low to high pressure
Table 1-3 -- Rule to help remember altimeter reading (pressure changes)
Effects of Changes in Temperature
The same rule applies to temperature changes (see Table 1-4). The altimeter of a plane
flying from a low temperature area into a high temperature area will read too low, and
moving from a higher temperature area to a lower temperature area will read too high.