Temperature
Altimeter Reads
High to low temperature
Too high
Low to high temperature
Too low
Table 1-4 -- Rule to help remember altimeter reading (temperature changes)
Altimeter Errors
The margin of error in the altimeter reading that is due to incorrect altimeter setting can
be determined for the lower levels of the atmosphere by applying the corrections in
Table 1-5.
Pressure Change
Altimeter Error
1 inch of mercury
1,000 feet
1/10 inch of mercury
100 feet
1/100 inch of mercury
10 feet
Table 1-5 -- Altimeter errors
Pressure Systems
Pressure systems are either high pressure (anticyclonic) areas or low pressure
(cyclonic) areas. A basic understanding of pressure systems, their distinguishing
characteristics, and the weather phenomena associated with them is necessary for
understanding concepts that will be presented later in this chapter.
Formation and Movement
One of the reasons that high and low pressure areas form is the uneven heating of the
earth s surface. Areas near the equator receive more heat, which causes the air to
expand and rise, which produces a low pressure area.
The atmosphere tends to maintain equal pressure over the entire earth. When this
equilibrium is upset (for example, by the formation of a high pressure area) air flows
from areas of high pressure to areas of low pressure attempting to maintain equal
1-7
