MART Vol. II MO/MP - NESA - Civil Air Patrol

4.1 Atmospheric pressurePressure at a given point is a measure of the weight of the column of airabove that point. As altitude increases, pressure diminishes as the weight of theair column decreases. This decrease in pressure has a pronounced effect onflight. The aircraft's altimeter is sensitive to these changes in pressure, anddisplays this pressure as altitude. When the aircraft's altimeter is set to thecurrent reported altimeter setting (ATIS/AWOS/ASOS/FSS) it indicates theaircraft's height above mean sea level (MSL). [If a local altimeter setting isunavailable, pilots usually set the altimeter to indicate the airport's MSL elevation.]Changes in pressure are registered in inches of mercury: the standard sealevelpressure is 29.92 inches at a standard temperature of 15º C (59º F). If CAPaircraft always operated at standard conditions, the altimeter would always beaccurate. An aircraft with an indicated (on the altimeter) altitude of 5,000' MSLwill really be 5000' above the ground (AGL). However, these standard conditionsrarely exist because the density of the atmosphere is always changing as altitudeand temperature changes. [The third factor - humidity - also effects density, butthe effect is smaller and it's very hard to determine.]Pressure altitude is an altitude measured from the point at which anatmospheric pressure of 29.92 inches of mercury is found. A good rule of thumbis that a 1,000' change of altitude results in a 1-inch (mercury) change on abarometer. Another way to determine pressure altitude is to enter 29.92 into thealtimeter's window and read the resulting altitude indication.When pressure altitude is corrected for non-standard temperature, densityaltitude can be determined. There are two ways to calculate this (besides ASOS).Chart method60