Chapter 8, Atmospheric Pressure and Altimetry 8-9
Figure 8-9. Pressure’s Effects on Density in the Atmosphere
8.3.4 Temperature’s Effects on Density
Density is inversely related to temperature. Assuming constant mass and pressure, an air parcel with a
higher temperature is less dense than an air parcel with a lower temperature (see Figure 8-10). This is
because the warmer air occupies a large volume.
Chapter 8, Atmospheric Pressure and Altimetry 8-10
Figure 8-10. Temperature’s Effects on Density
In the atmosphere, temperature has the most effect on density in the horizontal direction; that is, with
horizontal changes of location (e.g., New York City versus Miami), temperature has the greatest effect on
density.
8.3.5 Water Vapor’s Effects on Density
Density of an air parcel is inversely related to its quantity of water vapor. Assuming constant pressure,
temperature, and volume, air with a greater amount of water vapor is less dense than air with a lesser amount
of water vapor. This is because dry air molecules have a larger mass (weight) than water vapor molecules,
and density is directly related to mass (see Figure 8-11).
Figure 8-11. Water Vapor’s Effects on Density
8.4 Altimetry
The altimeter is essentially an aneroid barometer. The difference is the scale. The altimeter is graduated to
read increments of altitude rather than units of pressure. The standard for graduating the altimeter is the
standard atmosphere.
Chapter 8, Atmospheric Pressure and Altimetry 8-11
8.4.1 Altitude
Altitude seems like a simple term; it means the vertical elevation of an object above the surface of the Earth.
However, in aviation, it can have many meanings.
8.4.1.1 True Altitude
Since existing conditions in a real atmosphere are seldom standard, altitude indications on the altimeter are
seldom actual or true altitudes. True altitude is the actual vertical distance above MSL. If an altimeter does
not indicate true altitude, what does it indicate?
8.4.1.2 Indicated Altitude
Figure 8-10 shows the effect of mean temperature on the thickness of three columns of air. Pressures are
equal at the bottoms and tops of the three layers. Since an altimeter is essentially an aneroid barometer,
altitude indicated by the altimeter at the top of each column would be the same. To see this effect more
clearly, see Figure 8-12. In the warm air column, a pilot would fly at an altitude that is higher than the
indicated altitude. In the cold air column, the pilot would fly at an altitude lower than the indicated altitude.
Figure 8-12. True Versus Indicated Altitude
Height indicated on the altimeter also changes with changes in surface pressure. A movable scale on the
altimeter permits the pilot to adjust for surface pressure, but the pilot has no means of adjusting the altimeter
for mean temperature of the column of air below. Indicated altitude is the altitude above MSL indicated on
the altimeter when set at the local altimeter setting. But what is altimeter setting?
8.4.1.2.1 Altimeter Setting
Since the altitude scale is adjustable, a pilot can set the altimeter to read true altitude at some specified
height. Takeoff and landing are the most critical phases of flight; therefore, airport elevation is the most
Chapter 8, Atmospheric Pressure and Altimetry 8-12
desirable altitude for a true reading of the altimeter. The altimeter setting is the value to which the scale of
the pressure altimeter is set so the altimeter indicates true altitude at field elevation.
To ensure the altimeter reading is compatible with altimeter readings of other aircraft in the vicinity, a pilot
should ensure the altimeter setting is current. The pilot should adjust it frequently while in flight, according
to the nearest surface weather reporting station. Figure 8-13 shows the trouble a pilot can encounter if not
vigilant in adjusting the altimeter during flight. As the pilot flies from high pressure to low pressure, the
plane is lower than the altimeter indicates.
Figure 8-13. Pressure Change’s Effects on Altimeter Readings
Chapter 8, Atmospheric Pressure and Altimetry 8-13
Figure 8-14. Temperature Change’s Effects on Altimeter Readings
Figure 8-14 shows that as a pilot flies from warm to cold air, the altimeter reads too high—the pilot is lower
than the altimeter indicates. Over flat terrain, this lower-than-true reading is no great problem; other aircraft
in the vicinity are also flying indicated rather than true altitude, and everyone ’s altimeter readings are
compatible. If flying in cold weather over mountainous areas, however, a pilot needs to take this difference
between indicated and true altitude into account. The pilot needs to know that the true altitude assures
clearance of terrain and compute a correction to indicated altitude.
8.4.1.3 Corrected (Approximately True) Altitude
If a pilot could always determine the mean temperature of a column of air between the aircraft and the
surface, flight computers would be designed to use this mean temperature in computing true altitude.
However, the only guide a pilot has to temperature below is free air temperature at the pilot’s altitude.
Therefore, the flight computer uses outside air temperature (OAT) to correct indicated altitude to
approximately true altitude. The corrected (approximately true) altitude is indicated altitude corrected for
the temperature of the air column below the aircraft, the correction being based on the estimated deviation
of the existing temperature from standard atmosphere temperature. It is a close approximation to true
altitude and is labeled true altitude on flight computers. It is close enough to true altitude to be used for
terrain clearance, provided the pilot has the altimeter set to the value reported from a nearby reporting
station.
8.4.1.4 Pressure Altitude
In the standard at mosphere, sea level pressure is 29.92 inHg (1,013.2 mb). Pressure decreases at a fixed
rate upward through the standard atmosphere. Therefore, in the standard atmosphere, a given pressure exists
at any specified altitude. Pressure altitude is the altitude (above MSL) shown by the altimeter when set to
29.92 inHg. In other words, it is the altitude associated with a specific pressure measured by the static port
when the altimeter is set to 29.92. Since pressure is the same everywhere regardless of the specific pressure
Chapter 8, Atmospheric Pressure and Altimetry 8-14
altitude, a constant -pressure surface defines a constant -pressure altitud e. When a pilot flies a
constant-pressure altitude, the pilot is flying a constant-pressure surface.
As discussed earlier, constant-pressure surfaces have different heights across them. Therefore, when flying
at a specific pressure altitude (i.e. , constant-pressure surface) a pilot ’s true altitude will change with
distance. However, since pressure altitudes are flown at or above FL180 (in the United States), a pilot will
almost always be above the highest terrain features.
8.4.1.5 Density Altitude
Density altitude is the pressure altitude corrected for temperature deviations from the standard atmosphere.
Density altitude bears the same relation to pressure altitude as true altitude does to indicated altitude.
Density altitude is indirectly related to atmospheric density; as air density increases, the density altitude
decreases, and conversely, as air density decreases, the density altitude increases. Airports with higher field
elevations (e.g., Denver) have lower pressure, lower density, and, therefore, higher density altitudes than
airports with lower field elevations (e.g., New Orleans).
Density altitude equals field elevation during standard atmospheric conditions, but conditions are rarely
standard. Density altitude is higher (lower) than standard at airports that report lower (higher) than standard
pressures (e.g., 29.92 inHg) and/or higher (lower) -than-standard temperatures. Temperature is the most
important factor, since temperature has the greatest effect on density horizontally in the atmosphere. On hot
days, the air becomes less dense, causing high density altitudes. On cold days , the air is denser, causing
lower density altitudes. Dewpoint (water vapor) is also a contributing factor, but its effects are generally
negligible.
Density altitude is an index to aircraft performance. Higher density altitude decreases aircraft performance.
Lower density altitude increases aircraft performance. High density altitude is a hazard , since it reduces
aircraft performance in the following three ways:
1. It reduces power because the engine takes in less air to support combustion.
2. It reduces thrust because there is less air for the propeller to work with, or a jet has less mass of
gases to force out of the exhaust.
3. It reduces lift because the light air exerts less force on the airfoils.
A pilot cannot detect the effect of high-density altitude on the airspeed indicator. The aircraft lifts off,
climbs, cruises, glides, and lands at the prescribed indicated airspeeds; but at a specified indicated airspeed,
the pilot’s true airspeed and groundspeed increase proportionally as density altitude becomes higher.
The net results are that high density altitude lengthens a pilot ’s takeoff and landing rolls and reduces the
rate of climb. Before lift-off, the plane must attain a faster groundspeed, and, therefore, needs more runway;
and the reduced power and thrust add a need for still more runway. The plane lands at a faster groundspeed
and, therefore, needs more room to stop. At a prescribed indicated airspeed, it is flying at a faster true
airspeed, and, therefore, covers more distance in a given time, which means climbing at a shallower angle.
Adding to this are the problems of reduced power and rate of climb. Figure 8-15 shows the effect of density
altitude on takeoff distance and rate of climb.
Chapter 8, Atmospheric Pressure and Altimetry 8-15
Figure 8-15. High Density Altitude’s Effects on Flight
High density altitude also can be a problem at cruising altitude. When air temperature is higher (warmer)
than standard atmosphere, the higher density altitude lowers the service ceiling. For example, if temperature
at a pressure altitude of 10,000 ft is 20 °C, density altitude is 12,700 ft. A pilot’s aircraft will perform as
though it were at 12,700 ft indicated with a normal temperature of -8 °C.
To compute density altitude, a pilot can set the altimeter to 29.92 in (1,013.2 mb), read the pressure altitude
from the altimeter, obtain the OAT, and then use a flight computer to compute density altitude.