There are several layers to the atmosphere with the troposphere being closest to the Earth’s surface extending to about 60,000 feet. Following is the stratosphere, mesosphere, ionosphere, thermosphere, and finally the exosphere. The tropopause is the thin layer between the troposphere and the stratosphere. It varies in both thickness and altitude but is generally defined where the standard lapse (generally accepted at 2° C per 1,000 feet) decreases significantly (usually down to 1° C or less).
International Standard Atmosphere (ISA)
The International Civil Aviation Organization (ICAO) established the ICAO Standard Atmosphere as a way of creating an international standard for reference and performance computations. Instrument indications and aircraft performance specifications are derived using this standard as a reference. Because the standard atmosphere is a derived set of conditions that rarely exist in reality, pilots need to understand how deviations from the standard affect both instrument indications and aircraft performance.
In the standard atmosphere, sea level pressure is 29.92″ inches of mercury (Hg) and the temperature is 15° C (59° F). The standard lapse rate for pressure is approximately a 1″ Hg decrease per 1,000 feet increase in altitude. The standard lapse rate for temperature is a 2° C (3.6° F) decrease per 1,000 feet increase, up to the top of the stratosphere. Since all aircraft performance is compared and evaluated in the environment of the standard atmosphere, all aircraft performance instrumentation is calibrated for the standard atmosphere. Because the actual operating conditions rarely, if ever, fit the standard atmosphere, certain corrections must apply to the instrumentation and aircraft performance. For instance, at 10,000 ISA predicts that the air pressure should be 19.92″ Hg (29.92″ – 10″ Hg = 19.92″) and the outside temperature at -5°C (15° C – 20° C). If the temperature or the pressure is different than the International Standard Atmosphere (ISA) prediction an adjustment must be made to performance predictions and various instrument indications.
There are two measurements of the atmosphere that affect performance and instrument calibrations: pressure altitude and density altitude. Pressure altitude is the height above the standard datum pressure (SDP) (29.92″ Hg, sea level under ISA) and is used for standardizing altitudes for flight levels (FL). Generally, flight levels are at or above 18,000 feet (FL 180), providing the pressure is at or above 29.92″Hg. For calculations involving aircraft performance when the altimeter is set for 29.92″ Hg, the altitude indicated is the pressure altitude.
Density altitude is pressure altitude corrected for nonstandard temperatures, and is used for determining aerodynamic performance in the nonstandard atmosphere. Density altitude increases as the density decreases. Since density varies directly with pressure, and inversely with temperature, a wide range of temperatures may exist with a given pressure altitude, which allows the density to vary. However, a known density occurs for any one temperature and pressure altitude combination. The density of the air has a significant effect on aircraft and engine performance. Regardless of the actual altitude above sea level an aircraft is operating at, its performance will be as though it were operating at an altitude equal to the existing density altitude.
If a chart is not available the density altitude can be estimated by adding 120 feet for every degree Celsius above the ISA. For example, at 3,000 feet pressure altitude (PA), the ISA prediction is 9° C (15° C – [lapse rate of 2° C per 1,000 feet x 3 = 6° C]). However, if the actual temperature is 20° C (11° C more than that predicted by ISA) then the difference of 11° C is multiplied by 120 feet equaling 1,320. Adding this figure to the original 3,000 feet provides a density altitude of 4,320 feet (3,000 feet + 1,320 feet).
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