Action  Reaction  Action  Reaction  Although seemingly light, air does have weight and a one  square inch column of the atmosphere at sea level weighs  approximately 14.7 pounds. About one-half of the air by  weight is within the first 18,000 feet. The remainder of the  air is spread over a vertical distance in excess of 1,000 miles.  Air density is a result of the relationship between temperature  and pressure. Air density is inversely related to temperature  and directly related to pressure. For a constant pressure to be  maintained as temperature increases, density must decrease,  and vice versa. For a constant temperature to be maintained  as pressure increases, density must increase, and vice versa.  These relationships provide a basis for understanding  instrument indications and aircraft performance. There are several layers to the atmosphere with the  troposphere being closest to the Earth’s surface extending to  about 60,000 feet at the equator. 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). 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. Density altitude is pressure altitude corrected for nonstandard  temperature. As the density of the air increases (lower  density altitude), aircraft performance increases. Conversely,  as air density decreases (higher density altitude), aircraft  performance decreases. A decrease in air density means a  high density altitude; an increase in air density means a lower  density altitude. Density altitude is used in calculating aircraft  performance. Under standard atmospheric conditions, air at  each level in the atmosphere has a specific density; under  standard conditions, pressure altitude and density altitude  identify the same level. Density altitude, then, is the vertical  distance above sea level in the standard atmosphere at which  a given density is to be found. It can be computed using  The SDP is a theoretical level where the pressure of the  atmosphere is 29.92 "Hg and the weight of air is 14.7 psi.  As atmospheric pressure changes, the SDP may be below,  at, or above sea level. Pressure altitude is important as a  basis for determining aircraft performance, as well as for  assigning flight levels to aircraft operating at or above 18,000  feet. The pressure altitude can be determined by either of  two methods: (1) by setting the barometric scale of the  altimeter to 29.92 "Hg and reading the indicated altitude, or  (2) by applying a correction factor to the indicated altitude  according to the reported altimeter setting. Pressure altitude is the height above the standard datum  plane (SDP). The aircraft altimeter is essentially a sensitive  barometer calibrated to indicate altitude in the standard  atmosphere. If the altimeter is set for 29.92 "Hg SDP, the  altitude indicated is the pressure altitude-the altitude in the  standard atmosphere corresponding to the sensed pressure. 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 "Hg – 10 "Hg = 19.92 "Hg) 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.  Pressure Altitude Figure 4-6.  Newton’s Third Law of Motion: the Law of Reaction. Layers of the Atmosphere International Standard Atmosphere (ISA) Density Altitude 4-5