3.atmosphere-and-pressure. Atmosphere and Atmospheric Pressure
The atmosphere is the envelope of air that surrounds Earth and rests upon its surface. It is as much a part of Earth as the seas or the land, but air differs from land and water because it is a mixture of gases. By volume, dry air contains approximately 78 percent nitrogen, 21 percent oxygen, and about 1 percent other gases such as argon and carbon dioxide. Most of the oxygen, however, is contained below 35,000 feet. Water vapor is also present in varying amounts and has a significant effect on weather and aircraft performance.
Composition and Layers
Although the atmosphere extends well over 500 miles above Earth, almost all weather and routine flight occurs in the lowest layer, the troposphere. The troposphere extends from the surface to an average of 36,000 feet at mid-latitudes (higher over the equator, lower over the poles). Within this layer, temperature decreases with altitude at the standard rate of approximately 2 °C (3.5 °F) per 1,000 feet, and most of the moisture and turbulence are concentrated here. The boundary at the top of the troposphere is the tropopause, where temperature stops decreasing. Above it lies the stratosphere, which contains the ozone layer.
Atmospheric Pressure
Air has mass, and gravity pulls that mass toward Earth, producing weight. The weight of a column of air pressing on a unit area is atmospheric (barometric) pressure. Although the column extends to the top of the atmosphere, it is the dense air near the surface that accounts for most of the weight. Pressure is measured in several units:
- inches of mercury ("Hg) — used in altimeter settings
- millibars (mb) or hectopascals (hPa) — used in weather analysis
- pounds per square inch (psi)
At sea level, the standard atmospheric pressure is 29.92 "Hg, 1013.2 mb, or 14.7 psi. Pressure decreases with altitude — rapidly at first, then more slowly. A useful rule of thumb is that pressure decreases by approximately 1 "Hg per 1,000 feet in the lower atmosphere.
The International Standard Atmosphere (ISA)
Because atmospheric properties vary continuously, the International Standard Atmosphere (ISA) provides a fixed reference used to calibrate instruments and publish performance data. ISA defines:
- Sea-level pressure: 29.92 "Hg (1013.2 mb)
- Sea-level temperature: 15 °C (59 °F)
- Standard lapse rate: 2 °C per 1,000 ft until the tropopause
- Standard tropopause: 36,000 ft at −56.5 °C
For any pressure altitude, the standard temperature can be estimated as:
Standard Temp (°C) = 15 − (2 × altitude in thousands of feet)
For example, at 5,000 ft pressure altitude, ISA temperature is 15 − 10 = 5 °C.
Pressure Altitude and Density Altitude
Two altitude concepts are derived directly from pressure:
- Pressure altitude — the height above the standard datum plane (where pressure is 29.92 "Hg). Set the altimeter to 29.92 to read it directly. Pressure altitude is the value used in performance charts.
- Density altitude — pressure altitude corrected for nonstandard temperature. It is the altitude the airplane "feels" it is flying at, and it determines aircraft performance.
A quick approximation: Density Altitude ≈ Pressure Altitude + (120 × (OAT − ISA Temp)).
Example: at a field with pressure altitude 3,000 ft and OAT 30 °C, ISA temperature is 9 °C, so density altitude ≈ 3,000 + 120 × (30 − 9) = 5,520 ft. The airplane will perform as if operating at 5,520 ft on a standard day.
Effects on Flight
Atmospheric pressure and density directly affect every aspect of airplane performance:
- Lift and thrust decrease as air density decreases, because fewer air molecules pass over the wing and through the propeller and engine.
- Takeoff and landing distances increase at higher density altitudes; true airspeed at a given indicated airspeed is greater, so groundspeed at liftoff and touchdown is greater.
- Climb performance suffers because both the wing and the (normally aspirated) engine produce less.
- Indicated airspeed (IAS) is based on dynamic pressure, so as altitude increases, true airspeed (TAS) becomes greater than IAS for the same dynamic pressure.
The three variables that decrease air density are high altitude, high temperature, and high humidity — the "three Hs." Water vapor is lighter than dry air, so humid air is less dense than dry air at the same pressure and temperature.
Pilot Application
Before every flight, pilots evaluate atmospheric conditions to predict performance. PHAK Chapter 3 emphasizes that an understanding of pressure, temperature, and density is the foundation for interpreting altimeter settings, computing takeoff and landing data, and recognizing when a runway that is adequate on a cool morning may be marginal on a hot, humid afternoon. Required preflight action under 14 CFR 91.103 includes a review of runway lengths and aircraft performance — both of which depend directly on the atmospheric values discussed here.