6.1 Atmosphere, Pressure, Temperature, and Density

The Private Pilot Airplane knowledge test samples PA.I.C.K3a (atmospheric composition and stability) and PA.I.C.K3c (temperature) by asking you to reason about air you cannot see. You will not be handed a published density-altitude number for tomorrow’s form. You will be asked what 78 percent nitrogen and 21 percent oxygen imply, what 15 °C and 29.92 inHg are for, why a hot humid afternoon at a high-elevation field is a performance problem, and why an altimeter that still shows 6,500 feet can put you into terrain.

What the atmosphere is made of

In any given volume of dry air, nitrogen is 78 percent and oxygen is 21 percent. Argon, carbon dioxide, and traces of other gases make up the remaining one percent (PHAK Chapter 12). That dry mix is not the whole story. Water vapor occupies from zero to about five percent by volume. That small fraction is what makes weather: clouds, fog, icing, thunderstorms, and the moisture correction to density.

Nearly all of the weather that matters to a VFR airplane lives in the troposphere. PHAK Chapter 12 places the troposphere from roughly four to twelve miles deep at the poles and as high as about 48,000 feet over the equator. Temperature in the troposphere normally falls with height. At the tropopause that decrease stops. The tropopause acts as a lid that traps moisture and weather below it. PHAK ties the tropopause to the jet stream and to clear air turbulence. Above it, the stratosphere is largely stable, with little weather. The mesosphere and thermosphere do not drive PAR weather items.

The International Standard Atmosphere

Instruments and AFM/POH charts are calibrated to a hypothetical column called the International Standard Atmosphere (ISA). PHAK Chapters 11 and 12 and the Aviation Weather Handbook (FAA-H-8083-28B) give the sea-level anchors you must memorize.

Any temperature that is not that standard lapse is a nonstandard temperature. The difference between the actual outside air temperature and the ISA temperature at the same pressure altitude is the ISA deviation. At 5,000 feet pressure altitude the standard temperature is about 5 °C (15 − 2 × 5). An OAT of 25 °C is ISA +20. That plus-20 is not decoration on the ATIS. It is the reason the airplane will act as if the field were much higher than the elevation printed on the chart.

Do not invent a tighter tropopause height or a private-pilot-only “standard day” other than ISA. The FAA has not published one.

Four altitudes that are not the same number

PAR items fail when a student treats every altitude word as “what the altimeter says.”

• Indicated altitude is what the altimeter shows when the Kollsman window is set to the current local altimeter setting. Under 14 CFR 91.121 that is the setting you fly with at and below 17,999 feet.
• True altitude is the actual height above mean sea level. It matches indicated altitude only when the column is standard and the setting is correct.
• Pressure altitude is height above the standard datum plane — the theoretical surface where pressure is 29.92 inHg. Set 29.92 in the Kollsman window and read the altimeter; that reading is pressure altitude. PHAK 11 also lets you apply a correction to field elevation using the reported altimeter setting, or use a flight computer.
• Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere that has the same density as the air you are sitting in. The airplane performs as if it were at the density altitude, not at the field elevation and not at the indicated altitude.

Absolute altitude (height above the terrain) is a fifth word you will meet on orals. It is not density altitude.

Worked example: pressure altitude first

A field elevation is 4,500 feet MSL. The ATIS altimeter setting is 30.12 inHg. PHAK’s lower-atmosphere pressure lapse is about one inch per 1,000 feet. The setting is 0.20 inHg above 29.92, so the standard datum plane sits above the surface and pressure altitude is about 200 feet below field elevation:

Pressure altitude ≈ 4,500 − 200 = 4,300 feet.

Reverse the sign when the setting is low. Field elevation 5,500 feet, altimeter 29.42 inHg, is 0.50 inHg below 29.92, so add about 500 feet: pressure altitude ≈ 6,000 feet. That 6,000-foot pressure altitude — not the 5,500-foot elevation — is what you take to the density-altitude chart or the flight computer.

Worked example: density altitude and performance

PHAK Chapter 11 walks a takeoff that students routinely misread. With the altimeter set to 29.92, it indicates a pressure altitude of 5,000 feet. The AFM/POH ground run under standard temperature is 790 feet. If the temperature is 20 °C above standard, the air has expanded. Using the AFM temperature correction, a flight computer, or the PHAK density-altitude chart, the density level is above 7,000 feet, and the ground run may be closer to 1,000 feet.

The Aviation Weather Handbook gives the cruise version of the same idea: at a pressure altitude of 10,000 feet with an OAT of 20 °C, density altitude is 12,700 feet. The airplane at 10,000 indicated in that warm air performs like an airplane at 12,700 feet on a standard day.

High density altitude is worse performance. Lift is weaker because the wing has fewer molecules to work with. The propeller produces less thrust. The engine ingests less air and makes less power. Takeoff roll lengthens, climb rate decays, and true airspeed for a given indicated airspeed increases. Low density altitude (cold, high pressure, low elevation, dry) is denser air and better performance.

Temperature and moisture change density

Density falls when pressure falls, when temperature rises, or when moisture rises. PHAK is explicit that water vapor is lighter than dry air, so humid air is less dense than dry air at the same pressure and temperature. Humidity is usually a smaller actor than temperature or elevation, and PHAK states there is no published rule-of-thumb or chart that converts humidity into a density-altitude increment. You still expect a further performance loss on a muggy afternoon. Do not invent an “add 200 feet for humidity” number the FAA has not published.

The trap is to treat indicated altitude as density altitude. A mountain strip at 6,800 feet MSL on a 32 °C afternoon is not a 6,800-foot performance problem. After you set 29.92 and correct for the ISA deviation, density altitude may sit several thousand feet higher. The AFM takeoff chart cares about that higher number.

Altimeter setting errors: high to low, hot to cold

The altimeter is an aneroid barometer. It converts static pressure into an altitude by assuming the ISA column. Two errors dominate PAR.

Pressure. If you fly from high pressure toward low pressure and leave the Kollsman window unchanged, the altimeter overreads. True altitude is lower than indicated. AIM 7-2-2 states the old line: “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW.” The same paragraph states that an inch of mercury error equals 1,000 feet of altitude. Example: leaving 30.92 set when the correct setting is 29.92 puts you about 1,000 feet lower than the needles claim.

Temperature. The altimeter cannot see the temperature of the column below you. In colder-than-standard air the true pressure surfaces sit closer to the ground. At a constant indicated altitude you are lower than the altimeter shows. The matching memory aid is from hot to cold, look out below. The hazard is terrain clearance on a cold morning toward high terrain, not a colder OAT making density altitude worse. Cold air lowers density altitude and helps performance while it hurts true altitude versus indicated.

High-to-low and hot-to-cold are true-versus-indicated problems. Density altitude is a performance problem. Do not swap those two stories on the same question.

Scenario: Jordan at a high-elevation field

Jordan plans a noon departure from a 9,900-foot-elevation airport. ATIS: altimeter 30.12, OAT 22 °C, dew point 8 °C. Setting 29.92 will show a pressure altitude a couple of hundred feet below field elevation because 30.12 is above standard. Standard temperature near 10,000 feet is about −5 °C on the PHAK 2 °C/1,000 ft lapse (PHAK Figure 11-2 lists −4.8 °C at 10,000 feet). An OAT of 22 °C is more than 25 °C warmer than standard. Density altitude will be well above the elevation. Jordan does not glance at 9,900 on the sectional and call it the takeoff number. Jordan computes pressure altitude, then density altitude with the flight computer or AFM chart, and accepts a longer roll and a weaker climb. If Jordan later flies toward a falling altimeter setting without resetting, that is a separate “look out below” problem — true altitude, not density altitude.