Ch 1 Quiz: The Atmosphere

ISA MSL temperature is +15°C (288.15 K).

The atmosphere is approximately 78% Nitrogen (N₂), 21% Oxygen (O₂), and about 1% Argon and trace gases.

The Troposphere is the lowest layer of the atmosphere and contains approximately 75% of the atmospheric mass and virtually all water vapour and weather.

ISA MSL temperature is +15°C (288.15 K). This is the baseline from which all ISA temperature deviations are calculated.

The ISA Environmental Lapse Rate (ELR) in the troposphere is 1.98°C per 1000ft (approximately 6.5°C per 1000m).

The ISA tropopause is at 36,089ft (11,000m) in the standard atmosphere. Above this, temperature is constant at -56.5°C in the lower stratosphere.

In the lower stratosphere (up to about 20km/65,617ft), the ISA temperature remains constant at -56.5°C (-69.7°F). This is the isothermal layer.

Standard MSL pressure in the ISA is 1013.25 hPa (29.92 inHg). This is also the standard setting for the pressure altimeter when flying at or above the transition altitude.

The ozone (O₃) layer is concentrated in the stratosphere, primarily between 15–35 km altitude. It absorbs most of the Sun’s ultraviolet radiation.

Water vapour (H₂O) is the most abundant and effective greenhouse gas in the atmosphere, accounting for about 50% of the greenhouse effect.

At the tropopause (36,089ft), the ISA temperature is -56.5°C. This temperature remains constant throughout the lower stratosphere.

At FL350 (35,000ft), ISA temperature is approximately -54.5°C. ISA+10 means the actual temperature is -54.5 + 10 = -44.5°C. Note: above tropopause (36,089ft) base is -56.5°C. At 35,000ft (below tropopause), ISA = 15 – (1.98 × 35) = 15 – 69.3 = -54.3°C, so ISA+10 ≈ -44.3°C ≈ -46.5°C with exact lapse.

The Mesosphere (50–85km) has temperatures decreasing with altitude, reaching the coldest temperatures in the atmosphere (around -90°C) at the mesopause.

The tropopause is highest over the equator (~16–18km / 52,000–59,000ft) due to intense convective activity and is lowest over the poles (~8km / 26,000ft).

Long-range jet aircraft cruise in the upper troposphere / lower stratosphere (FL300–FL450), where smooth air and thin air reduce drag and fuel burn.

The DALR is approximately 3°C per 1000ft (9.8°C per 1000m). This is the rate at which unsaturated (dry) air cools as it rises adiabatically.

The atmosphere is stable when the ELR is less than the DALR (and SALR). A rising parcel of air would cool faster than its environment and sink back, suppressing convection.

The SALR is approximately 1.5°C per 1000ft (varies from ~4°C/1000m in warm air to ~9°C/1000m in cold air). It is less than the DALR because of latent heat release during condensation.

Conditional instability exists when SALR < ELR < DALR. The atmosphere is stable for dry (unsaturated) air but becomes unstable once air reaches saturation and rises moist-adiabatically.

The ionosphere (thermosphere layer, ~60–1000km) contains ionised particles that reflect HF radio waves, enabling long-distance communications.

Colder than ISA air is denser. At a given indicated airspeed, denser air means lower true airspeed. ISA negative deviations decrease TAS compared to standard conditions.

Approximately 50% of the total atmospheric mass lies below 5.5km (18,000ft) due to the exponential decrease of pressure with altitude.

The absolute temperature in Kelvin (K) = °C + 273.15. For example, ISA MSL temperature is 15°C = 288.15K.

The 500 hPa pressure level is located at approximately 5,500m (~18,000ft / FL180) in the standard atmosphere, and is an important level for synoptic meteorology.

Carbon Dioxide (CO₂), though only ~0.04% of the atmosphere, is a significant greenhouse gas that absorbs infrared radiation, contributing to global warming.

The Lifted Index (LI) compares the temperature of a parcel lifted to 500 hPa with the actual 500 hPa temperature. Negative LI values indicate potential thunderstorm activity.

Near sea level in the ISA, pressure decreases at approximately 1 hPa per 27–30ft (roughly 1 hPa per 8–9m). This is the basis for the altimeter pressure correction formula.

With ELR = 4°C/1000m, which is less than both SALR (~6°C/1000m) and DALR (9.8°C/1000m), the atmosphere is absolutely stable regardless of moisture content.

At mid-latitudes, the troposphere extends to approximately 10–12km (33,000–40,000ft). It is deeper at the equator (~16–18km) and shallower at the poles (~8km).

In the ISA lower stratosphere (11–20km), temperature is constant at -56.5°C. Above 20km, it begins to increase due to ozone absorption of UV radiation.

Absolute instability occurs when ELR > DALR (>9.8°C/1000m). Any parcel of air, whether saturated or not, will continue to rise once displaced upward.

The aneroid barometer measures atmospheric pressure directly using a sealed, flexible metal capsule that expands and contracts with pressure changes.

The ICAO Standard Atmosphere is representative of average conditions at approximately 45° North latitude in temperate climates.

In air colder than ISA (ISA-20), air is denser and columns of air are shorter. The aircraft’s true altitude will be LOWER than indicated altitude — ‘Cold, Low.’ This is a dangerous error near terrain.

The troposphere is primarily heated from below by longwave (infrared) radiation re-emitted by the Earth’s surface after absorbing shortwave solar radiation. This is why temperature decreases with altitude in the troposphere.

In the ISA, pressure at the tropopause (36,089ft / 11,000m) is approximately 226.3 hPa.

Atmospheric density decreases with increasing altitude. Both pressure and temperature (mostly) decrease, but the pressure effect dominates, causing density (ρ = P/RT) to decrease.

Ozone is a molecule consisting of three oxygen atoms (O₃). It is formed in the stratosphere by photochemical reactions and absorbs UV-B and UV-C radiation.

ISA provides the basis for calculating true airspeed (from CAS/EAS via density), density altitude (performance calculations), and aircraft performance data in the AFM/POH.

The stratosphere (~12–50km) is followed by the mesosphere (~50–85km) and then the thermosphere. The layers are separated by the stratopause and mesopause respectively.

Absolute zero is 0 Kelvin (-273.15°C), the theoretical temperature at which all molecular motion ceases. It is the basis of the Kelvin temperature scale used in ISA calculations.

The tropopause over polar regions is at approximately 8km (~26,000ft), much lower than the mid-latitude ISA standard of 36,089ft or the equatorial ~16–18km.

Dry air is approximately 78% Nitrogen (N₂), 21% Oxygen (O₂), 0.93% Argon (Ar), 0.04% CO₂, and trace amounts of other gases.

The stratosphere above approximately 20km shows increasing temperature with altitude due to ozone absorbing UV solar radiation. This temperature inversion makes the stratosphere very stable.

A temperature inversion (temperature increasing with height) creates a very stable layer. Warm air above traps cooler air below, suppressing vertical mixing and convection. This can trap pollutants and moisture.

The total mass of the Earth’s atmosphere is approximately 5.15 × 10¹⁸ kg, which represents about 0.00009% of Earth’s total mass.

ISA deviation = actual temperature – ISA temperature at the same pressure altitude. Used in performance calculations, TAS correction, and understanding density altitude.

Gay-Lussac’s Law: P/T = constant (at constant volume). This means pressure increases proportionally with absolute temperature. Important for understanding atmospheric pressure changes.

Above 20km in the stratosphere, ozone absorbs increasing amounts of UV radiation, causing temperature to increase with altitude, reaching about 0°C at the stratopause (~50km).

Potential temperature (θ) is the temperature a parcel of dry air would have if moved adiabatically to a reference pressure of 1000 hPa. It is conserved in adiabatic processes and useful for comparing air masses.

The ICAO ISA assumes the atmosphere is: dry (no moisture effects), in hydrostatic equilibrium (pressure balances gravity), calm (no wind), and chemically uniform composition.

The Dry Adiabatic Lapse Rate (~9.8°C/1000m / 3°C/1000ft) is always greater than the Saturated Adiabatic Lapse Rate (~6°C/1000m / 1.5°C/1000ft) because latent heat released during condensation partly offsets the cooling of a rising saturated parcel.

ISA temperature at altitude = 15 – (1.98 × altitude_in_thousands_ft) = 15 – (1.98 × 25) = 15 – 49.5 = -34.5°C. (Using exact 1.98°C/1000ft lapse rate)

In the Northern Hemisphere, the Coriolis Effect deflects moving air (and objects) to the right. In the Southern Hemisphere, deflection is to the left. This creates the characteristic cyclonic/anticyclonic flow patterns.

The homosphere (0–80km) has uniform chemical composition maintained by turbulent mixing. Above 80km (the turbopause), the heterosphere begins where gases separate by molecular weight.

The mesopause (top of the mesosphere, ~85km) is the coldest point in the atmosphere at approximately -90°C (-130°F). Noctilucent clouds form near this level.

With the altimeter sub-scale set to 1013.25 hPa, the altimeter reads Pressure Altitude (expressed as Flight Levels above the Transition Level). All aircraft above the Transition Level use this setting.

Sound propagates through the atmosphere due to its elastic and compressible nature — pressure disturbances can travel as compression waves. The speed of sound depends on temperature: a = √(γRT).

The ISA provides a universal, reproducible reference standard for: altimeter calibration, aircraft performance calculations, engine performance, and comparison of meteorological data.

At 40,000ft (above the tropopause of 36,089ft), the ISA temperature is constant at -56.5°C throughout the lower stratosphere. This is the isothermal layer.

From lowest to highest: Troposphere (0–12km), Stratosphere (12–50km), Mesosphere (50–85km), Thermosphere (85–600km+). Separated by the tropopause, stratopause, and mesopause.

Density altitude = Pressure altitude ± ISA deviation correction. It represents the altitude in the ISA at which the actual density would be found. High density altitude reduces engine and aerodynamic performance.

In India, the DGCA has set the transition altitude at 5,000ft AMSL (or aerodrome elevation +500ft, whichever is higher). Above the Transition Level, QNE (1013.25 hPa) is used.

CO₂ constitutes approximately 0.04% (400 ppm) of dry atmospheric air. While a small fraction, its greenhouse properties make it significant for climate.

The WMO defines the tropopause as the lowest level where the lapse rate decreases to 2°C/km or less, maintained over a depth of at least 2km. This is the thermal tropopause.

An adiabatic process involves no exchange of heat with the surroundings. A rising (or sinking) air parcel that neither gains nor loses heat changes temperature only due to expansion (cooling) or compression (warming).

At ISA MSL (15°C), the speed of sound is approximately 340 m/s = 661 knots (Mach 1). Speed of sound varies with √T, so it decreases with altitude as temperature falls.

The thermosphere and ionosphere have highly variable composition due to ionisation by solar radiation, auroral activity, and the separation of gases by molecular weight in the heterosphere.

Humid (moist) air is LESS dense than dry air at the same temperature and pressure. Water vapour (H₂O, molecular weight 18) is lighter than the nitrogen (28) and oxygen (32) it displaces.

FL390 = 39,000ft is above the tropopause (36,089ft). In the ISA lower stratosphere, temperature is constant at -56.5°C regardless of altitude above the tropopause.

The Coriolis parameter f = 2Ω sin(φ), where φ is latitude. At the equator (φ=0), sin(0)=0, so f=0. This means there is no Coriolis deflection at the equator, which is why tropical systems require other mechanisms to organise rotation.

CO₂ has a molecular weight of 44 g/mol. Argon is 40, O₂ is 32, N₂ is 28. CO₂ is the heaviest of the common atmospheric gases. (Water vapour H₂O = 18 g/mol, lighter than all of them.)

The hydrostatic equation: dp/dz = -ρg relates pressure gradient (dp/dz) to air density (ρ) and gravity (g). It describes how pressure decreases with increasing altitude in a static atmosphere.

ISA deviation = Actual Temp – ISA Temp = -30°C – (-25°C) = -5°C = ISA-5. The actual temperature is 5°C colder than standard. FL200 ISA = 15 – (1.98×20) = 15 – 39.6 = -24.6°C ≈ -25°C.

In the ISA, pressure at approximately FL180 (18,000ft / 5,500m) is about 500 hPa. This is a significant synoptic level used in upper-air charts.

Absolute humidity is the mass of water vapour per unit volume of moist air, expressed in g/m³. It is distinct from relative humidity (a ratio) or specific humidity (mass per mass).

This is the Föhn effect. Air rising over a mountain loses moisture (as precipitation on the windward side), then descends. The DALR (descent) is greater than the SALR (ascent), so the air arrives on the lee side warmer and drier.

Relative Humidity (RH) = (actual vapour pressure / saturation vapour pressure) × 100%. At the dew point, RH = 100% (air is saturated). As temperature increases with constant moisture, RH decreases.

ISA MSL air density is 1.225 kg/m³. This is used as the reference density for calculating indicated airspeed, equivalent airspeed, and aircraft performance.

The troposphere is primarily heated by: longwave radiation from the Earth’s surface (most important), convective heat transfer, latent heat release from condensation, and finally conduction from the surface (least important over distance).