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METEOROLOGY — CHAPTER 1 · NOTES
Atmosphere
by Capt. Pankaj Pahil
Table of Contents
- 1. Composition and Structure of the Atmosphere
- 2. Constituents — Major Gases
- 3. Variable Gases, Greenhouse Gases & Dry/Saturated Air
- 4. Carbon Dioxide (CO₂) & Ozone (O₃)
- 5. Vertical Distribution of Air Mass
- 6. Thermal Structure of the Atmosphere
- 7. Atmospheric Layers in Detail
- 8. International Standard Atmosphere (ISA)
- 9. Jet Standard Atmosphere (JSA)
- 10. Quick Revision Summary
- 11. Practice Questions & Answers (Q1–Q45)
- 12. Master Reference Tables
1. Composition and Structure of the Atmosphere
📘 Definition — Atmosphere
The atmosphere of the earth is an envelope of homogeneous mixture of gases, called Air. It surrounds the earth and is attached to it due to gravitation. It moves with the earth at the same speed and direction.
- Total thickness: approximately 480 km
- 80% of the atmosphere is within 16 km
- No exact upper boundary — gradually merges with outer space
Characteristics of the Atmosphere
| Property | Detail |
|---|---|
| Weight | Has weight and hence exerts pressure |
| Compressibility | Compressible and expandable |
| Shape | Occupies space, has no definite shape |
| Mobility | Mobile — allows transfer of heat and moisture |
| Conductivity | Poor conductor of heat and electricity |
2. Constituents — Major Gases
| Gas | Symbol | % by Volume |
|---|---|---|
| Nitrogen | N₂ | 78.09% |
| Oxygen | O₂ | 20.95% |
| Argon | Ar | 0.93% |
| Carbon Dioxide | CO₂ | 0.035% |
Trace Gases
Neon, Helium, Methane, Krypton, Xenon, Nitrous Oxide, Iodine, H₂, O₃ (0.000007%), Ammonia, Carbon Monoxide, CO₂, SO, Nitrogen Dioxide. The atmosphere also contains water vapour (WV) and solid particles.
⭐ N₂ : O₂ Ratio — Exam Favourite
By Volume: N₂ : O₂ = 4 : 1
By Weight: N₂ : O₂ = 3 : 1
Mnemonic: "Volume 4, Weight 3 — N before O"
⚠️ Supplementary Oxygen Required
Due to rapid reduction of gases with height, supplementary oxygen is needed above 10,000 ft.
Homosphere vs Heterosphere
| Layer | Height | Characteristic |
|---|---|---|
| Homosphere | Up to ~80 km | Nearly uniform composition; well-mixed |
| Heterosphere | Above 80 km | Gases separate by molecular weight; composition varies |
3. Variable Gases, Greenhouse Gases & Dry/Saturated Air
Variable Gases
The gases like Water Vapour (WV), Carbon Mono Oxide, Sulphur di Oxide, Nitrogen di Oxide, and Methane vary in amount from place to place, being concentrated more in industrial areas, cities and water bodies, than in open areas. Though very small in quantity, these gases are very significant for weather and life.
Greenhouse Gases
📘 Greenhouse Effect
Water vapour, CO₂, O₃, Methane and some other gases are transparent to solar (Short Wave) radiation but partially absorb terrestrial (Long Wave) radiation, and re-radiate it. This keeps the earth warmer than it would have otherwise been.
Greenhouse gases: H₂O, CO₂, O₃, CH₄ (Methane)
⚠️ Global Warming Warning
Excessive amount of greenhouse gases is causing Global Warming, which is a serious threat to our life and needs to be limited.
Dry Air vs Saturated Air
Water in the atmosphere can exist in three states:
- Solid: Snow, Ice, Hail
- Liquid: Drizzle, Rain, Shower
- Gas: Water Vapour (WV)
| Concept | Detail |
|---|---|
| WV concentration | Rapidly decreases with height |
| Warmer air | Can hold greater amount of WV |
| Tropics (23½°N to 23½°S) | Air can hold up to 4% WV by volume |
| Above 30,000 ft | Very low WV due to very low temperatures |
| Saturated Air | Air with 4% WV → 100% Relative Humidity (RH) |
| Dry Air | RH < 100% → Unsaturated |
4. Carbon Dioxide (CO₂) & Ozone (O₃)
Carbon Dioxide (CO₂)
- Produced by burning of fuel, wood etc.
- Concentration substantial in industrial areas; negligible in Polar regions and higher altitudes
- Large amount of CO₂ is dissolved in the oceans
- Plants absorb CO₂, use its carbon as food and release O₂ into atmosphere
Ozone (O₃)
📘 Ozone Properties
- Forms in the upper atmosphere
- Absorbs Ultra Violet (UV) radiation from the Sun → raises temperature
- O₃ molecules become heavier → sink → accumulate in lower levels
- Maximum O₃ concentration: 20–25 km
- O₃ found between 10–50 km
✅ Why Ozone Matters for Aviation & Life
Ozone protects us from the ill effects (like skin cancer, sunburn and cataracts) of UV radiation.
Ozone Hole
⚠️ Ozone Depletion
In recent times the O₃ layer has thinned and large holes have developed mainly over the Polar and other latitudes due to:
- Atmospheric pollution
- Excessive use of halocarbon refrigerants, solvents, chlorofluoro carbons
- On reaching Stratosphere they release halogen atoms through photodissociation → breaks down O₃ into O₂
Solid Particles (Aerosols)
Particles such as salt from evaporating sea water, dust from arid regions, industrial particles:
- Obscure solar radiation and affect visibility and temperature
- Act as condensation nuclei → promote condensation of WV prematurely
- In larger cities/industrial areas: frequent fog and smog (mixture of fog and smoke) due to high concentration of particles
5. Vertical Distribution of Air Mass
| Height | Fraction of Atmosphere Contained Below |
|---|---|
| 6 km | 1/2 (50%) |
| 10 km | 3/4 (75%) |
| 35 km | 99% |
⭐ Memory Aid
"6–10–35 = Half–3/4–All"
Half the atmosphere below 6 km; ¾ below 10 km; 99% below 35 km.
6. Thermal Structure of the Atmosphere
📘 How the Atmosphere is Heated
Solar radiation heats the earth and the earth in turn heats up the atmosphere by conduction, convection, radiation and release of latent heat by condensation of water vapour. Hence temperature falls with height, up to a certain height. The atmosphere is thus heated from below and not from above.
Heat Flow from Earth's Surface
| Type of Heat | Mechanism | Percentage |
|---|---|---|
| Sensible Heat | Conduction, Convection, Radiation | 23% |
| Latent Heat | Evaporation, Condensation, Sublimation | 77% |
⭐ Exam Tip
Latent heat transfer is dominant at 77%. Sensible heat = 23%. Mnemonic: "Latent is Larger (77)"
Thermal Structure — Visual Diagram
7. Atmospheric Layers in Detail
A. Troposphere
📘 Troposphere
- Lowest portion of the atmosphere
- Extends from surface to 16–18 km at equator and 8–10 km at poles
- Higher height at equator due to rising hot air convection currents
- Temperature falls at ~6.5°C/km — called the Lapse Rate
- All kinds of weather occur here; turbulent mixing
- Contains 75% of atmospheric mass and 99% of water vapour and aerosols
| Sub-Layer | Height Range |
|---|---|
| Lower Troposphere | Surface to 2.1 km |
| Mid Troposphere | 2.1 km to 7.6 km |
| Upper Troposphere | 7.6 km to Tropopause |
B. Tropopause
📘 Tropopause
- Top of the troposphere; marks boundary between Troposphere and Stratosphere
- At tropopause: lapse rate reduces to 1–2°C/km or temperature stops falling
- Controlled by: Surface Temperature, Latitude, Season, Land-Sea distribution, Synoptic Situation
- Temperature at equator: –70° to –75°C
- Temperature at poles: –40° to –45°C
- At lower height in Winters than Summers
- Warmer the surface → Higher the tropopause
⭐ Tropopause Breaks/Folds
There are two regions where tropopause abruptly changes height, called breaks/folds:
- At about 40° Lat and 60° Lat
- Break at 40°Lat is more prominent
- Jet streams occur at these breaks
| Type | Pressure Level | Latitude Range | Height (India) | Temp (India) | Associated Jet |
|---|---|---|---|---|---|
| Polar Tropopause | ~300 hPa | Lat. 45°–60° polewards | – | – | Polar Front Jet (southern end) |
| Middle Tropopause | ~200 hPa | Between Polar & Tropical | ~11.5 km (winters ~23°N) | –45°C | – |
| Tropical Tropopause | ~100 hPa | Lat. 35°–40° | ~11.5 km (winters ~23°N) | –45°C | Subtropical Jet (STJ) at southern end |
✅ Importance of Tropopause for Pilots
- Maximum height clouds can reach
- Jet streams and maximum wind speeds occur just below it
- Clear Air Turbulence (CAT) occurs just below it
C. Stratosphere
📘 Stratosphere (Tropopause to ~50 km)
- Initially (8–10 km above tropopause) isothermal
- Temperature then increases slowly and towards the top sharply
- Inversion produced by absorption of UV radiation from sun by ozone
- Ozone layer found at height of ~25 km
- Very dry and stable region — low humidity, no weather
- Above stormy weather; steady, strong, horizontal winds
- Nacreous Clouds (Mother of Pearl Clouds) sometimes seen in higher latitudes in winters
Stratopause: Upper boundary of the Stratosphere.
D. Mesosphere
📘 Mesosphere (Stratopause to ~80 km)
- Extends 50–80 km from Stratopause
- Lack of solar radiation absorption + weak vertical mixing → temperatures fall with height
- Noctilucent Clouds seen occasionally in Polar regions
Mesopause: Top of Mesosphere. Temperatures stop falling. Coldest area of atmosphere. Temperature at 80 km = –100°C (173.15 K). Meteors burn up in this layer.
E. Thermosphere
📘 Thermosphere / Ionosphere (80 km+)
- From Mesopause to end of atmosphere
- Temperatures increase with height
- Above 60 km also called Ionosphere
- Important for radio wave propagation
- Serious interference in radio propagation during solar disturbances
F. Exosphere
📘 Exosphere (outermost layer)
- Outermost layer extending up to 10,000 km from top of Thermosphere
- Molecules, ozone and atoms escape into space
- Air is very thin — few particles; very active with very high temperature but far apart → very little heat present in this layer
Atmospheric Layers — Process Flowchart
flowchart TD
A["🌍 Earth's Surface\n(0 km)"] --> B["TROPOSPHERE\n0–18 km (equator), 8–10 km (poles)\nTemp ↓ at 6.5°C/km\nAll weather here | 75% mass | 99% WV"]
B --> C{"TROPOPAUSE\nTemp stops falling\nJet streams & CAT below it"}
C --> D["STRATOSPHERE\n~18–50 km\nOzone at 25 km absorbs UV\nTemp ↑ (inversion)\nStable, dry, no weather"]
D --> E{"STRATOPAUSE"}
E --> F["MESOSPHERE\n~50–80 km\nTemp ↓ with height\nNoctilucent clouds (polar)\nMeteors burn here"]
F --> G{"MESOPAUSE\n–100°C / 173 K\nColdest point"}
G --> H["THERMOSPHERE / IONOSPHERE\n80 km+\nTemp ↑ with height\nRadio wave propagation"]
H --> I["EXOSPHERE\nTo 10,000 km\nMolecules escape to space"]
style A fill:#90caf9
style B fill:#64b5f6
style C fill:#fff9c4
style D fill:#a5d6a7
style E fill:#fff9c4
style F fill:#ffcc80
style G fill:#fff9c4
style H fill:#ef9a9a
style I fill:#ce93d8
8. International Standard Atmosphere (ISA)
📘 What is ISA?
A standard average atmosphere specified for various purposes like design and testing of aircraft, evaluation of aircraft performance, calibration of altimeters. The most widely used is the ICAO ISA (International Standard Atmosphere).
ISA Specifications
| Parameter | Value |
|---|---|
| Air Condition | Dry |
| Temperature at MSL | 15°C (288.15 K) |
| Pressure at MSL | 1013.25 hPa |
| Density at MSL | 1225 g/m³ |
| Acceleration due to gravity | 980.665 cm/s² |
| Lapse Rate (up to 11 km / 36,090 ft) | 6.5°C/km (1.98°C/1000 ft ≈ 2°C/1000 ft) |
| Temperature 11–20 km (up to 65,617 ft) | –56.5°C (constant / isothermal) |
| Temperature 20–30 km | Rises at 1°C/km with temp –44.5°C at 32 km (104,987 ft) |
ISA Deviation Formula
ISA Deviation = Actual Temperature − ISA Temperature
Worked Example 1:
Actual temp at 2 km = 05°C
ISA temp at 2 km = 15 – (6.5 × 2) = 15 – 13 = 2°C
ISA Deviation = 05 – 02 = +03°C
Worked Example 2:
Pressure at MSL = 1002.25 hPa
ISA Pressure at MSL = 1013.25 hPa
ISA Deviation = 1002.25 – 1013.25 = –11 hPa
Worked Example 3:
Actual temp at 19 km = –60°C
ISA temp at 19 km = –56.5°C (constant 11–20 km)
ISA Deviation = –60 – (–56.5) = –3.5°C → answer –03.5°C
⭐ Key ISA Numbers to Remember
15-25-56.5-36090
MSL Temp = 15°C | MSL Pressure = 1013.25 hPa | Isothermal at –56.5°C | Isothermal starts at 36,090 ft (11 km)
9. Jet Standard Atmosphere (JSA)
📘 Why JSA?
ISA values are satisfactory for aircraft operating below 30,000 ft, but NOT above. For space flights and very high altitude flying and engine manufacturing, additional specific values are required. JSA was introduced to meet these requirements.
| Parameter | ISA | JSA |
|---|---|---|
| MSL Temperature | 15°C (288.15 K) | 15°C (288.15 K) |
| Lapse Rate | 6.5°C/km (1.98°C/1000 ft) | 2°C/1000 ft |
| Tropopause | Yes (at 11 km) | No Tropopause |
🚀 Quick Revision Summary — Chapter 1: Atmosphere
| Key Fact | Value |
|---|---|
| Total atmosphere thickness | ~480 km |
| 80% atmosphere within | 16 km |
| N₂ : O₂ by Volume | 4:1 | By Weight: 3:1 |
| Supplementary O₂ above | 10,000 ft |
| Homosphere up to | ~80 km |
| Max WV (tropics) | 4% by volume |
| Saturated Air RH | 100% |
| Ozone max concentration | 20–25 km |
| Ozone range | 10–50 km |
| Sensible heat | 23% |
| Latent heat | 77% |
| Troposphere height (equator) | 16–18 km |
| Troposphere height (poles) | 8–10 km |
| Lapse rate | ~6.5°C/km |
| WV + aerosols in troposphere | 99% |
| Atmospheric mass in troposphere | 75% |
| Tropopause temp (equator) | –70° to –75°C |
| Tropopause temp (poles) | –40° to –45°C |
| Stratosphere to | ~50 km |
| Ozone layer height | ~25 km |
| Mesosphere range | 50–80 km |
| Mesopause temperature | –100°C (173.15 K) |
| Ionosphere above | 60 km |
| Exosphere up to | 10,000 km |
| ISA MSL Temp | 15°C (288.15 K) |
| ISA MSL Pressure | 1013.25 hPa |
| ISA MSL Density | 1225 g/m³ |
| ISA Lapse Rate | 6.5°C/km up to 11 km |
| ISA Isothermal zone | –56.5°C, 11–20 km |
| JSA Lapse Rate | 2°C/1000 ft (no tropopause) |
Practice Questions & Answers
Q1Lowest layer of atmosphere is ……… (a) Troposphere (b) Tropopause (c) Stratosphere
✅ Correct Answer: (a) Troposphere
The Troposphere is the lowest layer, starting at earth's surface and extending to the tropopause. All weather occurs here.
Distractor Analysis(b) Tropopause is a boundary, not a layer. (c) Stratosphere is above the Troposphere.
📌 Instructor's NoteRemember the order: T-S-M-T-E = Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere from bottom to top.
Q2Height of Tropopause at equator is ……… (a) 8–10 km (b) 16–18 km (c) 14 km
✅ Correct Answer: (b) 16–18 km
At the equator, rising hot air due to convection pushes the tropopause higher — up to 16–18 km.
Distractor Analysis(a) 8–10 km is the tropopause height at the poles. (c) 14 km is intermediate but incorrect.
📌 Instructor's NoteEquator = Hot = Higher tropopause. Poles = Cold = Lower tropopause. "Hotter higher, colder lower."
Q3Height of Tropopause at Poles is ……… (a) 12 km (b) 13 km (c) 08 km
✅ Correct Answer: (c) 08 km
At the poles, the atmosphere is colder; therefore convective activity is minimal and the tropopause is lower, around 8–10 km.
Distractor Analysis(a) and (b) are too high for polar regions.
📌 Instructor's NotePolar tropopause ~300 hPa; Tropical tropopause ~100 hPa (lower pressure = higher altitude).
Q4Higher the surface temperature ……… would be the tropopause. (a) Higher (b) Lower (c) Same
✅ Correct Answer: (a) Higher
Warmer surfaces generate more convective activity, pushing the tropopause to a greater height.
Distractor Analysis(b) Lower — incorrect; this is the effect at colder surfaces/poles. (c) Same — temperature definitely affects tropopause height.
📌 Instructor's Note"Warmer surface = Higher tropopause" — think of hot air balloons rising higher when it's warmer.
Q5Height of tropopause ……… (a) Is constant (b) Varies with altitude (c) Varies with Latitude
✅ Correct Answer: (c) Varies with Latitude
Tropopause height depends on latitude (equator ~16–18 km; poles ~8 km) and also on season, surface temperature, synoptic situation.
Distractor Analysis(a) Is constant — definitely not; it varies significantly. (b) Varies with altitude — altitude is not the controlling factor.
📌 Instructor's NoteTropopause height is controlled by: Surface Temp, Latitude, Season, Land-Sea distribution, Synoptic situation.
Q6Above 8 km the lower temperatures are over ……… (a) Equator (b) Mid Latitudes (c) Poles
✅ Correct Answer: (c) Poles
Above 8 km, polar tropopause is at a lower height. Above the tropopause (stratosphere), temperatures invert. Since polar tropopause is lower (~8 km), above 8 km at poles we are already in the lower stratosphere where temperature reversal occurs, making it warmer — but the question refers to lower temperatures in the troposphere above 8 km. Actually, since poles have reversal above 8 km (into stratosphere), this is a nuanced answer. The text states "there is reversal of temperature and density above 8 km" at poles.
Distractor Analysis(a) Equator tropopause is at 16–18 km, so above 8 km equator is still in troposphere getting colder with height. (c) Poles — above 8 km at poles, we are in stratosphere where temp starts rising.
📌 Instructor's NoteKey: Tropopause temp at poles (–40 to –45°C) is warmer than at equator (–70 to –75°C). Equator has colder tropopause despite being hotter at surface.
Q7Atmosphere is heated by ……… (a) Solar Radiation (b) Heat from earth surface (c) From above
✅ Correct Answer: (b) Heat from earth surface
The atmosphere is heated from below by the earth's surface (via conduction, convection, radiation and latent heat). The atmosphere is a poor absorber of direct solar radiation.
Distractor Analysis(a) Solar radiation heats the EARTH, not directly the lower atmosphere. (c) From above is incorrect — atmosphere is heated from below.
📌 Instructor's NoteRemember: Temperature falls with height in troposphere BECAUSE heating is from below.
Q8The temperature at 80 km is ……… (a) 173 K (b) 100 K (c) –137°C
✅ Correct Answer: (a) 173 K (= –100°C)
The text explicitly states: "The temperature at 80 km is about –100°C (173.15 K)." 173 K ≈ –100°C. This is the Mesopause, the coldest point in the atmosphere.
Distractor Analysis(b) 100 K = –173°C, far too cold. (c) –137°C = 136 K, not matching the textbook value.
📌 Instructor's NoteMesopause = –100°C = 173.15 K. Mnemonic: "80 km → 173 K → Meteors burn".
Q9CO₂ and H₂O are also called ……… (a) Green House Gases (b) Rare Earth Gases (c) [other]
✅ Correct Answer: (a) Green House Gases
CO₂ and H₂O (water vapour) along with O₃ and Methane are greenhouse gases — they are transparent to shortwave solar radiation but absorb longwave terrestrial radiation.
Distractor Analysis(b) Rare Earth Gases (Noble gases like Argon, Neon, Helium) are a different category entirely.
📌 Instructor's NoteGreenhouse gases: W-C-O-M = Water vapour, CO₂, O₃, Methane.
Q10Troposphere is generally ……… (a) Stable (b) Unstable (c) Neutral
✅ Correct Answer: (b) Unstable
The Troposphere has a lapse rate of ~6.5°C/km. With turbulent mixing, convective activity and all weather occurring here, it is generally unstable.
Distractor Analysis(a) Stable — that's the Stratosphere. (c) Neutral — not the defining characteristic.
📌 Instructor's NoteTroposphere = Unstable; Stratosphere = Stable. Easy contrast for exams.
Q11Stratosphere is ……… (a) Unstable (b) Neutral (c) Stable
✅ Correct Answer: (c) Stable
The Stratosphere has a temperature inversion (temperature increases with height due to ozone absorbing UV), making it very stable with no convective weather.
Distractor Analysis(a) Unstable — that's the Troposphere. (b) Neutral — not the dominant characteristic.
📌 Instructor's NoteStratosphere = "Stratified" = Layered = Stable. Also: no weather, steady winds.
Q12Tropopause is discontinuous at about ……… (a) 30°lat (b) 40°lat (c) 80°lat
✅ Correct Answer: (b) 40°lat
The breaks/folds in the tropopause occur at ~40° and ~60° latitude, with the break at 40°lat being more prominent. Jet streams occur at these breaks.
Distractor Analysis(a) 30°lat — not a significant break. (c) 80°lat — too poleward, not where the prominent break is.
📌 Instructor's Note"Break at 40, Fold at 60." Jet streams at these breaks — important for upper-level wind planning.
Q13Most of atmospheric mass is contained in ……… (a) Troposphere (b) Stratosphere (c) Heterosphere
✅ Correct Answer: (a) Troposphere
The Troposphere contains 75% of the atmospheric mass and 99% of water vapour and aerosols.
Distractor Analysis(b) Stratosphere is relatively thin in mass. (c) Heterosphere is the outer part with very low density.
📌 Instructor's NoteTroposphere = 75% mass, 99% WV & aerosols. Two key numbers.
Q14Stratosphere extends from Tropopause to ……… (a) 50 km (b) 60 km (c) 40 km
✅ Correct Answer: (a) 50 km
The Stratosphere extends up to ~50 km from the earth's surface (approximately 30–32 km above the tropopause at equator).
Distractor Analysis(b) 60 km is too high — enters Mesosphere region. (c) 40 km is not the correct upper boundary.
📌 Instructor's NoteLayer tops: Tropo ~18, Strato ~50, Meso ~80. Easy: 18–50–80.
Q15The middle atmosphere layer with temperature inversion and stability is ……… (a) Troposphere (b) Tropopause (c) Stratosphere
✅ Correct Answer: (c) Stratosphere
The Stratosphere has a temperature inversion (UV absorption by ozone warms upper layers) making it very stable. It lies between the Tropopause and Stratopause.
Distractor Analysis(a) Troposphere is unstable with a normal lapse rate. (b) Tropopause is a boundary, not a layer.
📌 Instructor's NoteStratosphere = Stable + Inversion = Ozone = UV absorption. All linked.
Q16Mother of Pearl clouds occur in ……… (a) Mesosphere (b) Thermosphere (c) Stratosphere
✅ Correct Answer: (c) Stratosphere
Nacreous Clouds (Mother of Pearl Clouds) are sometimes seen in the Stratosphere in higher latitudes in winters.
Distractor Analysis(a) Mesosphere — Noctilucent clouds occur here. (b) Thermosphere — no clouds form here.
📌 Instructor's NoteNacreous = Stratosphere | Noctilucent = Mesosphere. "Na-Stra, Noc-Meso".
Q17The temperature in ISA at 17 km is ……… (a) –56.5°C (b) –65.5°C (c) –35.5°C
✅ Correct Answer: (a) –56.5°C
In ISA, from 11 km to 20 km (65,617 ft), temperature is assumed constant at –56.5°C. 17 km falls within this isothermal zone.
Distractor Analysis(b) –65.5°C and (c) –35.5°C are both incorrect — no lapse rate change occurs in this zone.
📌 Instructor's NoteISA isothermal zone: 11 km to 20 km = –56.5°C throughout. Any question with height 11–20 km → answer is –56.5°C.
Q18By weight, approximate ratio of O₂ to N₂ in the atmosphere is ……… (a) 1:3 (b) 1:4 (c) 1:5
✅ Correct Answer: (a) 1:3 (i.e., O₂ : N₂ = 1:3 by weight)
N₂ : O₂ by weight = 3:1, so O₂ : N₂ by weight = 1:3.
Distractor Analysis(b) 1:4 — that is the VOLUME ratio (O₂:N₂ = 1:4). Don't confuse weight and volume ratios.
📌 Instructor's NoteVolume: 4:1 | Weight: 3:1 (N₂:O₂). Volume ratio is larger because N₂ is less dense per unit volume.
Q19By volume, the approximate ratio of O₂ to N₂ in the atmosphere is ……… (a) 1:3 (b) 1:4 (c) 1:5
✅ Correct Answer: (b) 1:4 (O₂ : N₂ = 1:4 by volume)
N₂ : O₂ by volume = 4:1, therefore O₂ : N₂ = 1:4. N₂ = 78.09%, O₂ = 20.95% — ratio is almost 4:1.
Distractor Analysis(a) 1:3 is the weight ratio. (c) 1:5 is incorrect.
📌 Instructor's NoteVolume ratio is 4:1 (larger). Weight ratio is 3:1 (smaller). "Volume is Vast (4)."
Q20By volume, the proportion of CO₂ in the atmosphere is ……… (a) 3% (b) 0.3% (c) 0.03%
✅ Correct Answer: (c) 0.03% (0.035%)
CO₂ constitutes approximately 0.035% by volume in the atmosphere.
Distractor Analysis(a) 3% — vastly overstated. (b) 0.3% — 10 times too high.
📌 Instructor's NoteCO₂ = 0.035% ≈ 0.03% — tiny amount but significant for greenhouse effect.
Q21In ISA, the mean sea level temperature is ……… (a) 15°C (b) 10°C (c) 25°C
✅ Correct Answer: (a) 15°C
ISA MSL temperature = 15°C (288.15 K). This is a fundamental ISA value.
Distractor Analysis(b) 10°C — incorrect. (c) 25°C — too warm, not ISA standard.
📌 Instructor's NoteISA MSL: Temp = 15°C, Pressure = 1013.25 hPa, Density = 1225 g/m³. Memorise all three together.
Q22Maximum concentration of ozone is at a height of ……… (a) 10–15 km (b) 20–25 km (c) 30–35 km
✅ Correct Answer: (b) 20–25 km
Ozone is found between 10–50 km with maximum concentration at 20–25 km in the lower Stratosphere.
Distractor Analysis(a) 10–15 km — lower end of ozone range, not maximum. (c) 30–35 km — past the maximum.
📌 Instructor's NoteOzone range: 10–50 km | Max: 20–25 km | Ozone layer: ~25 km. "20-25 is Ozone's favourite zone."
Q23Additional oxygen is needed while flying above ……… (a) 5000 ft (b) 7000 ft (c) 10000 ft
✅ Correct Answer: (c) 10000 ft
Due to rapid reduction of gases with height, supplementary oxygen is needed above 10,000 ft.
Distractor Analysis(a) 5000 ft and (b) 7000 ft — below the threshold requiring supplemental oxygen.
📌 Instructor's Note10,000 ft = critical altitude for supplemental O₂. Also relevant for cabin pressurization regulations.
Q24CO₂ and H₂O keep the atmosphere ……… (a) Warm (b) Cold (c) Have no effect
✅ Correct Answer: (a) Warm
CO₂ and H₂O are greenhouse gases that absorb terrestrial longwave radiation and re-radiate it, keeping the earth warmer than it would otherwise be.
Distractor Analysis(b) Cold — incorrect, they trap heat. (c) Have no effect — incorrect, they are the primary drivers of the greenhouse effect.
📌 Instructor's NoteThink of greenhouse gases as a "blanket" around the earth, trapping warmth.
Q25Noctilucent clouds occur in ……… (a) Thermosphere (b) Mesosphere (c) Stratosphere
✅ Correct Answer: (b) Mesosphere
Noctilucent Clouds are seen on rare occasions in the upper Mesosphere, in Polar regions.
Distractor Analysis(a) Thermosphere — no clouds form there. (c) Stratosphere — Nacreous/Mother of Pearl clouds occur here, not Noctilucent.
📌 Instructor's NoteNoctilucent = Mesosphere (Polar) | Nacreous = Stratosphere (higher latitudes, winters). Know both!
Q26Temperature at 2 km is 05°C. What is ISA deviation? Hint: (Actual – ISA) (a) –05°C (b) –02°C (c) 03°C
✅ Correct Answer: (c) 03°C (i.e. +03°C)
ISA temp at 2 km = 15 – (6.5 × 2) = 15 – 13 = 2°C. ISA Deviation = 5 – 2 = +3°C.
Distractor Analysis(a) –5°C wrong sign and magnitude. (b) –2°C — wrong direction entirely.
📌 Instructor's NoteISA Deviation = Actual – ISA. Positive deviation = warmer than ISA. Always compute ISA temp first using lapse rate.
Q27Pressure at MSL is 1002.25 hPa. Find the ISA deviation. Hint: (Actual – ISA) (a) –11 hPa (b) 10 hPa (c) 12 hPa
✅ Correct Answer: (a) –11 hPa
ISA MSL Pressure = 1013.25 hPa. Deviation = 1002.25 – 1013.25 = –11 hPa. Negative means pressure is lower than ISA standard.
Distractor Analysis(b) 10 hPa and (c) 12 hPa — wrong sign and magnitude.
📌 Instructor's NoteSame formula applies for pressure deviation as temperature deviation. ISA MSL Pressure = 1013.25 hPa.
Q28In actual atmosphere temp. at 19 km is –60°C. Find the ISA deviation. Hint: (Actual – ISA) (a) –4.5°C (b) –05.5°C (c) –03.5°C
✅ Correct Answer: (c) –03.5°C
ISA temp at 19 km = –56.5°C (isothermal zone 11–20 km). Deviation = –60 – (–56.5) = –60 + 56.5 = –3.5°C.
Distractor Analysis(a) –4.5°C and (b) –5.5°C are arithmetic errors — make sure signs are handled correctly.
📌 Instructor's NoteFor any altitude between 11–20 km: ISA temp is always –56.5°C. Don't try to apply lapse rate in this zone.
Q29Nacreous clouds occur in ……… (a) Thermosphere (b) Mesosphere (c) Upper Stratosphere
✅ Correct Answer: (c) Upper Stratosphere (Stratosphere)
Nacreous Clouds (Mother of Pearl Clouds) are sometimes seen in the Stratosphere in higher latitudes in winters.
Distractor Analysis(a) Thermosphere — no clouds. (b) Mesosphere — Noctilucent clouds.
📌 Instructor's NoteNacreous clouds are iridescent and beautiful — found in the stratosphere over polar regions in winter.
Q30The atmosphere up to 80 km has a nearly similar composition and is called the Homosphere. Its uniform composition is due to ……… (a) Pressure (b) Gravitation of earth (c) Mixing due to turbulence
✅ Correct Answer: (c) Mixing due to turbulence
The atmosphere is generally well mixed up to ~80 km due to turbulent mixing, maintaining a nearly uniform composition.
Distractor Analysis(a) Pressure alone doesn't cause uniform mixing. (b) Gravitation causes vertical stratification, not uniform mixing.
📌 Instructor's NoteHomosphere = uniform mixing by turbulence. Heterosphere = no mixing = separation by molecular weight.
Q31Half of the atmospheric air mass is contained ……… below (a) 20,000 ft (b) 15,000 ft (c) 10,000 ft
✅ Correct Answer: (b) 15,000 ft (~6 km ≈ 19,685 ft; closest is ~18,000 ft; textbook states 6 km)
The textbook states half the atmospheric mass is below 6 km. 6 km ≈ 19,685 ft. The closest answer would be approximately 18,000–20,000 ft. Option (a) 20,000 ft is the closest match.
Distractor AnalysisThe question is about 6 km = ~20,000 ft, so (a) 20,000 ft is correct. 10,000 ft = ~3 km (too low); 15,000 ft = ~4.6 km (too low).
📌 Instructor's Note6 km = ~20,000 ft = half the atmosphere below this level. Key for pressurisation and O₂ planning.
Q32In jet standard atmosphere the Lapse Rate is ……… (a) 2°C/1000 ft (b) 2°C/km (c) 5°C/km
✅ Correct Answer: (a) 2°C/1000 ft
The Jet Standard Atmosphere specifies a lapse rate of 2°C/1000 ft (with no tropopause).
Distractor Analysis(b) 2°C/km is the ISA lapse rate in the stratospheric rise zone (20–30 km) — 1°C/km actually. (c) 5°C/km is not a standard rate.
📌 Instructor's NoteJSA: 2°C/1000 ft, no tropopause. ISA: 6.5°C/km (≈2°C/1000 ft below 11 km, then isothermal).
Q33The rate of fall of temperatures with height, called ……… (a) Isothermal rate (b) Inversion Rate (c) Lapse Rate
✅ Correct Answer: (c) Lapse Rate
Lapse rate is the rate at which temperature decreases with increasing altitude, approximately 6.5°C/km in the ISA troposphere.
Distractor Analysis(a) Isothermal rate — temperature does NOT change with height (isothermal). (b) Inversion Rate — temperature INCREASES with height (inverse of normal).
📌 Instructor's NoteLapse Rate = normal fall of temperature with height. Inversion = temperature rise with height. Isothermal = constant temperature.
Q34In actual atmosphere the lapse rate could ……… (a) Assume any value (b) Fall up to 8 km. (c) Rise up to 50 km
✅ Correct Answer: (a) Assume any value
In the actual (real) atmosphere — unlike ISA — the lapse rate can assume any value; it can be positive, zero (isothermal), or negative (inversion) depending on atmospheric conditions.
Distractor Analysis(b) and (c) describe fixed behaviours. The actual atmosphere is highly variable.
📌 Instructor's NoteISA lapse rate = fixed at 6.5°C/km. ACTUAL lapse rate = variable, can be anything. This distinction is key for stability analysis.
Q35Tropical Tropopause extends from the equator to Lat. 35°–40°. Over India it is at ……… (a) 20–21 km (b) 14–15 km (c) 16–16.5 km
✅ Correct Answer: (c) 16–16.5 km (or option b 14–15 km per textbook)
Over India, the Tropical Tropopause is at approximately 11.5 km (temperature –45°C) per the textbook. In winters it may be at 23°N. The Tropical Tropopause extends to Lat. 35°–40° and at 100 hPa level, which is approximately 16–16.5 km at equatorial latitudes.
Distractor AnalysisThe question specifies "over India" — India is in tropical latitudes (8°N–37°N); the tropical tropopause over India varies with season.
📌 Instructor's NoteTropical Tropopause: ~100 hPa level | Over India: ~11.5 km at 23°N in winters. India experiences both tropical and subtropical tropopause influences.
Q36Lapse rate in the troposphere is produced by ……… and in the stratosphere by ……… (a) Evaporation; condensation (b) Condensation; solar radiation (c) Terrestrial radiation; solar radiation (d) Solar radiation; convection
✅ Correct Answer: (c) Terrestrial radiation; solar radiation
In the Troposphere: cooling with height is due to terrestrial radiation heating from below. In Stratosphere: temperature inversion is due to solar radiation (UV) being absorbed by ozone — causing warming with height.
Distractor AnalysisOther combinations mix up the mechanisms. Condensation releases latent heat but is not the primary cause of the lapse rate.
📌 Instructor's NoteTroposphere heated from Earth (below) → cools upward. Stratosphere heated by ozone absorbing UV (from Sun above) → warms upward.
Q37Most of the water vapour in the atmosphere is confined up to ……… (a) stratosphere (b) 30,000 ft (c) Mid troposphere (d) lower troposphere
✅ Correct Answer: (d) lower troposphere
Water vapour concentration rapidly decreases with height. Most WV is concentrated in the lower troposphere where temperatures are highest. Above 30,000 ft, WV is very low.
Distractor Analysis(a) Stratosphere — very little WV. (b) 30,000 ft — WV is minimal at this height. (c) Mid troposphere — less than lower troposphere.
📌 Instructor's Note99% WV and aerosols in troposphere; most concentrated in lower troposphere. This is why most weather and cloud formation occurs there.
Q38Negative lapse rate of temperature is ……… (a) Isothermal rate (b) Temperature rise with lowering height (c) Temperature rise with height (d) Temperature fall with height
✅ Correct Answer: (c) Temperature rise with height
A "negative lapse rate" means temperature increases with altitude — this is called an inversion. This occurs in the Stratosphere and in inversions within the Troposphere.
Distractor Analysis(a) Isothermal = zero lapse rate (no temperature change). (d) Temperature fall with height = positive lapse rate (normal). (b) is phrased incorrectly.
📌 Instructor's NotePositive lapse rate = temp falls with height (normal). Zero lapse rate = isothermal. Negative lapse rate = inversion (temp rises with height).
Q39In ICAO ISA the atmosphere is assumed to be isothermal ……… (a) in stratosphere (b) 11 to 16 km (c) 11 to 20 km (d) 11 to 32 km
✅ Correct Answer: (c) 11 to 20 km
In ICAO ISA, temperature is assumed constant at –56.5°C from 11 km (36,090 ft) to 20 km (65,617 ft). This is the isothermal (stratospheric) zone.
Distractor Analysis(b) 11–16 km — too narrow. (d) 11–32 km — too wide; from 20 km temperature starts rising again. (a) "in stratosphere" is vague and incorrect if taken to mean the whole stratosphere.
📌 Instructor's NoteISA Isothermal: 11–20 km at –56.5°C. After 20 km temp rises. After 32 km another zone begins.
Q40One of the characteristics of our atmosphere is ……… (a) Poor conductor of heat and electricity (b) Equator is warmer than poles above 10 km (c) Lapse rate is positive in stratosphere (d) Density is constant above 8 km
✅ Correct Answer: (a) Poor conductor of heat and electricity
The atmosphere is a poor conductor of heat and electricity — one of its key characteristics listed at the beginning of the chapter.
Distractor Analysis(b) Above 10 km, poles are actually warmer than equator at the tropopause (poles –40 to –45°C vs equator –70 to –75°C). (c) Lapse rate in stratosphere is negative (inversion). (d) Density is NOT constant above 8 km.
📌 Instructor's NoteAtmosphere = poor conductor. Counterintuitively, tropopause at POLES is warmer (–40°C) than EQUATOR (–75°C) — despite surface being opposite!
Q41Heat transfer in the atmosphere is maximum due to ……… (a) convection (b) radiation (c) sensible heat (d) latent heat
✅ Correct Answer: (d) latent heat
Latent Heat (Evaporation, Condensation, Sublimation) accounts for 77% of heat transfer from earth to atmosphere — much more than sensible heat at 23%.
Distractor Analysis(a) Convection is a mechanism within sensible heat, not the maximum overall. (b) Radiation contributes but is part of the 23%. (c) Sensible heat is only 23%.
📌 Instructor's NoteLatent heat = 77% (dominant!). Sensible heat = 23%. "Latent is Larger at 77%."
Q42The knowledge of the height of tropopause is important for a pilot because ……… (a) weather is mainly confined up to this level (b) clouds rarely reach up to this height due to jetstream (c) stratosphere starts at this height where all solar radiation is absorbed
✅ Correct Answer: (a) weather is mainly confined up to this level
The tropopause is the maximum height clouds can reach. All weather, turbulence, cloud formations are below it. Knowing its height helps pilots plan for weather-free cruising altitudes.
Distractor Analysis(b) Clouds do grow to just below tropopause — not because of jet streams. (c) Solar radiation is absorbed by ozone in the stratosphere, not at the tropopause itself.
📌 Instructor's NoteTropopause = weather ceiling. Above it = stable, clear, steady stratospheric winds. CAT and Jet streams just BELOW it.
Q43In ISA atmosphere the tropopause occurs at a height of ……… (a) 8–10 km (b) 11 km (c) 16–18 km
✅ Correct Answer: (b) 11 km
In ICAO ISA, the tropopause is fixed at 11 km (36,090 ft), where the lapse rate stops and isothermal zone begins (–56.5°C).
Distractor Analysis(a) 8–10 km = actual polar tropopause. (c) 16–18 km = actual equatorial tropopause. ISA is a standard — fixed at 11 km.
📌 Instructor's NoteISA tropopause = 11 km = 36,090 ft. This is a fixed constant in ISA, unlike the actual variable tropopause.
Q44Most of the transfer of heat in the atmosphere is due to ……… (a) conduction (b) convection (c) Both A and B (d) latent heat
✅ Correct Answer: (d) latent heat
As stated in the chapter: Latent Heat transfer = 77% (dominant). Sensible heat (including conduction, convection, radiation) = 23%.
Distractor Analysis(a) Conduction — atmosphere is a poor conductor. (b) Convection — important but part of 23%. (c) Both — only 23% combined.
📌 Instructor's NoteThis question is almost identical to Q41. Reinforce: Latent heat dominates atmospheric heat transfer.
Q45There is reversal of temperature at poles and becomes negative at ……… (a) 8 km (b) 12 km (c) 20 km (d) 30 km
✅ Correct Answer: (a) 8 km
Since the polar tropopause is at ~8–10 km, above 8 km at the poles we enter the stratosphere where temperature reverses (starts rising). The text states: "since in troposphere temperatures fall with height up to tropopause, above 8 km the poles start warming up and become warmer than the equator."
Distractor Analysis(b) 12 km and higher — the reversal starts at the polar tropopause (~8 km), not higher.
📌 Instructor's NoteAbove 8 km at poles: temperature reversal → poles become warmer than equator (at that level). This is why jet streams form at tropopause breaks.
Master Reference Tables
1. All Numerical Values — Chapter 1
| Parameter | Value | Source |
|---|---|---|
| Atmosphere total thickness | ~480 km | Composition |
| 80% atmosphere below | 16 km | Composition |
| N₂ by volume | 78.09% | Constituents |
| O₂ by volume | 20.95% | Constituents |
| Argon by volume | 0.93% | Constituents |
| CO₂ by volume | 0.035% | Constituents |
| O₃ trace concentration | 0.000007% | Constituents |
| N₂:O₂ by Volume | 4:1 | Ratios |
| N₂:O₂ by Weight | 3:1 | Ratios |
| Supplemental O₂ required above | 10,000 ft | Physiology |
| Homosphere up to | ~80 km | Structure |
| Max WV (tropics) | 4% by volume | WV |
| WV minimal above | 30,000 ft | WV |
| Tropics latitude band | 23½°N to 23½°S | WV |
| Ozone range | 10–50 km | Ozone |
| Ozone max concentration | 20–25 km | Ozone |
| Ozone layer (Stratosphere) | ~25 km | Ozone |
| Airmass below 6 km | 1/2 (50%) | Vertical dist. |
| Airmass below 10 km | 3/4 (75%) | Vertical dist. |
| Airmass below 35 km | 99% | Vertical dist. |
| Sensible heat transfer | 23% | Thermal |
| Latent heat transfer | 77% | Thermal |
| Troposphere height (equator) | 16–18 km | Layers |
| Troposphere height (poles) | 8–10 km | Layers |
| ISA Lapse rate | 6.5°C/km (≈1.98°C/1000 ft) | Layers |
| Atmospheric mass in troposphere | 75% | Layers |
| WV+aerosols in troposphere | 99% | Layers |
| Lower Troposphere | Surface – 2.1 km | Layers |
| Mid Troposphere | 2.1 – 7.6 km | Layers |
| Upper Troposphere | 7.6 km – Tropopause | Layers |
| Tropopause lapse rate | 1–2°C/km | Tropopause |
| Tropopause temp (equator) | –70° to –75°C | Tropopause |
| Tropopause temp (poles) | –40° to –45°C | Tropopause |
| Tropopause breaks at | ~40° and ~60° Lat | Tropopause |
| Polar Tropopause pressure | ~300 hPa | Tropopause |
| Tropical Tropopause pressure | ~100 hPa | Tropopause |
| Middle Tropopause pressure | ~200 hPa | Tropopause |
| Stratosphere extent | ~18–50 km | Layers |
| Mesosphere extent | 50–80 km | Layers |
| Mesopause temperature | –100°C (173.15 K) | Layers |
| Ionosphere above | 60 km | Layers |
| Exosphere extent | Up to 10,000 km | Layers |
| ISA MSL Temperature | 15°C (288.15 K) | ISA |
| ISA MSL Pressure | 1013.25 hPa | ISA |
| ISA MSL Density | 1225 g/m³ | ISA |
| ISA gravity | 980.665 cm/s² | ISA |
| ISA isothermal zone | 11–20 km at –56.5°C | ISA |
| ISA isothermal start | 11 km = 36,090 ft | ISA |
| ISA isothermal end | 20 km = 65,617 ft | ISA |
| ISA temp rise zone (20–30 km) | 1°C/km, –44.5°C at 32 km | ISA |
| JSA Lapse rate | 2°C/1000 ft | JSA |
| JSA valid below | 30,000 ft (ISA ok); JSA for above | JSA |
2. Mnemonics Summary
| Mnemonic | What it Helps Remember |
|---|---|
| T-S-M-T-E | Atmospheric layers bottom to top: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere |
| Volume 4, Weight 3 | N₂:O₂ ratio by Volume=4:1, by Weight=3:1 |
| Latent is Larger (77%) | Latent heat transfer = 77%, Sensible = 23% |
| 6–10–35 = Half–3/4–All | Vertical air mass distribution |
| Hotter = Higher (Tropopause) | Warmer surface → higher tropopause |
| 15–1013.25–1225 | ISA MSL: Temp, Pressure, Density |
| 11 km = 36,090 ft = –56.5°C | ISA tropopause and isothermal start |
| Na-Stra, Noc-Meso | Nacreous clouds = Stratosphere; Noctilucent = Mesosphere |
| W-C-O-M | Greenhouse gases: Water vapour, CO₂, O₃, Methane |
| 20–25 is Ozone's zone | Maximum ozone concentration at 20–25 km |
| ISA Dev = Actual – ISA | Formula for ISA deviation (positive = warmer than standard) |
| Break at 40, Fold at 60 | Tropopause discontinuities at ~40° and ~60° latitude |
3. Formula Sheet
Key Formulae — Chapter 1
ISA Temperature at altitude h (km), for h ≤ 11 km:
T(h) = 15 – 6.5 × h [°C]
Example: h = 5 km → T = 15 – 32.5 = –17.5°C
ISA Deviation:
ISA Deviation = T_actual – T_ISA
Positive = warmer than ISA | Negative = colder than ISA
ISA Isothermal zone (11–20 km):
T = –56.5°C (constant; no lapse rate applies)
Kelvin–Celsius conversion:
K = °C + 273.15
–56.5°C = 216.65 K | –100°C = 173.15 K | 15°C = 288.15 K
4. Answer Key — Chapter 1 Questions (Q1–Q45)
| Q | Ans | Q | Ans | Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|---|---|---|---|
| 1 | a | 2 | b | 3 | c | 4 | a | 5 | c |
| 6 | c | 7 | b | 8 | a | 9 | a | 10 | b |
| 11 | c | 12 | b | 13 | a | 14 | a | 15 | c |
| 16 | c | 17 | a | 18 | a | 19 | b | 20 | c |
| 21 | a | 22 | b | 23 | c | 24 | a | 25 | b |
| 26 | c | 27 | a | 28 | c | 29 | c | 30 | c |
| 31 | a | 32 | a | 33 | c | 34 | a | 35 | c |
| 36 | c | 37 | d | 38 | c | 39 | c | 40 | a |
| 41 | d | 42 | a | 43 | b | 44 | d | 45 | a |
Capt. Pankaj Pahil
Key facts — Atmosphere
The figures from this chapter worth carrying into the exam hall.
- ISA sea level pressure
- The International Standard Atmosphere takes mean sea level pressure as 1013.25 hPa (29.92 inches of mercury).
- ISA sea level temperature
- ISA mean sea level temperature is +15°C, with a density of 1.225 kg/m³.
- ISA lapse rate
- Temperature in the ISA falls by 1.98°C per 1000 ft — used as 2°C per 1000 ft in practice — up to the tropopause.
- ISA tropopause
- The ISA tropopause is at 36 090 ft (11 km), where the temperature is −56.5°C and stops falling with height.
- Composition of the atmosphere
- Dry air is about 78% nitrogen and 21% oxygen by volume, the remaining 1% being argon, carbon dioxide and trace gases.
- Vertical divisions
- From the surface upward the atmosphere divides into troposphere, stratosphere, mesosphere and thermosphere, each separated from the next by a pause.
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