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DGCA Aviation Meteorology · 125 questions

Atmosphere & ISA: DGCA Aviation Meteorology Questions and Answers

125 practice questions on Atmosphere & ISA with answers and explanations, plus the key concepts and formulas, from the TrueHeading question bank.

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About this topic

Use this page to revise Atmosphere & ISA for the DGCA Aviation Meteorology paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 125 questions in the TrueHeading bank for this topic.

In short: Temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³.

Open the full set in the student zone to answer every question, track your accuracy and take timed mock tests.

Key concepts: Atmosphere & ISA

8 ideas to know cold.

What are the ISA values at mean sea level and how does temperature change with height?

Temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³.

  • Falls 6.5 °C per km (≈ 1.98 °C per 1000 ft) up to the tropopause at 11 km (36 090 ft)
  • Constant −56.5 °C from 11 km to 20 km
  • Then rises 1 °C per km up to 32 km
−60 °C+20 °CStratosphereisothermal −56.5 °Cup to about 20 kmTroposphere−6.5 °C / km≈ −2 °C / 1000 ftTropopause11 km · 36 090 ftMSL +15 °C1013.25 hPa

What is the make-up of the atmosphere?

About 78 % nitrogen, 21 % oxygen, 0.9 % argon, 0.04 % carbon dioxide, plus water vapour (0–4 %).

  • The mix is the same to about 80 km, apart from water vapour and ozone
  • Most water vapour and all weather lie in the troposphere

Where is the tropopause and why does it matter?

The boundary between troposphere and stratosphere, where temperature stops falling.

  • About 16–17 km at the equator, 8–9 km at the poles, 11 km in ISA
  • Higher in summer than winter, and over warm air
  • Jet streams, clear-air turbulence and cirrus gather near it

Which flight levels match the standard pressure levels?

Approximate pressure-altitude values:

  • 850 hPa ≈ FL 050
  • 700 hPa ≈ FL 100
  • 500 hPa ≈ FL 180
  • 300 hPa ≈ FL 300
  • 250 hPa ≈ FL 340
  • 200 hPa ≈ FL 390

Tip: Pressure halves about every 18 000 ft (5.5 km).

How is air density calculated?

From the equation of state for dry air.

ρ=pR TR=287 J kg−1K−1\rho=\frac{p}{R\,T}\qquad R=287\ \text{J\,kg}^{-1}\text{K}^{-1}
  • Density falls with height and with higher temperature
  • Humid air is less dense than dry air (water vapour is lighter than N₂ and O₂)

How many feet is one hPa near the surface?

About 27–30 ft per hPa. Exam questions normally use 30 ft.

  • Gets bigger with height (about 50 ft per hPa at 18 000 ft)
  • Used for QFE/QNH and pressure altitude calculations

What are the ISA sea level values and lapse rate?

15 °C, 1013.25 hPa, 1.225 kg/m³. Temperature falls 1.98 °C / 1 000 ft (6.5 °C/km) up to 11 km (36 090 ft).

TISA=15−1.98×h1000T_{ISA}=15-1.98\times\dfrac{h}{1000}
  • Above 11 km the temperature is constant at −56.5 °C to 20 km
  • Tropopause height ≈ 8 km at poles, 16–18 km at the equator
  • ISA deviation = actual − ISA temperature

What is the composition of the atmosphere?

78 % nitrogen, 21 % oxygen, ~1 % argon, trace CO₂. Water vapour is variable (0–4 %).

  • Troposphere contains most of the weather
  • Ozone layer is in the stratosphere
  • Pressure falls by half every ~18 000 ft

Practice questions: Atmosphere & ISA

Tap “Show answer” after you have tried each one.

Q1The ISA temperature at FL40 is closest to:

  1. −7 °C
  2. 7 °C
  3. 14 °C
  4. 11 °C
Show answer

Answer: B. 7 °C

ISA: 15 °C at sea level, falling 1.98 °C per 1 000 ft up to the tropopause (36 090 ft). 15 − 1.98 × 4 = 7.1 °C.

Q2The ISA temperature at FL100 is closest to:

  1. 2 °C
  2. −1 °C
  3. −5 °C
  4. −11 °C
Show answer

Answer: C. −5 °C

ISA: 15 °C at sea level, falling 1.98 °C per 1 000 ft up to the tropopause (36 090 ft). 15 − 1.98 × 10 = −4.8 °C.

Q3The ISA temperature at FL90 is closest to:

  1. 4 °C
  2. −3 °C
  3. 1 °C
  4. −9 °C
Show answer

Answer: B. −3 °C

ISA: 15 °C at sea level, falling 1.98 °C per 1 000 ft up to the tropopause (36 090 ft). 15 − 1.98 × 9 = −2.8 °C.

Q4At FL160 the outside air temperature is −27 °C. The ISA deviation is closest to:

  1. ISA −10
  2. ISA +10
  3. ISA −20
  4. ISA −1
Show answer

Answer: A. ISA −10

ISA at FL160 = 15 − 1.98 × 16 = −16.7 °C. Deviation = OAT − ISA = −27 − (−16.7) ≈ −10 °C.

Q5At FL310 the outside air temperature is −54 °C. The ISA deviation is closest to:

  1. ISA −18
  2. ISA +8
  3. ISA −8
  4. ISA −1
Show answer

Answer: C. ISA −8

ISA at FL310 = 15 − 1.98 × 31 = −46.4 °C. Deviation = OAT − ISA = −54 − (−46.4) ≈ −8 °C.

Q6The ISA temperature at FL390 is:

  1. −56.5 °C
  2. −92.2 °C
  3. −51.2 °C
  4. −60.0 °C
Show answer

Answer: A. −56.5 °C

In the ISA the temperature is constant at −56.5 °C from the tropopause (11 km / 36 090 ft) up to 20 km.

Q7The approximate ISA pressure at FL180 is:

  1. about 500 hPa
  2. about 400 hPa
  3. about 300 hPa
  4. about 700 hPa
Show answer

Answer: A. about 500 hPa

Standard figures: FL100 ≈ 700 hPa, FL180 ≈ 500 hPa, FL300 ≈ 300 hPa, FL390 ≈ 200 hPa, FL530 ≈ 100 hPa.

Q8Pressure altitude 9000 ft, OAT −13 °C (ISA temperature at that level is −2.8 °C). Using density altitude = pressure altitude + 120 × ISA deviation, the density altitude is about:

  1. 11 443 ft
  2. 10 222 ft
  3. 9 000 ft
  4. 7 778 ft
Show answer

Answer: D. 7 778 ft

ISA deviation = −13 − (−2.8) = −10.2 °C. DA = 9000 + 120 × −10.2 ≈ 7 778 ft.

Q9Pressure altitude 7500 ft, OAT 15 °C (ISA temperature at that level is 0.2 °C). Using density altitude = pressure altitude + 120 × ISA deviation, the density altitude is about:

  1. 5 718 ft
  2. 9 282 ft
  3. 11 064 ft
  4. 7 500 ft
Show answer

Answer: B. 9 282 ft

ISA deviation = 15 − (0.2) = 14.8 °C. DA = 7500 + 120 × 14.8 ≈ 9 282 ft.

Q10The lowest layer of the atmosphere, in which most weather occurs, is the:

  1. troposphere
  2. mesosphere
  3. tropopause
  4. stratosphere
Show answer

Answer: A. troposphere

Weather and almost all water vapour are in the troposphere.

Q11The ISA sea-level air density is:

  1. 0.125 kg/m³
  2. 12.25 kg/m³
  3. 1.013 kg/m³
  4. 1.225 kg/m³
Show answer

Answer: D. 1.225 kg/m³

ISA density at sea level is 1.225 kg/m³ (0.002377 slug/ft³).

Q12The height of a pressure level in cold air compared with warm air is:

  1. higher in dry air only
  2. lower in cold air
  3. the same
  4. higher in cold air
Show answer

Answer: B. lower in cold air

Cold air is denser, so the pressure falls more rapidly with height and a given pressure level lies lower.

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