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DGCA Aviation Meteorology · 71 questions

Stability & Adiabatics: DGCA Aviation Meteorology Questions and Answers

71 practice questions on Stability & Adiabatics 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 Stability & Adiabatics for the DGCA Aviation Meteorology paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 71 questions in the TrueHeading bank for this topic.

In short: DALR 3 °C per 1000 ft (9.8 °C/km). SALR about 1.5 °C per 1000 ft (varies). ELR averages 2 °C per 1000 ft.

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Key concepts: Stability & Adiabatics

6 ideas to know cold.

DALR, SALR and ELR: what are the values?

DALR 3 °C per 1000 ft (9.8 °C/km). SALR about 1.5 °C per 1000 ft (varies). ELR averages 2 °C per 1000 ft.

  • A rising parcel cools at DALR until saturated, then at the SALR
  • The SALR is smaller because condensation releases latent heat
ELR steeperthan 3 °C: unstableELR below SALRstableconditionallyunstableDALR 3 °C/1000 ftSALR ~1.5 °C/1000 fttemperature → (colder to the left)height

Stable, unstable or conditionally unstable?

Compare the environmental lapse rate (ELR) with the parcel’s:

  • Absolutely stable: ELR < SALR
  • Conditionally unstable: SALR < ELR < DALR
  • Absolutely unstable: ELR > DALR

Tip: Dry parcel colder than its surroundings sinks back. Warmer than its surroundings, it keeps rising.

What weather goes with stable and unstable air?

Stable: layer cloud (St, As, Ns), fog, poor visibility, smooth air. Unstable: heap cloud (Cu, Cb), showers, good visibility, turbulence.

What is the foehn effect?

Air rises on the windward side, cools at DALR then SALR (rain falls). It descends on the lee side warming at the DALR, so it arrives warmer and drier.

  • Example: 20 °C air rises 6000 ft with cloud base at 2000 ft. Dry ascent: −6 °C. Saturated ascent (4000 ft): −6 °C. Top: 8 °C.
  • Descending 6000 ft at the DALR adds 18 °C, so it reaches 26 °C on the lee side.

What are the lapse rates?

DALR 3 °C/1 000 ft (9.8 °C/km); SALR about 1.5 °C/1 000 ft (varies 1–2); ELR is the actual lapse rate.

  • Stable: ELR < SALR
  • Unstable: ELR > DALR
  • Conditionally unstable: SALR < ELR < DALR
  • Inversion: temperature increases with height (very stable)

How do you find the cloud base from temperature and dew point?

Cloud base height (ft) ≈ 400 × (T − Td), or 125 m per °C of spread.

Base (ft)≈400 (T−Td)\text{Base (ft)}\approx 400\,(T-T_d)
  • T 25 °C, Td 13 °C: 12 × 400 = 4 800 ft above the station
  • Convection cloud forms at the condensation level
  • Dew point falls about 0.5 °C/1 000 ft in rising air

Practice questions: Stability & Adiabatics

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

Q1Air at sea level on the windward side has a temperature of 20 °C and a dew point of 8 °C. It is forced over a ridge 12000 ft high. Use DALR 3 °C/1 000 ft, SALR 1.5 °C/1 000 ft and condensation level = spread × 400 ft. Find the temperature at sea level on the lee side.

  1. 35.3 °C
  2. 30.8 °C
  3. 20.0 °C
  4. 26.3 °C
Show answer

Answer: B. 30.8 °C

Condensation level 4800 ft: temperature 5.6 °C. Cooling at the SALR to 12000 ft gives −5.2 °C. Descending dry on the lee side warms at the DALR: −5.2 + 3 × 12 = 30.8 °C. The lee side ends 10.8 °C warmer than the windward side.

Q2Air at sea level on the windward side has a temperature of 24 °C and a dew point of 17 °C. It is forced over a ridge 12000 ft high. Use DALR 3 °C/1 000 ft, SALR 1.5 °C/1 000 ft and condensation level = spread × 400 ft. Find the temperature at sea level on the lee side.

  1. 42.3 °C
  2. 33.3 °C
  3. 24.0 °C
  4. 37.8 °C
Show answer

Answer: D. 37.8 °C

Condensation level 2800 ft: temperature 15.6 °C. Cooling at the SALR to 12000 ft gives 1.8 °C. Descending dry on the lee side warms at the DALR: 1.8 + 3 × 12 = 37.8 °C. The lee side ends 13.8 °C warmer than the windward side.

Q3Air at sea level on the windward side has a temperature of 20 °C and a dew point of 13 °C. It is forced over a ridge 6000 ft high. Use DALR 3 °C/1 000 ft, SALR 1.5 °C/1 000 ft and condensation level = spread × 400 ft. Find the temperature at sea level on the lee side.

  1. 20.0 °C
  2. 20.3 °C
  3. 24.8 °C
  4. 29.3 °C
Show answer

Answer: C. 24.8 °C

Condensation level 2800 ft: temperature 11.6 °C. Cooling at the SALR to 6000 ft gives 6.8 °C. Descending dry on the lee side warms at the DALR: 6.8 + 3 × 6 = 24.8 °C. The lee side ends 4.8 °C warmer than the windward side.

Q4Air at sea level on the windward side has a temperature of 27 °C and a dew point of 18 °C. It is forced over a ridge 13000 ft high. Use DALR 3 °C/1 000 ft, SALR 1.5 °C/1 000 ft and condensation level = spread × 400 ft. Find the temperature at the top of the ridge.

  1. 2.1 °C
  2. −2.4 °C
  3. 27.0 °C
  4. −12.0 °C
Show answer

Answer: A. 2.1 °C

Condensation level 3600 ft: temperature 16.2 °C. Cooling at the SALR to 13000 ft gives 2.1 °C. Descending dry on the lee side warms at the DALR: 2.1 + 3 × 13 = 41.1 °C. The lee side ends 14.1 °C warmer than the windward side.

Q5Air at sea level on the windward side has a temperature of 23 °C and a dew point of 17 °C. It is forced over a ridge 13000 ft high. Use DALR 3 °C/1 000 ft, SALR 1.5 °C/1 000 ft and condensation level = spread × 400 ft. Find the temperature at the top of the ridge.

  1. 23.0 °C
  2. −0.1 °C
  3. −16.0 °C
  4. −4.6 °C
Show answer

Answer: B. −0.1 °C

Condensation level 2400 ft: temperature 15.8 °C. Cooling at the SALR to 13000 ft gives −0.1 °C. Descending dry on the lee side warms at the DALR: −0.1 + 3 × 13 = 38.9 °C. The lee side ends 15.9 °C warmer than the windward side.

Q6The temperature is 22.0 °C at 2000 ft and 10.0 °C at 8000 ft. Taking DALR 3 °C and SALR 1.5 °C per 1 000 ft, the layer between is:

  1. neutrally stable (isothermal)
  2. absolutely stable
  3. conditionally unstable
  4. absolutely unstable
Show answer

Answer: C. conditionally unstable

ELR = (22.0 − 10.0) ÷ 6 = 2.0 °C per 1 000 ft. ELR 2.0 °C/1 000 ft lies between the SALR (1.5) and the DALR (3): stable for unsaturated air, unstable once saturated.

Q7The temperature is 20.0 °C at 2000 ft and 9.2 °C at 8000 ft. Taking DALR 3 °C and SALR 1.5 °C per 1 000 ft, the layer between is:

  1. absolutely stable
  2. neutrally stable (isothermal)
  3. conditionally unstable
  4. absolutely unstable
Show answer

Answer: C. conditionally unstable

ELR = (20.0 − 9.2) ÷ 6 = 1.8 °C per 1 000 ft. ELR 1.8 °C/1 000 ft lies between the SALR (1.5) and the DALR (3): stable for unsaturated air, unstable once saturated.

Q8The temperature is 18.0 °C at 2000 ft and 13.2 °C at 8000 ft. Taking DALR 3 °C and SALR 1.5 °C per 1 000 ft, the layer between is:

  1. absolutely unstable
  2. neutrally stable (isothermal)
  3. conditionally unstable
  4. absolutely stable
Show answer

Answer: D. absolutely stable

ELR = (18.0 − 13.2) ÷ 6 = 0.8 °C per 1 000 ft. ELR 0.8 °C/1 000 ft is less than the SALR (1.5), so parcels (dry or saturated) are always cooler than their surroundings after lifting.

Q9The temperature is 20.0 °C at 1000 ft and 4.0 °C at 5000 ft. Taking DALR 3 °C and SALR 1.5 °C per 1 000 ft, the layer between is:

  1. absolutely unstable
  2. conditionally unstable
  3. neutrally stable (isothermal)
  4. absolutely stable
Show answer

Answer: A. absolutely unstable

ELR = (20.0 − 4.0) ÷ 4 = 4.0 °C per 1 000 ft. ELR 4.0 °C/1 000 ft exceeds the DALR (3): a lifted parcel stays warmer than the surrounding air.

Q10Unstable air is typically associated with:

  1. stratus and drizzle
  2. cumuliform cloud, showers, good visibility and turbulence
  3. radiation fog
  4. smooth flying and haze
Show answer

Answer: B. cumuliform cloud, showers, good visibility and turbulence

Strong vertical motion gives cumulus and cumulonimbus with turbulence below and in cloud.

Q11Subsidence (sinking) of a layer of air tends to make it:

  1. more stable, with a warming and drying effect
  2. colder and wetter
  3. unchanged
  4. more unstable and moist
Show answer

Answer: A. more stable, with a warming and drying effect

Subsiding air warms at the DALR and humidity falls; a subsidence inversion forms.

Q12The foehn effect is caused by:

  1. cold air flowing downhill at night
  2. sea breeze convergence
  3. moist air rising on the windward side
  4. air descending the lee side of a mountain and warming at the dry adiabatic rate
Show answer

Answer: D. air descending the lee side of a mountain and warming at the dry adiabatic rate

Air loses moisture as precipitation on the windward side, and warms at the DALR on descent, so it is warmer and drier on the lee side.

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