Atmospheric Stability, Lapse Rates & Adiabatic Processes — Summary Notes

1. Adiabatic Process

An adiabatic process is a temperature change in an air parcel caused by expansion or compression without external heat exchange.

Rising air

• Atmospheric pressure decreases with height.
• The air parcel expands.
• Expansion causes adiabatic cooling.
• No heat is exchanged with the surrounding atmosphere.

Descending air

• Atmospheric pressure increases.
• The parcel is compressed.
• Compression causes adiabatic warming.
• No external heat is added.

Memory:

Rising → Expansion → Cooling
Descending → Compression → Warming

2. Lapse Rates — The Three Rates You Must Know

Most important distinction

• ELR = actual atmosphere
• DALR = theoretical rate for dry/unsaturated parcel
• SALR = rate for saturated parcel

The ELR is variable and represents the actual temperature profile of the surrounding atmosphere.

3. Dry Adiabatic Lapse Rate — DALR

The DALR is the rate at which an unsaturated air parcel:

• Cools while rising.
• Warms while descending.

Value

DALR = 1°C/100 m

Equivalent:

• 10°C/km
• 3°C/1000 ft

The DALR is constant, regardless of the parcel’s pressure or temperature.

Why?

Unsaturated air has no condensation releasing latent heat, so its temperature changes at the fixed dry adiabatic rate.

4. Saturated Adiabatic Lapse Rate — SALR

The SALR applies to a saturated air parcel.

It is slower than DALR because condensation releases latent heat, which offsets some of the cooling caused by expansion.

Typical values in the source

Approximately:

• 0.6°C/100 m
• 1.5–1.8°C/1000 ft
• ≈5–6°C/km

Important

SALR is not constant.

It varies with:

• Temperature
• Moisture content

Warm, moist air

More water vapour → more latent heat released → SALR considerably lower than DALR.

Very cold air

At approximately −40°C, very little water vapour is present.

Therefore:

• Little condensation/sublimation
• Little latent heat release
• SALR approaches DALR

5. Why Saturated Air Cools More Slowly

When saturated air rises:

Expansion → cooling → condensation → latent heat release

The released latent heat partly offsets the cooling.

Therefore:

SALR < DALR

This is the fundamental reason saturated air cools more slowly than unsaturated air.

6. Saturated Air During Descent

When saturated air descends:

Compression → warming

But if liquid droplets are present:

Warming → evaporation → latent heat absorbed

Evaporation absorbs heat and therefore reduces the rate of warming.

Hence:

Saturated air warms at SALR

while:

Dry air warms at DALR

So:

SALR < DALR for both saturated cooling on ascent and saturated warming on descent.

7. Environmental Lapse Rate — ELR

The Environmental Lapse Rate (ELR) is the actual temperature change with height in the surrounding atmosphere.

Characteristics

• It is variable.
• It depends on the actual atmospheric temperature structure.
• It is measured using instruments such as a radiosonde.
• It is not a fixed theoretical value.

Stability

Atmospheric stability is determined by comparing:

ELR ↔ DALR ↔ SALR

Surface temperature or surface pressure alone cannot determine atmospheric stability.

8. The Master Stability Table

This is the most important table in the chapter.

One-line memory

ELR high → unstable
ELR low → stable

9. Absolute Instability

Condition

ELR > DALR

The environment is cooling with height faster than a rising dry parcel.

Therefore:

• Rising parcel cools relatively slowly.
• Parcel remains warmer than surroundings.
• Parcel remains less dense.
• It continues rising spontaneously.
• Vertical currents become strong.

Weather

Favors:

• Cumuliform clouds
• Strong convection
• Showers
• Turbulence
• Moderate/heavy precipitation

Typical occurrence

The source notes that this can occur in a shallow surface layer on hot, sunny days, sometimes called a superadiabatic layer.

10. Absolute Stability

Condition

ELR < SALR

This is the most stable condition.

A parcel forced upward:

• Cools faster than the environment.
• Becomes colder.
• Becomes denser.
• Tends to return/sink toward its original level.

Weather

Associated with:

• Stratiform clouds
• Fog
• Continuous/light precipitation
• Suppressed convection
• Generally smoother air

Memory

ELR < SALR = Absolutely Stable

11. Conditional Instability

Condition

DALR > ELR > SALR

This means the atmosphere behaves differently depending on whether the parcel is dry or saturated.

If parcel is dry

DALR > ELR

Therefore:

• Parcel cools faster.
• Parcel becomes colder/denser.
• Parcel tends to sink.
• Stable when dry.

If parcel becomes saturated

ELR > SALR

Therefore:

• Parcel cools more slowly.
• Parcel remains warmer.
• Parcel continues to rise.
• Unstable when saturated.

Memory

Conditional instability = Stable dry + Unstable saturated

12. Neutral Stability

Neutral stability occurs when the parcel and environment cool at exactly the same rate.

Dry air

ELR = DALR

Saturated air

ELR = SALR

The displaced parcel:

• Has the same temperature as its surroundings.
• Has the same density.
• Neither accelerates upward nor returns downward.

13. Temperature Inversion

An inversion occurs when:

Temperature increases with height.

This is opposite to the normal tropospheric temperature profile.

ELR during inversion

Because temperature increases upward:

ELR is negative.

Therefore:

ELR < SALR

and the atmosphere is absolutely stable.

14. Effects of an Inversion

An inversion acts like a lid.

It:

• Suppresses convection.
• Suppresses vertical mixing.
• Prevents vertical air movement.
• Traps fog.
• Traps haze/smoke/pollutants.
• Can produce poor visibility.

Aviation significance

A strong low-level inversion can also produce significant wind shear.

15. Radiation / Ground / Nocturnal Inversion

A low-level radiation inversion is particularly common during winter.

Why winter?

• Long nights → more time for ground cooling.
• Clear skies → greater radiational heat loss.
• Calm/light winds → less turbulent mixing.
• High-pressure conditions often favour clear skies and light winds.
• Snow-covered ground can intensify cooling.

Aviation hazards

Radiation inversions can trap moisture and produce:

• Fog
• Low stratus
• Poor visibility
• Strong wind shear

16. Inversion and Wind Shear

This is an important exam point.

A strong low-level inversion can separate:

Calm, friction-affected air near surface

from

Stronger wind above the inversion

This creates a rapid change in:

• Wind speed
• Wind direction

That is wind shear.

The resulting shear can generate:

• Eddies
• Turbulence
• Moderate to severe turbulence

This is particularly hazardous during:

• Takeoff
• Landing
• Climb
• Descent

Key association

Strong low-level inversion → Wind shear → Turbulence

17. Isothermal Layer

An isothermal layer is one in which temperature remains constant with height.

Therefore:

ELR = 0°C/100 m

Since:

ELR < SALR

the layer is absolutely stable.

Compare

18. Stabilization

Warming aloft

When warm air is advected into upper levels:

• Air aloft warms.
• ELR decreases.
• Stability increases.
• An inversion may develop.

This process is called stabilization.

Memory

Warm aloft → Stable

19. Destabilization

Cooling aloft

When cold air is advected into upper levels:

• Upper air cools.
• ELR becomes steeper.
• Temperature decreases more rapidly with height.
• Stability decreases.
• Vertical motion becomes easier.

This is called destabilization.

Surface warming also promotes instability.

Memory

Cold aloft → Unstable

20. Stable vs Unstable Air

21. Unstable Air and Clouds

Unstable air promotes vertical currents.

Therefore it favours:

• Cumulus (CU)
• Cumulonimbus (CB)
• Vertical cloud development
• Showers
• Turbulence

The vertical extent of a cumuliform cloud depends largely on the depth of the unstable layer.

A stable layer such as an inversion can cap the cloud.

The tropopause is the ultimate cap for cumulonimbus development.

22. Visibility in Stable and Unstable Air

Stable air

Poor visibility is common because stable air traps:

• Fog
• Haze
• Smoke
• Pollutants
• Moisture

Unstable air

Visibility is generally good, but can deteriorate inside:

• Rain showers
• Snow showers
• Hail showers

23. Important Numerical Example — ELR

Given:

• Surface temperature = 15°C
• Temperature at 1000 m = 13°C

Temperature difference:

15 − 13 = 2°C

Therefore:

ELR = 2°C / 1000 m

= 0.2°C/100 m

Compare:

• ELR = 0.2°C/100 m
• SALR ≈ 0.6°C/100 m

Therefore:

ELR < SALR

Answer:

Absolutely Stable

24. Numerical Example — DALR

Given:

• Surface temperature = 30°C
• Height = 2 km
• Dry air

DALR:

10°C/km

Temperature decrease:

2 × 10 = 20°C

Temperature at 2 km:

30 − 20 = 10°C

Answer:

10°C

25. Numerical Example — Dry Parcel

Given:

• Initial temperature = 35°C
• Ascent = 1 km
• Parcel remains unsaturated.

DALR:

10°C/km

Cooling:

1 × 10 = 10°C

Final temperature:

35 − 10 = 25°C

Answer:

25°C

26. ISA Lapse Rate

The source identifies:

ISA lapse rate = 0.65°C/100 m

Equivalent:

≈1.98°C/1000 ft

This lies between the typical DALR and SALR:

1.0 > 0.65 > 0.6

Therefore it represents conditional instability under the values used in the source.

27. Example — ELR = 0.65°C/100 m

Given:

• DALR = 1.0°C/100 m
• ELR = 0.65°C/100 m
• SALR ≈ 0.6°C/100 m

Relationship:

DALR > ELR > SALR

Therefore:

When dry

Stable

When saturated

Unstable

Overall

Conditional Instability

28. Example — ELR = 6.8°C/km

Given:

• ELR = 6.8°C/km
• DALR ≈ 9.8–10°C/km
• SALR ≈ 6°C/km

Therefore:

DALR > ELR > SALR

So the air is:

Stable when dry, unstable when saturated → Conditional Instability

If the question only asks for the state of an unsaturated parcel, describe it as stable.

29. Example — ELR = 4.5°C/km

Given:

• ELR = 4.5°C/km
• SALR = 5.5°C/km

Therefore:

ELR < SALR

Answer:

Absolute Stability

A saturated rising parcel cools faster than its environment and becomes colder/denser, so it tends to sink.

30. Parcel Returning to Original Level

For an unsaturated parcel:

Ascent

• Expands
• Cools at DALR

Descent

• Compresses
• Warms at DALR

If the parcel:

• Remains unsaturated
• Returns through the same vertical distance

then:

Cooling during ascent = warming during descent

Therefore:

The parcel returns to its original temperature.

31. Quick Stability Decision Method

Whenever an exam question gives an ELR:

Step 1 — Identify the ELR

Calculate:

ELR = temperature change / height change

Step 2 — Remember

DALR ≈ 1°C/100 m

SALR ≈ 0.6°C/100 m

Step 3 — Compare

If:

ELR > DALR

→ Absolutely Unstable

If:

ELR = DALR

→ Neutral, dry

If:

DALR > ELR > SALR

→ Conditionally Unstable

If:

ELR = SALR

→ Neutral, saturated

If:

ELR < SALR

→ Absolutely Stable

32. High-Yield Memory Table

33. Absolute vs Conditional vs Neutral — Final Comparison

This is the core table to memorize for exam questions.

34. Final Rapid Revision

Adiabatic

Rise → expand → cool

Sink → compress → warm

DALR

1°C/100 m = 10°C/km = 3°C/1000 ft

SALR

≈0.6°C/100 m = ≈5–6°C/km

SALR < DALR

Stability

ELR > DALR → Absolute instability

DALR > ELR > SALR → Conditional instability

ELR < SALR → Absolute stability

Neutral

ELR = DALR → dry neutral

ELR = SALR → saturated neutral

Inversion

Temperature increases with height

ELR is negative

Extremely stable

Fog/haze trapped

Strong low-level inversion → wind shear

Air movement

Warm aloft → stabilization

Cold aloft → destabilization

Surface heating → destabilization

Condensation → latent heat release → slower cooling

Evaporation → latent heat absorption → slower warming