MET 07- Stability and Instability

An adiabatic process is defined as a thermodynamic change in a parcel of air where the temperature increases or decreases due to compression or expansion, respectively, but with no external exchange of heat with the surrounding environment.
• Rising Air: Air that rises expands due to decreasing atmospheric pressure, causing the temperature within the air parcel to decrease (adiabatic cooling). This happens without external heat exchange.
• Descending Air: Air that descends is compressed by increasing pressure, causing the temperature to increase (adiabatic warming) without external heat exchange.
⭐️ ⭐️ Key Data to Remember (FAA/ICAO Context):
• Process Definition: Temperature changes due to expansion or compression, but no heat is added to or removed from the air parcel by external sources.
• Dry Adiabatic Rate (DALR): 3
∘
C/1000 ft (or 1
∘
C/100 m) for unsaturated air.
• Saturated Adiabatic Rate (SALR): Slower than the DALR (average 1.5
∘
C/1000 ft or 0.6
∘
C/100 m) because latent heat is released during condensation, partially offsetting expansional cooling.

The stability of the atmosphere is determined by comparing the Environmental Lapse Rate (ELR)—the actual rate of temperature decrease with height—to the theoretical adiabatic lapse rates of rising air parcels.
• The Dry Adiabatic Lapse Rate (DALR) for unsaturated air is approximately 1
∘
C/100 m (or 3
∘
C/1000 ft).
• When the environmental lapse rate (ELR) is greater than the DALR (ELR > DALR), the atmosphere is classified as Absolutely Unstable.
• In this highly unstable condition, any rising air parcel, whether dry or saturated, cools more slowly than the surrounding air. The rising air remains warmer and less dense than its environment, causing it to continue accelerating upward spontaneously.
⭐️ ⭐️ Key Data to Remember (Absolute Instability):
• Condition: ELR > DALR.
• DALR Value: 1
∘
C/100 m.
• Weather: Favors strong vertical currents, resulting in cumuliform clouds, moderate to heavy showers, and turbulence.

A Temperature Inversion is the meteorological condition where the air temperature increases with increasing height in the atmosphere, which is the reverse of the normal temperature profile in the troposphere.
• Normal Profile (Lapse Rate): Normally, temperature decreases with height, and this decrease is defined as the lapse rate.
• Inversion Structure: An inversion is extremely stable because warm (less dense) air is situated above cool (more dense) air, acting like a lid on vertical air motions.
• Aviation Context: Inversions are common near the ground on clear, calm nights (radiation inversion), or aloft, such as a subsidence inversion in high-pressure systems. Inversions suppress convection and cloud formation.
⭐️ ⭐️ Key Data to Remember:
• Definition: Temperature increases with altitude.
• Stability: Extremely stable condition.
• Effect: Acts as a lid, suppressing vertical mixing, turbulence, and cloud formation.

Atmospheric stability is determined by the Environmental Lapse Rate (ELR). When warm air is advected (moves horizontally) into the upper part of an air layer, the air aloft warms.
When the air aloft warms, the rate at which temperature decreases with height (the lapse rate) decreases, or the temperature may even begin to increase with height (forming an inversion). This process, known as stabilization, makes the atmosphere more resistant to vertical movement, thereby increasing the stability within that layer.
⭐️ ⭐️ Key Data to Remember:
• Warming Aloft: Advection of warm air to upper levels increases stability.
• Lapse Rate Effect: This action decreases the ELR, potentially causing an inversion, which is an extremely stable condition.
• Opposite Effect (Instability): Stability decreases if the air aloft cools or the surface air warms.

Atmospheric stability for saturated air is determined by comparing the Environmental Lapse Rate (ELR) with the Saturated Adiabatic Lapse Rate (SALR).
• For saturated air to be unstable, a rising air parcel must remain warmer and lighter than its surrounding environment, causing it to continue accelerating upward spontaneously.
• This occurs when the air parcel cools more slowly than the ambient environment. Since the parcel cools at the SALR and the environment cools at the ELR, the condition for instability is when the ELR is greater than the SALR (ELR>SALR).
• Therefore, if SALR<ELR, the air is unstable when saturated, favoring strong vertical currents and the formation of cumuliform clouds.
⭐️ ⭐️ Key Data to Remember:
• Instability Condition: ELR>SALR (Environment cools faster than the saturated parcel).
• Result: The saturated air parcel remains warmer and continues to rise.
• Weather: Cumuliform clouds, showery precipitation, and turbulence.
• Note: If the air is stable when unsaturated but unstable when saturated (DALR>ELR>SALR), the condition is called Conditional Instability.

To determine atmospheric stability, we must first calculate the Environmental Lapse Rate (ELR) for the given air layer and compare it to the adiabatic lapse rates.
1. Calculate the Environmental Lapse Rate (ELR):
ELR=
Height Change
Surface Temperature−Temperature at 1000 m
​
=
1000 m
15
∘
C−13
∘
C
​
=0.2
∘
C/100 m
2. Identify Adiabatic Lapse Rates (Typical Values):
◦ Dry Adiabatic Lapse Rate (DALR): 1.0
∘
C/100 m (or 3
∘
C/1000 ft) for unsaturated air.
◦ Saturated Adiabatic Lapse Rate (SALR): Approximately 0.6
∘
C/100 m (or 1.8
∘
C/1000 ft) for saturated air.
3. Determine Stability Condition: The calculated ELR (0.2
∘
C/100 m) is less than the SALR (0.6
∘
C/100 m).
ELR<SALR
When the ELR is less than the SALR, the atmosphere is classified as Absolutely Stable. This means that any air parcel forced to rise, whether dry or saturated, will cool faster than the surrounding environment, become denser, and tend to sink back to its original position.
⭐️ ⭐️ Key Data to Remember:
• ELR: 0.2
∘
C/100 m.
• Stability Condition: ELR<SALR (Absolute Stability).
• Result: Stable air resists vertical motion.

Temperature inversions, particularly the low-level radiation inversion (also known as a ground or nocturnal inversion), are most common during the winter season.
This is due to favorable atmospheric conditions specific to winter:
• Long Nights: Provides an extended period for the ground surface to cool significantly through terrestrial radiation.
• Clear Skies and Calm Wind: Clear skies maximize the loss of outgoing infrared energy to space, and calm conditions prevent turbulent mixing of the cold surface air with warmer air aloft, allowing a strong inversion to develop.
• Association with High Pressure: Inversions are frequently found in regions of high pressure (anticyclones, ridges, or cols), where descending air (subsidence) results in clear skies and calm or light winds. The cooling effect can be particularly intense when the ground is snow-covered.
In the aviation context, these inversions are hazardous as they often trap moisture, leading to the formation of fog or low stratus, and are frequently associated with strong windshear.

This calculation relies on the principle of the Dry Adiabatic Lapse Rate (DALR), which dictates the rate at which unsaturated air cools when rising adiabatically (without exchanging heat with the surrounding environment).
1. Dry Adiabatic Lapse Rate (DALR): The DALR is a fixed value of 1
∘
C per 100 meters (or 10
∘
C per 1000 meters) for dry (unsaturated) air.
2. Altitude Conversion: 2 km is equal to 2000 meters.
3. Total Temperature Decrease (ΔT):
ΔT=10
∘
C/km×2 km=20
∘
C
4. Temperature at Altitude:
T
2 km
​
=T
surface
​
−ΔT=30
∘
C−20
∘
C=10
∘
C
The temperature at 2 km (or 2000 m) under DALR conditions is 10
∘
C.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• DALR: 1
∘
C/100 m or 3
∘
C/1000 ft.
• Adiabatic Process: Cooling occurs due to expansion as pressure decreases during ascent.

inversion – stable – sinking air – poor visibility because of FOG
correct answer – wind shear

when moist air will get heated —- water particle inside the air get evaporates and slow
downs the cooling process.

ISOTHERMAL layer – tropopause – stable
stratosphere – inversion – stable
troposphere – lapse rate – unstable

Unstable air is characterized by vertical air currents and the formation of cumuliform clouds (heap clouds). This environment generally promotes good visibility. However, the key restriction to visibility in unstable air comes from the precipitation type it produces: showers of rain, snow, or hail. Low stratus, haze, and drizzle are typically associated with stable air conditions.
⭐️ ⭐️ Key Data to Remember:
• Air Type: Unstable (convective) air.
• Associated Clouds: Cumuliform (CU, CB).
• Precipitation Type: Showers (sudden, varying intensity).
• Visibility: Generally good, restricted only within the showers themselves.

A lapse rate describes the rate at which temperature decreases with height. When referring to air that is “still,” or the temperature profile of the ambient environment, this rate is known as the Environmental Lapse Rate (ELR).
The ELR is highly variable, depending on location and time. It is measured by instruments like the radiosonde. Since 1.2
∘
C/100 m is not the fixed theoretical rate for dry adiabatic processes (DALR=1.0
∘
C/100 m) nor the average saturated rate (SALR≈0.6
∘
C/100 m), it must be a measurement of the actual environmental temperature change.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• ELR: The actual rate of temperature change with altitude in the stationary surrounding atmosphere. It is variable.
• DALR: Fixed rate of cooling/warming for unsaturated air (1.0
∘
C/100 m).
• SALR: Variable rate of cooling/warming for saturated air (average 0.6
∘
C/100 m).

The Dry Adiabatic Lapse Rate (DALR) is the rate of temperature change experienced by a rising or descending parcel of air that remains unsaturated (dry, relative humidity <100%).
During ascent, the air parcel expands due to decreasing ambient pressure, causing it to cool adiabatically. Since the air is unsaturated, there is no release of latent heat to offset this cooling.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Definition: Cooling (or warming) rate for unsaturated air undergoing an adiabatic process.
• Value: Constant rate of 3
∘
C/1000 ft (or 10
∘
C/1000 m).
• Contrast: This rate is faster than the Saturated Adiabatic Lapse Rate (SALR).

The relationship DALR>ELR>SALR defines Conditional Instability.
• Condition: In this state, the stability of the atmosphere is dependent upon whether the air parcel is saturated or unsaturated.
• Unsaturated Air (DALR): If an air parcel is lifted while unsaturated, it cools at the Dry Adiabatic Lapse Rate (DALR) (approx. 1.0
∘
C/100 m). Since DALR>ELR, the unsaturated parcel cools faster than the environment, becomes colder, and tends to sink, meaning the air is stable when unsaturated.
• Saturated Air (SALR): If the air parcel is lifted high enough to reach saturation (cloud formation), it then cools at the Saturated Adiabatic Lapse Rate (SALR) (average 0.6
∘
C/100 m). Since ELR>SALR, the saturated parcel cools more slowly than the environment, remains warmer, and continues to rise spontaneously, meaning the air is unstable when saturated.
⭐️ ⭐️ Key Data to Remember:
• Relationship: DALR>ELR>SALR.
• Definition: Stable when dry (unsaturated); unstable when wet (saturated).
• Resulting Weather: If lifting occurs and saturation is reached, vertical development of clouds (cumuliform) and showers is favored.
• Note: The term “conditional stability” is not a recognized meteorological term.

When an air mass descends, it encounters increasing atmospheric pressure, causing it to be compressed. This compression leads to adiabatic warming.
If the descending air is saturated and contains liquid water droplets, the warming process causes these droplets to evaporate.
• Evaporation is a cooling process that requires heat absorption (latent heat of vaporization).
• This absorption of latent heat counteracts the compressional warming.
• Consequently, saturated air warms at the Saturated Adiabatic Lapse Rate (SALR), which is slower than the Dry Adiabatic Lapse Rate (DALR) used for unsaturated air. The net result is that saturated air heats up less than dry air over the same descent distance.
⭐️ ⭐️ Key Data to Remember:
• Descending Air: Always warms (adiabatic process).
• Saturated Air Effect: Contains liquid droplets that evaporate.
• Mechanism: Evaporation absorbs latent heat, offsetting compressional heating.
• Rate: SALR (slower warming) < DALR (faster warming).

Dry air is considered Absolutely Unstable when the Environmental Lapse Rate (ELR) is greater than the Dry Adiabatic Lapse Rate (DALR).
1. Dry Air (Unsaturated): An air parcel classified as “dry” or unsaturated cools at the constant DALR (3
∘
C/1000 ft or 1.0
∘
C/100 m) when forced to rise adiabatically.
2. Condition for Instability: For the air to be unstable, the rising parcel must be warmer and less dense than the surrounding environment at every level, causing it to accelerate upward spontaneously.
3. Comparison: If ELR>DALR, the surrounding environment is cooling faster with altitude than the rising dry air parcel. The parcel remains warmer than its environment and continues to rise.
⭐️ ⭐️ Key Data to Remember (Absolute Instability):
• Condition: ELR>DALR.
• Physical Result: Rising air remains warmer and accelerates upward.
• Weather: Favors strong vertical currents and cumuliform clouds.
• Contrast: If ELR<DALR (or ELR<SALR), the air is absolutely stable.

The stability of the atmosphere—its resistance or favorability to vertical movement—is determined by comparing the behavior of a vertically displaced parcel of air (which changes temperature at known adiabatic rates) to the temperature profile of the surrounding atmosphere.
• Environmental Lapse Rate (ELR): The ELR represents the actual rate of temperature decrease (or increase) with height in the surrounding, non-moving environment. The ELR is highly variable and is measured by a radiosonde.
• Stability Test: Atmospheric stability is determined solely by comparing the ELR against the Dry Adiabatic Lapse Rate (DALR) and the Saturated Adiabatic Lapse Rate (SALR).
◦ If the ELR is steeper than the DALR (ELR>DALR), the air is absolutely unstable.
◦ If the ELR is less than the SALR (ELR<SALR), the air is absolutely stable.
◦ If DALR>ELR>SALR, the air is conditionally unstable.
Knowledge of the surface temperature or surface pressure alone is insufficient because stability depends entirely on the vertical temperature structure (the lapse rate) above the surface. The DALR is a fixed, theoretical rate (approximately 1
∘
C/100 m) for unsaturated air and is used as a constant reference point in stability calculations, not as the element determined to define stability.

The Saturated Adiabatic Lapse Rate (SALR) represents the cooling rate of a rising, saturated parcel of air. This rate is not constant but is generally much slower than the Dry Adiabatic Lapse Rate (DALR) (10
∘
C/Km) because the condensation of water vapor releases latent heat, which offsets the cooling due to expansion.
• Approximate Value: The SALR near mean sea level (where temperatures are higher and moisture content is greater) is significantly slower than DALR.
• Aviation Metric: The average SALR is approximately 1.5
∘
C/1000 ft, which converts closely to 5
∘
C/Km for exam purposes. (Other sources state 0.6
∘
C/100 m or 1.8
∘
C/1000 ft, which is 6
∘
C/Km).
⭐️ ⭐️ Key Data to Remember:
• DALR (Dry Air): 10
∘
C/Km (or 3
∘
C/1000 ft).
• SALR (Saturated Air): Highly variable, but significantly less than the DALR due to latent heat release. The rate is often simplified to approximately 5
∘
C to 6
∘
C per Km.

The stability of the atmosphere is determined by comparing the Environmental Lapse Rate (ELR), which is the actual temperature change of the stationary air mass with height, to the adiabatic lapse rates (DALR and SALR) of a moving air parcel.
1. Identify the Rates: The Dry Adiabatic Lapse Rate (DALR) for unsaturated air is a constant 1
∘
C/100 m (or 3
∘
C/1000 ft). The given ELR is 1
∘
C/100 m.
2. Determine Stability: When the ELR is exactly the same as the DALR (ELR=DALR), the layer exhibits Neutral Stability for unsaturated air.
3. Neutral Stability Definition (Dry Air): An unsaturated air parcel forced to rise will cool at the DALR (e.g., 1
∘
C/100 m). Since the surrounding environment is also cooling at 1
∘
C/100 m, the rising parcel maintains the same temperature and density as the surrounding air, and thus neither accelerates upward nor sinks back down, remaining at the level it was displaced to.
⭐️ ⭐️ Key Data to Remember:
• Condition: ELR=DALR.
• DALR Value: 1
∘
C/100 m.
• Definition: The air is neutrally stable when dry (unsaturated).

The Dry Adiabatic Lapse Rate (DALR) is defined as the rate at which unsaturated air cools when it rises adiabatically (or warms when it descends adiabatically).
• The DALR has a constant/fixed value of 1
∘
C/100 m (or 3
∘
C/1000 ft).
• The DALR is the same regardless of how close the air is to saturation and is independent of pressure and temperature.

A temperature inversion is the meteorological condition where the temperature increases with increasing altitude. Since the lapse rate (LR) is conventionally defined as the rate at which temperature decreases with height, a reversal of this trend (a temperature increase) means the Environmental Lapse Rate (ELR) is, in fact, negative.
• Standard Lapse Rate: Decrease of temperature with height (Positive LR).
• Inversion: Increase of temperature with height (Negative LR).
Inversions are highly relevant to aviation because they represent an extremely stable atmospheric condition, acting like a lid on vertical air motions, often trapping fog, haze, or pollutants below.

This condition is formally known as Conditional Instability [1, 28; 2, 129; 4, 370, 372; 5, 574; 6, 629].
Conditional instability exists when the Environmental Lapse Rate (ELR) is less than the Dry Adiabatic Lapse Rate (DALR) but greater than the Saturated Adiabatic Lapse Rate (SALR) (DALR>ELR>SALR) [1, 29; 2, 130; 4, 373; 6, 631].
• When unsaturated (dry): The air parcel cools faster than the environment (DALR>ELR), remaining colder and tending to sink back down. Thus, the layer is stable when unsaturated [1, 28; 2, 129].
• When saturated: The air parcel cools more slowly than the environment due to latent heat release (ELR>SALR), remaining warmer and continuing to rise. Thus, the layer is unstable when saturated [1, 28; 2, 129; 4, 372; 6, 629].
⭐️ ⭐️ Key Data to Remember:
• Condition: DALR>ELR>SALR.
• Definition: Stable when dry (unsaturated); unstable when wet (saturated) [2, 129; 6, 629].
• Note: The term “conditional stability” is not a recognized meteorological term.

In an unstable air mass, vertical currents favor the formation and growth of cumuliform clouds (heap clouds). The vertical extent of these clouds, such as cumulus or cumulonimbus, is primarily determined by the depth of the unstable layer.
• Mechanism: Rising, unstable air parcels continue to ascend as long as they remain warmer than the surrounding environment. Vertical development ceases when the rising air reaches a level where the ambient atmosphere becomes stable.
• Limiting Factor: The depth of instability is often capped by a stable layer, such as a temperature inversion or the tropopause (the ultimate cap for cumulonimbus clouds). If the conditionally unstable layer is “several miles deep,” the cloud may develop into a giant cumulonimbus.
⭐️ ⭐️ Key Data to Remember:
• Cloud Type: Cumuliform clouds form in unstable air.
• Vertical Limit: Determined by the top of the unstable layer/depth of convection.
• Cap: Often an inversion or the tropopause.

The Environmental Lapse Rate (ELR) is the actual temperature profile of the atmosphere, and it is highly variable. The Dry Adiabatic Lapse Rate (DALR), which describes the cooling rate of unsaturated air, is a fixed constant of 1
∘
C/100 m (or 3
∘
C/1000 ft).
The ELR can be greater than the DALR.
• Condition: When ELR>DALR, the atmosphere is in a state of Absolute Instability.
• Mechanism: In this condition, a rising air parcel cools more slowly than the surrounding air (which is lapsing rapidly). Because the parcel remains warmer and less dense than the environment, it accelerates upward spontaneously.
• Occurrence: This condition is usually confined to a shallow layer near the surface on hot, sunny days, where the lapse rate may be called superadiabatic.
⭐️ ⭐️ Key Data to Remember:
• DALR Value: 1
∘
C/100 m.
• ELR Variability: The ELR is variable and represents the ambient air temperature change.
• Result of ELR>DALR: Absolute Instability.

To determine stability, the Environmental Lapse Rate (ELR) must be compared to the adiabatic lapse rates.
1. Reference Rates:
◦ Dry Adiabatic Lapse Rate (DALR): 1.0
∘
C/100 m (or 3
∘
C/1000 ft) for unsaturated (dry) air.
◦ Saturated Adiabatic Lapse Rate (SALR): Approximately 0.6
∘
C/100 m (or 1.8
∘
C/1000 ft) for saturated air.
◦ Given ELR: 0.65
∘
C/100 m.
2. Comparison: The ELR (0.65
∘
C/100 m) falls between the DALR and the average SALR. This condition (DALR>ELR>SALR) defines Conditional Instability.
3. Stability Interpretation (Conditional Instability):
◦ When Dry (Unsaturated): Since ELR<DALR (0.65
∘
C/100 m<1.0
∘
C/100 m), a rising, unsaturated air parcel cools faster than the surrounding air, becomes denser, and sinks back to its original position. Therefore, the air is stable when dry.
◦ When Saturated: Since ELR>SALR, a rising, saturated air parcel cools slower than the surrounding air (due to latent heat release) and continues to rise, meaning the air is unstable when saturated.
⭐️ ⭐️ Key Data to Remember:
• ELR=0.65
∘
C/100 m is the International Standard Atmosphere (ISA) Lapse Rate.
• The relationship defines conditional instability: stable for dry air, unstable for saturated air.

Atmospheric stability refers to the atmosphere’s resistance to vertical motion. Stability is determined by the relationship between the Environmental Lapse Rate (ELR) and the adiabatic lapse rates (DALR or SALR).
1. Cooling Aloft: When cold air is advected (moved horizontally) into the upper part of an air layer, the air aloft cools.
2. Effect on ELR: When the upper air cools, the difference in temperature between the surface and the upper level increases, causing the ELR to steepen (increase).
3. Result: A steepening ELR means the temperature drops more rapidly with height. When the ELR approaches or exceeds the adiabatic rates, the atmosphere becomes more susceptible to spontaneous vertical movement. This condition is known as instability, which means stability decreases. This process is known as destabilization.
⭐️ ⭐️ Key Data to Remember:
• Cooling Aloft: Advection of cold air aloft promotes instability.
• Warming Below: Heating of the surface air also promotes instability.
• Result: Decreased stability favors strong vertical currents and cumuliform cloud development.

An isothermal layer is defined as a layer of the atmosphere where the temperature remains constant with increasing height. This means the Environmental Lapse Rate (ELR) is 0
∘
C per 100 m (zero lapse rate).
Atmospheric stability is determined by comparing the ELR to the Saturated Adiabatic Lapse Rate (SALR). The SALR is typically around 0.6
∘
C/100 m.
Since the ELR is zero, it is less than the SALR (ELR<SALR). This condition defines Absolute Stability.
In an absolutely stable layer, any air parcel (whether dry or saturated) forced to rise will immediately cool faster than the surrounding environment, become denser, and tend to sink back to its original level.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Isothermal Layer: ELR=0.
• Stability Condition: ELR<SALR.
• Result: Absolute Stability; convection is suppressed.
• Aviation Relevance: Inversions (where temperature increases, giving a negative ELR) and isothermal layers are extremely stable and act like a lid on vertical air motion.

While an inversion represents an extremely stable atmosphere that suppresses general convection, a strong low-level inversion, particularly a radiation inversion, creates a boundary layer that results in severe vertical wind shear.
• Mechanism: Strong radiation inversions develop during calm or light surface winds. However, the wind just above this inversion is decoupled from surface friction and can be relatively strong (sometimes forming a Low-Level Jet).
• Effect: The interface between the calm air below and the strong wind flow above is a zone of rapid change in wind speed and/or direction (wind shear). This shear generates eddies and turbulence.
• Aviation Hazard: An aircraft climbing or descending through this zone of sharp wind shear will experience turbulence, which can be moderate to severe, especially during takeoff or landing where airspeed margins are low.
⭐️ ⭐️ Key Data to Remember:
• Inversion = Extreme stability (suppresses vertical motion).
• Strong low-level inversion → Wind Shear → Turbulence.
• Inversions also trap smoke, haze, and fog, leading to poor visibility.

When the air climbs , water vapour gets condensed by cooling and releases some heat.
that heat slows the cooling rate.

The primary reason air cools as it rises is due to the phenomenon of adiabatic cooling.
1. Pressure Decrease: As a parcel of air moves upward in the atmosphere, it enters regions where the surrounding atmospheric pressure is lower.
2. Expansion: To equalize the pressure, the air parcel must expand.
3. Cooling: This expansion requires the air molecules to use their own internal kinetic energy to push the parcel walls outward. This internal energy loss results in slower molecular motion, which is measured as a decrease in temperature. The rising air cools due to expansion, not because it mixes with colder surrounding air or transfers heat externally.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Process: Adiabatic cooling (no heat exchange with the environment).
• Mechanism: Expansion of the air parcel due to decreasing ambient pressure.
• Rate (Unsaturated Air): The cooling rate is defined by the Dry Adiabatic Lapse Rate (DALR): 1
∘
C/100 m or 3
∘
C/1000 ft.

A temperature inversion occurs when the temperature of the atmosphere increases with increasing height. This means the Environmental Lapse Rate (ELR) is negative.
The atmosphere is defined as Absolutely Stable when the ELR is less than the Saturated Adiabatic Lapse Rate (SALR) (ELR<SALR). Since the ELR is zero or negative during an inversion, this condition is met.
• Result: Inversions represent an extremely stable atmospheric condition. Any air parcel (whether saturated or unsaturated) forced to rise will be immediately colder and denser than the surrounding environment, causing it to sink back to its original position.
• Aviation Context: Absolute stability inhibits vertical air motion and convection, often trapping fog, haze, and pollutants below the inversion layer.

A temperature Inversion is defined as a layer of the atmosphere where the temperature increases with increasing height. This condition results in a negative Environmental Lapse Rate (ELR).
Atmospheric stability is determined by comparing the ELR to the adiabatic rates. Since an inversion produces an ELR (negative or zero) that is less than the Saturated Adiabatic Lapse Rate (SALR), the air mass is considered Absolutely Stable. Stable air strongly resists upward or downward vertical displacement, acting as a lid on convection and vertical air motions.
⭐️ ⭐️ Key Data to Remember:
• Definition: Temperature increases with height (ELR is negative).
• Condition: ELR<SALR.
• Result: Absolute Stability, suppressing convection and vertical movement.

The stability of the atmosphere is determined by comparing the Environmental Lapse Rate (ELR) to the adiabatic lapse rates.
1. Reference Rates: The Dry Adiabatic Lapse Rate (DALR) for unsaturated air is a fixed constant, approximately 10
∘
C/km (or 1.0
∘
C/100 m). The value DALR=9.8
∘
C/km is a close representation of this constant.
2. Comparison (ELR vs. DALR): The given ELR is 6.8
∘
C/km. Since the ELR (6.8
∘
C/km) is less than the DALR (9.8
∘
C/km), the atmosphere is stable for unsaturated (dry) air. An unsaturated air parcel forced to rise cools faster than the environment, becomes denser, and tends to sink back to its original position.
This atmospheric state, where DALR>ELR but ELR is typically greater than the Saturated Adiabatic Lapse Rate (SALR≈6.0
∘
C/km), defines Conditional Instability. Since “conditional instability” is not an option, and the air resists vertical movement unless saturated, the general condition is typically described as stable (for the unsaturated state).
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Stable Condition (Unsaturated Air): ELR<DALR.
• Unstable Condition (Unsaturated Air): ELR>DALR (Absolute Instability).
• The relationship DALR>ELR>SALR (Conditional Instability) means the air is stable when dry but unstable when saturated.

Conditional instability occurs when the Environmental Lapse Rate (ELR) is less than the Dry Adiabatic Lapse Rate (DALR) but greater than the Saturated Adiabatic Lapse Rate (SALR).
This relationship is expressed as: DALR>ELR>SALR.
1. Reference Values:
◦ DALR (Unsaturated Air) ≈1.0
∘
C/100 m.
◦ SALR (Saturated Air) ≈0.6
∘
C/100 m (average near ground).
2. Evaluation of 0.65
∘
C/100 m:
◦ The value 0.65
∘
C/100 m is the standard lapse rate in the International Standard Atmosphere (ISA).
◦ Since 1.0
∘
C/100 m>0.65
∘
C/100 m>0.6
∘
C/100 m, this condition is met.
⭐️ ⭐️ Key Data to Remember (Conditional Instability):
• Dry State: Stable, because unsaturated air cools faster than the environment (DALR>ELR).
• Saturated State: Unstable, because saturated air cools slower than the environment (ELR>SALR).
• ISA ELR: 0.65
∘
C/100 m (or 1.98
∘
C/1000 ft) is generally indicative of conditional instability in the troposphere.
• 1
∘
C/100 m: This value, where ELR=DALR, represents Neutral Stability for dry air.

The condition ELR<SALR defines Absolute Stability.
• Definition: An atmosphere is absolutely stable when the Environmental Lapse Rate (ELR), the actual temperature change with height, is less than the Saturated Adiabatic Lapse Rate (SALR).
• Consequence: Under this condition, any air parcel that is forced to rise, whether it is unsaturated (cooling at the DALR) or saturated (cooling at the SALR), will cool faster than the surrounding environment.
• Result: The rising parcel remains colder and denser (heavier) than the ambient air at all levels. It therefore resists vertical movement and tends to sink back to its original position when the lifting force is removed.
• Aviation Context: Absolutely stable air strongly resists upward vertical motion, favoring the formation of stratiform (layer) clouds and promoting smooth flying conditions, except near mechanical lifting or inversions.

The rate at which saturated air cools, the Saturated Adiabatic Lapse Rate (SALR), is typically less than the Dry Adiabatic Lapse Rate (DALR) because condensation releases latent heat, offsetting the cooling due to expansion.
The magnitude of this latent heat release is directly dependent on the amount of water vapor present in the saturated air.
• At very cold temperatures (such as −40
∘
C), the total amount of water vapor the air can hold is extremely low.
• Consequently, when cooling causes condensation (or sublimation), very little latent heat is released.
• Because so little heat is released, the SALR is nearly as high as the DALR (approximately 1
∘
C/100 m or 3
∘
C/1000 ft). The SALR approaches the DALR at high altitude and high latitude, where temperatures are coldest.
⭐️ ⭐️ Key Data to Remember:
• SALR approaches DALR when air is very cold (low temperature/high altitude).
• Reason: Minimal water vapor content results in minimal latent heat release.
• Relationship: SALR is lowest (greatest difference from DALR) in warm, moist air.

Atmospheric stability is determined by comparing the Environmental Lapse Rate (ELR) to the adiabatic lapse rates (DALR and SALR).
The condition for Absolute Stability is met when the Environmental Lapse Rate (ELR) is less than the Saturated Adiabatic Lapse Rate (SALR).
In this scenario:
ELR(4.5
∘
C/km)<SALR(5.5
∘
C/km)
If the rising air parcel is saturated, it cools at the SALR (5.5
∘
C/km). Since the surrounding environment is cooling slower (4.5
∘
C/km), the parcel becomes colder and denser than the environment and tends to sink back to its original position.
In absolutely stable air, all vertical motion is suppressed, leading to layer (stratiform) clouds and generally smooth flying conditions.
⭐️ ⭐️ Key Data to Remember (Absolute Stability):
• Condition: ELR<SALR.
• Physical Result: Rising air parcels (both dry and saturated) cool faster than the environment and tend to sink.
• Weather: Stratiform clouds, fog, continuous light precipitation, and suppressed convection.

The Dry Adiabatic Lapse Rate (DALR) is the fixed, theoretical rate at which unsaturated (dry) air cools as it rises due to expansion, or warms as it descends due to compression. This process is adiabatic, meaning there is negligible heat exchange with the surrounding environment.
The value of the DALR is a constant:
• 1
∘
C per 100 m.
• 10
∘
C per 1000 m (or 1 km).
• 3
∘
C per 1000 ft.
⭐️ ⭐️ Key Data to Remember:
• DALR Nature: Fixed and constant, regardless of pressure or temperature.
• DALR Value: Approximately 10
∘
C/km.
• Aviation Relevance: The DALR is a key reference point used when comparing against the Environmental Lapse Rate (ELR) to determine atmospheric stability.

The air parcel is defined as “dry” (unsaturated) and is forced to rise adiabatically. Therefore, it cools at the constant Dry Adiabatic Lapse Rate (DALR).
The accepted value for the DALR is 1
∘
C per 100 m or 10
∘
C per 1000 m (1 km).
1. Initial Temperature: 35
∘
C
2. Ascent: 1 km
3. Cooling: 1 km×10
∘
C/km=10
∘
C
4. Final Temperature: 35
∘
C−10
∘
C=25
∘
C
⭐️ ⭐️ Key Data to Remember:
• Process: Rising unsaturated air cools adiabatically due to expansion.
• Rate: DALR=10
∘
C/Km (fixed value).

The process which fundamentally determines how the temperature of an air parcel changes in the vertical plane, and thus governs the stability characteristics of the atmosphere, is the Adiabatic process.
• Adiabatic Definition: An adiabatic process is one where the temperature of an air mass changes due to compression or expansion without any exchange of heat with the surrounding environment.
• Mechanism: As air rises, atmospheric pressure decreases, causing the air to expand and cool (adiabatic cooling). Conversely, downward-moving air is compressed by increasing pressure, causing it to warm (adiabatic heating).
• Lapse Rates: The rates of adiabatic temperature change (Dry Adiabatic Lapse Rate (DALR) and Saturated Adiabatic Lapse Rate (SALR)) are the fixed and variable constants, respectively, used to compare against the Environmental Lapse Rate (ELR) to determine atmospheric stability and the potential for vertical motion. Expansional cooling in upward-moving air is the major cause of cloud formation.
⭐️ ⭐️ Key Data to Remember:
• Adiabatic Cooling: Occurs due to expansion as air rises.
• Adiabatic Warming: Occurs due to compression as air sinks.
• Relevance: The comparison between the ELR (existing temperature distribution) and the adiabatic rates (DALR/SALR) determines the stability of the atmosphere.

When the Environmental Lapse Rate (ELR) is equal to either the Dry Adiabatic Lapse Rate (DALR) or the Saturated Adiabatic Lapse Rate (SALR), the atmosphere is in a state of Neutral Stability, also referred to as Indifferent Stability.
• Condition: ELR=DALR (for unsaturated/dry air) or ELR=SALR (for saturated air).
• Mechanism: An air parcel forced to rise cools at the exact same rate as the surrounding environmental air.
• Result: The lifted air parcel has the same temperature and density as the environment at the new level. Once the lifting force is removed, the parcel neither rises further nor sinks back; it remains where it was displaced.
⭐️ ⭐️ Key Data to Remember:
• ELR=DALR means neutral stability for dry air.
• ELR=SALR means neutral stability for saturated air.

The scenario describes an adiabatic process where an unsaturated (dry) air parcel is lifted and then returns to its original position. Adiabatic changes occur without the exchange of heat with the surrounding environment.
1. Ascent: As the air parcel is forced to rise, it moves into regions of lower pressure, causing it to expand and cool at the constant Dry Adiabatic Lapse Rate (DALR) (1
∘
C/100 m or 3
∘
C/1000 ft).
2. Descent: When the parcel settles back (subsides), it moves into regions of higher pressure, causing it to be compressed and warm at the same DALR.
Since the air remains unsaturated throughout the entire cycle (rising only “just under the condensation level”) and the upward displacement distance equals the downward displacement distance, the total adiabatic cooling during the rise is exactly canceled out by the total adiabatic warming during the descent. Therefore, the temperature of the air parcel returns precisely to its initial value.
⭐️ ⭐️ Key Data to Remember:
• Process: Adiabatic (no heat exchange).
• Rate: Unsaturated air cools/warms at the constant DALR.
• Result: If unsaturated air returns to its starting altitude, its temperature returns to its starting temperature.