# Adiabatic processes

> Clear explanation of adiabatic processes, lapse rates (DALR and SALR), and atmospheric stability for aviation meteorology exams.

- **Subject:** 050 — Meteorology
- **Section:** 050-03 — Thermodynamics
- **Topic code:** 050-03-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/thermodynamics/adiabatic-processes/

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An adiabatic process is one in which **there is no heat transfer** to or from the system. The temperature change is due solely to the expansion or compression of the gas.

## Physical Mechanism

### 1. Adiabatic Expansion (Cooling)
*   When an air parcel **rises**, atmospheric pressure decreases.
*   The parcel **expands** (increases its volume).
*   To expand, the parcel performs work consuming its own internal energy.
*   Result: The temperature of the parcel **decreases**.

### 2. Adiabatic Compression (Warming)
*   When an air parcel **descends**, atmospheric pressure increases.
*   The parcel **compresses** (decreases its volume).
*   The environment performs work on the parcel.
*   Result: The temperature of the parcel **increases**.

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## Lapse Rates

![Adiabatic Lapse Rate](https://upload.wikimedia.org/wikipedia/commons/d/d4/Adiabatic_lapse_rate.svg)

### DALR (Dry Adiabatic Lapse Rate)
*   It is the cooling rate of **unsaturated** (dry) air when it rises.
*   **Constant Value:** $3^\circ C / 1000 \text{ ft}$ ($1^\circ C / 100 \text{ m}$).

### SALR (Saturated Adiabatic Lapse Rate)
*   It is the cooling rate of **saturated** air (inside a cloud) when it rises.
*   **Variable Value:** Average $\approx 1.8^\circ C / 1000 \text{ ft}$ ($0.6^\circ C / 100 \text{ m}$).
*   **Why is it lower than the DALR?** Because as water vapor condenses, it releases **latent heat**, which counteracts part of the cooling due to expansion.

### ELR (Environmental Lapse Rate)
*   It is the actual temperature profile of the atmosphere at a given time and place.
*   **ISA Value:** $2^\circ C / 1000 \text{ ft}$ ($0.65^\circ C / 100 \text{ m}$).
*   Determines the stability of the atmosphere.

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## Atmospheric Stability
Stability is determined by comparing the ELR with the DALR and SALR.

| Condition | Relationship | Description |
| :--- | :--- | :--- |
| **Absolute Instability** | **ELR > DALR** | The air is unstable whether dry or saturated. The parcel will always be warmer than the environment and will continue to rise. |
| **Conditional Instability** | **SALR < ELR < DALR** | The air is stable if dry, but unstable if saturated. This is the most common condition. |
| **Absolute Stability** | **ELR < SALR** | The air is stable whether dry or saturated. The parcel will always be colder than the environment and will tend to sink. (Includes isotherms and inversions). |
| **Neutral Stability** | **ELR = DALR** (dry) <br> **ELR = SALR** (saturated) | The parcel has the same temperature as the environment and tends neither to rise nor fall. |

### Air Mass Stability

*   **Warm Air Mass (warmer than the surface):**
    *   Cooled from below $\rightarrow$ Becomes **Stable**.
    *   Stratiform clouds (ST, SC), poor visibility, light turbulence, possible fog.
*   **Cold Air Mass (colder than the surface):**
    *   Heated from below $\rightarrow$ Becomes **Unstable**.
    *   Cumuliform clouds (CU, CB), showers, good visibility (except in showers), moderate/strong turbulence.

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## Thermodynamic Diagrams (Tephigram)


![Tephigram Diagram](https://upload.wikimedia.org/wikipedia/commons/4/48/Tephigram.png)

These are thermodynamic diagrams used to analyze stability and predict condensation levels.
*   If the parcel state curve (DALR/SALR) is to the **right** of the environmental temperature curve (ELR), the parcel is warmer $\rightarrow$ **Unstable** (rises).
*   If it is to the **left**, the parcel is colder $\rightarrow$ **Stable** (resists ascent).

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