Adiabatic processes

050-03-03Pablo Asensio Martínez2026-04-013 min

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.

Lapse Rates

Adiabatic Lapse Rate

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.

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)
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.

Thermodynamic Diagrams (Tephigram)

Tephigram Diagram

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).