4.2 Adiabatic processes and potential temperature

Adiabatic processes are key to understanding atmospheric temperature changes. As air parcels move vertically, they expand or compress without exchanging heat, leading to cooling or warming. This affects cloud formation, precipitation, and overall atmospheric stability.

Dry and moist adiabatic processes differ based on air saturation. Dry processes follow a steeper lapse rate, while moist processes involve latent heat release. Understanding these differences is crucial for predicting weather patterns and atmospheric behavior.

Adiabatic Processes

Adiabatic processes in thermodynamics

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• Involve no heat exchange between a system (air parcel) and its surroundings
  • Occur when air parcels move vertically in the atmosphere with minimal heat exchange due to air's low thermal conductivity (insulating properties)
• Adiabatic cooling and warming significantly influence atmospheric temperature changes
  • Rising air parcels expand and cool adiabatically as pressure decreases (adiabatic cooling in updrafts)
  • Sinking air parcels compress and warm adiabatically as pressure increases (adiabatic warming in downdrafts)
• Crucial for understanding atmospheric stability, cloud formation (cumulus), and precipitation processes (rain, snow)

Potential temperature conservation

• Potential temperature ($\theta$) is the temperature an air parcel would have if brought adiabatically to a reference pressure level (usually 1000 hPa)
  • Defined as: $\theta = T \left(\frac{p_0}{p}\right)^{\frac{R}{c_p}}$
    • $T$ is the actual temperature (K)
    • $p_0$ is the reference pressure (1000 hPa)
    • $p$ is the actual pressure (hPa)
    • $R$ is the gas constant for dry air (287 J/kg/K)
    • $c_p$ is the specific heat capacity at constant pressure (1004 J/kg/K)
• Conserved during adiabatic processes, remaining constant as an air parcel moves vertically without heat exchange
• Useful for comparing temperatures of air parcels at different heights and identifying atmospheric stability (stable vs unstable layers)

Dry and Moist Adiabatic Processes

Dry vs moist adiabatic processes

• Dry adiabatic processes occur in unsaturated air (relative humidity < 100%)
  • Temperature changes follow the dry adiabatic lapse rate (DALR) of approximately -9.8℃/km
  • Rising unsaturated air cools at the DALR, sinking unsaturated air warms at the DALR
• Moist adiabatic processes occur in saturated air (relative humidity = 100%)
  • Temperature changes follow the moist adiabatic lapse rate (MALR), which varies with temperature and pressure
  • MALR is less than the DALR due to latent heat release during condensation (cloud formation)
  • Rising saturated air cools at the MALR, sinking saturated air warms at the MALR
• Transition between dry and moist adiabatic processes occurs at the lifted condensation level (LCL)
  • LCL is the height at which a rising air parcel becomes saturated and condensation begins (cloud base)

Atmospheric stability analysis

• Atmospheric stability depends on the environmental lapse rate compared to adiabatic lapse rates
  • Stable: Environmental lapse rate < MALR (vertical motion suppressed)
  • Conditionally unstable: MALR < Environmental lapse rate < DALR (instability depends on saturation)
  • Unstable: Environmental lapse rate > DALR (vertical motion enhanced)
• Potential temperature assesses atmospheric stability
  • Increasing $\theta$ with height indicates a stable atmosphere (inversion)
  • Decreasing $\theta$ with height indicates an unstable atmosphere (allows convection)
  • Constant $\theta$ with height indicates a neutrally stable atmosphere (allows vertical mixing)
• Analyzing atmospheric stability using adiabatic processes and potential temperature is essential for predicting convection (thunderstorms), turbulence (clear air), and other weather phenomena (fog, haze)