Short notes 07 : Stability and Instability

Adiabatic Process

  • Definition: Temperature change of an air parcel due to expansion or compression, with no heat exchange with surroundings.
  • Rising air: Pressure ↓ → expansion → adiabatic cooling.
  • Descending air: Pressure ↑ → compression → adiabatic warming.
  • Key point: Cooling/heating is due to pressure change, not mixing.

Adiabatic Lapse Rates

  • DALR (Dry / Unsaturated air):
    • Constant
    • 1°C / 100 m ≈ 3°C / 1000 ft ≈ 10°C / km
    • Applies when RH < 100%.
  • SALR (Saturated air):
    • Variable, always less than DALR.
    • Average near surface: ~0.6°C / 100 m (≈5–6°C / km).
    • Slower due to latent heat release during condensation.
    • SALR → DALR at cold temperatures / high altitude (little moisture).

Environmental Lapse Rate (ELR)

  • Definition: Actual temperature change with height of the surrounding (still) air.
  • Highly variable, measured by radiosonde.
  • ISA ELR: 0.65°C / 100 m.

Atmospheric Stability (Key Logic)

Stability depends only on comparison between ELR, DALR, SALR.

Absolute Instability

  • Condition: ELR > DALR
  • Rising air (dry or saturated) stays warmer than environment.
  • Weather: Strong convection, CU/CB, showers, turbulence.
  • Often shallow near surface on hot days (superadiabatic).

Absolute Stability

  • Condition: ELR < SALR
  • Any rising air becomes colder & denser → sinks.
  • Weather: Stratiform clouds, fog, haze, smooth air.
  • Includes: Isothermal layers & inversions.

Conditional Instability

  • Condition: DALR > ELR > SALR
  • Dry (unsaturated): Stable.
  • Saturated: Unstable.
  • Result: CU/CB if lifting reaches saturation.
  • Note: Term is conditional instability (not “conditional stability”).

Neutral Stability

  • Dry air: ELR = DALR
  • Saturated air: ELR = SALR
  • Displaced parcel neither rises nor sinks.

Temperature Inversion

  • Definition: Temperature increases with height (ELR negative).
  • Stability: Extremely stable.
  • Effects:
    • Suppresses vertical motion.
    • Traps fog, haze, pollution.
    • Common in winter, clear, calm nights, high pressure.
  • Aviation hazard: Wind shear near top of inversion.

Isothermal Layer

  • Definition: Temperature constant with height (ELR = 0).
  • Stability: Absolutely stable.
  • Examples: Tropopause, parts of stratosphere.

Stability Changes (Cause → Effect)

  • Warming aloft → ELR decreases → stability increases.
  • Cooling aloft or surface heating → ELR steepens → instability increases.

Moist Air Effect

  • Rising moist air:
    • Condensation releases latent heat.
    • Slows cooling → SALR < DALR.
  • Descending saturated air:
    • Droplets evaporate → absorb heat.
    • Warming slower than dry air.

Cloud & Visibility Link

  • Unstable air:
    • Cumuliform clouds.
    • Generally good visibility, reduced only in showers.
  • Stable air:
    • Stratiform clouds, drizzle.
    • Poor visibility (fog, haze).

Exam One-Line Traps

  • Stability depends on lapse rates, not surface temperature alone.
  • Inversion = absolute stability.
  • ELR = DALR → neutral (dry).
  • ELR < SALR → absolute stability.
  • ELR > DALR → absolute instability.
  • Strong inversion → wind shear, not turbulence-free.
  • Unsaturated air returning to original level regains original temperature.
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