Conditional Instability Explained

Conditional instability in the atmosphere occurs when the air's stability depends on whether it is saturated.

Understanding Lapse Rates

To understand conditional instability, we need to know the different lapse rates:

• Environmental Lapse Rate ($ELR$): The actual rate at which the surrounding atmospheric temperature decreases with altitude.
• Dry Adiabatic Lapse Rate ($DALR$): The rate at which a *dry* (unsaturated) air parcel cools as it rises (approximately 9.8°C per 1000 meters).
• Saturated Adiabatic Lapse Rate ($SALR$): The rate at which a *saturated* air parcel cools as it rises. This rate is variable but always less than the $DALR$ (typically around 4-7°C per 1000 meters) because latent heat is released during condensation.

Conditions for Atmospheric Instability

The stability of the atmosphere depends on comparing these lapse rates:

• Absolute Instability: Occurs when $ELR$ > $DALR$. Rising air parcels are always warmer and less dense than their surroundings, regardless of saturation.
• Absolute Stability: Occurs when $ELR$ < $SALR$. Rising air parcels cool faster than the surrounding air, becoming colder and denser, causing them to sink back down.
• Conditional Instability: Occurs when $SALR$ < $ELR$ < $DALR$. In this scenario:
  • An unsaturated rising parcel cools at the $DALR$, becoming colder than the environment, making it stable.
  • A saturated rising parcel cools at the $SALR$, remaining warmer than the environment, making it unstable.

Conclusion on Conditional Instability

Therefore, conditional instability is observed when the environmental lapse rate is greater than the saturated adiabatic lapse rate but lower than the dry adiabatic lapse rate.