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Stability & Instability

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Malia Damit
Malia Damit

This document discusses the factors that determine the stability or instability of the atmosphere. It explains that a stable atmosphere occurs when a rising air parcel cools more rapidly than the surrounding environment, causing it to sink back down. An unstable atmosphere happens when a rising parcel cools more slowly than the environment, allowing it to continue ascending. Specifically, stability depends on how the environmental lapse rate compares to the dry and saturated adiabatic lapse rates of a rising air parcel.

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Stability & Instability
Does forced uplift trigger continued uplift? Yes - unstable atmosphere No - stable atmosphere Does rising air cool more or less rapidly than environment it is rising through? Cooler parcel -> denser -> sinks back toward initial position Warmer parcel -> lighter -> continues rising
Parcel cooling rate depends only on whether parcel is saturated or not Unsaturated parcel cools at constant dry adiabatic lapse rate (DALR) - no condensation occurring Nearly constant at 1.0C / 100m Saturated parcel cools at slower saturated adiabatic lapse rate (SALR) condensation is occurring
Unsaturated parcel cools at constant dry adiabatic lapse rate (DALR)
Saturated parcel cools at slower saturated adiabatic lapse rate (SALR) condensation is occurring Condensation releases latent heat into atmosphere, reducing rate of cooling Typical value of about 0.6C / 100m
Saturated parcel cools at slower saturated adiabatic lapse rate (SALR)
Environmental lapse rate (ELR) Vertical change in temperature moving through a free atmosphere Highly variable, depends on many different factors
Adiabatic processes Adiabatic processes are reversible Adiabatic processes in atmosphere mainly related to vertical motion Vertical motion subjects air parcels to changes in pressure Rising air parcels expand due to reduced pressure and cool Sinking air parcels contract due to increased pressure and warm
Stability ELR < SALR Rising parcel always cools more rapidly than environment Becomes denser than the environment and sinks Environment becomes more stable when lower levels cooled or upper levels warmed
 
Instability Absolute instability: DALR < ELR Rising parcel always cools more slowly than environment Becomes less dense than environment and continues to rise
 
Conditional instability Conditional instability: SALR < ELR < DALR Unsaturated parcel cooling at DALR cools more rapidly than environment Saturated parcel cooling at SALR cools more slowly than environment Stability or instability conditional on whether parcel is saturated or not
 
Summary .                                                                                                                      

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Stability & Instability

  • 2. Does forced uplift trigger continued uplift? Yes - unstable atmosphere No - stable atmosphere Does rising air cool more or less rapidly than environment it is rising through? Cooler parcel -> denser -> sinks back toward initial position Warmer parcel -> lighter -> continues rising
  • 3. Parcel cooling rate depends only on whether parcel is saturated or not Unsaturated parcel cools at constant dry adiabatic lapse rate (DALR) - no condensation occurring Nearly constant at 1.0C / 100m Saturated parcel cools at slower saturated adiabatic lapse rate (SALR) condensation is occurring
  • 4. Unsaturated parcel cools at constant dry adiabatic lapse rate (DALR)
  • 5. Saturated parcel cools at slower saturated adiabatic lapse rate (SALR) condensation is occurring Condensation releases latent heat into atmosphere, reducing rate of cooling Typical value of about 0.6C / 100m
  • 6. Saturated parcel cools at slower saturated adiabatic lapse rate (SALR)
  • 7. Environmental lapse rate (ELR) Vertical change in temperature moving through a free atmosphere Highly variable, depends on many different factors
  • 8. Adiabatic processes Adiabatic processes are reversible Adiabatic processes in atmosphere mainly related to vertical motion Vertical motion subjects air parcels to changes in pressure Rising air parcels expand due to reduced pressure and cool Sinking air parcels contract due to increased pressure and warm
  • 9. Stability ELR < SALR Rising parcel always cools more rapidly than environment Becomes denser than the environment and sinks Environment becomes more stable when lower levels cooled or upper levels warmed
  • 10.  
  • 11. Instability Absolute instability: DALR < ELR Rising parcel always cools more slowly than environment Becomes less dense than environment and continues to rise
  • 12.  
  • 13. Conditional instability Conditional instability: SALR < ELR < DALR Unsaturated parcel cooling at DALR cools more rapidly than environment Saturated parcel cooling at SALR cools more slowly than environment Stability or instability conditional on whether parcel is saturated or not
  • 14.  
  • 15. Summary .