Stability & Instability

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Mass movement (Paper 1)

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Stability & Instability

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

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