1. Assessing Atmospheric Stability… …Use of the Skew-T/log P diagram… (what does it all mean?)

2. What is stability? The atmosphere tends to resist vertical motion due to hydrostatic balance: dP/dZ = -ρg This is why horizontal motions are generally >> than vertical motions. Stability: The degree to which the atmosphere resists vertical motion.

5. Causes of vertical motion: Forced / mechanical: fronts lake breeze orography jet streams short waves Thermal: differential surface heating warm advection in lower levels and/or cold advection aloft cooling on top of moist layers.

6. Stable: Lifted parcel of air is negatively buoyant. Unstable: Lifted parcel of air becomes positively buoyant

9. Instability = Convection A convective cell:

10. pic of cu field

11. Adiabatic Process From the first law of thermodynamics: dE= dQ + dW, where dQ = 0… …Changes in the internal temperature of an air parcel (hypothetical “bubble” of air) are due entirely to expansion (rising…cooling) and compression (sinking…warming) Applying the hydrostatic equation to the gas law yields: γd = -g/Cp or 10 C ⁰ / Km, or 5.4 F ⁰/1K’ ISA standard lapse rate ≈ 3.6 F ⁰ / 1K’

12. Expansion/Compression example

13. Moist Ascent: pseudo-adiabatic process Adiabatic cooling by expansion is partially offset by release of latent heat of condensation: γ m = -g/C p + L v dq v /C p dZ The moist (or saturation) lapse rate varies depending on: Dew Point (how much water vapor is present) Buoyancy ( rate of condensation)

14. Saturation lapse rates range from about 1 F ⁰/1K’ in very warm, moist tropical air…and approach the dry adiabatic lapse rate in very cold, dry environments. (gives saturation adiabats a curve with altitude)

15. Evaluating Stability Uses parcel theory: Parcel of air: A hypothetical bubble of air, homogeneous in temperature and dew point. Treated as a discrete feature. Assumes no mixing with “outside” environmental air. T environment

16. Parcel can ascend either dry or pseudo adiabatically:

18. Lift the parcel to a desired altitude:

19. A parcel’s ascent is usually a combination of dry…then moist ascent:

20. Next, compare the temperature of the parcel to the temperature of the environment at the top of the ascent: In this example, the temperature of a parcel lifted from the surface to 500 mb would be 4 C ⁰ warmer than the temperature of the environment. It would be positively buoyant, it would be unstable…

21. Quantifying stability: Stability Indices Lifted Index (LI) >0 = stable; 0 = neutral; <0 unstable Showalter Index (SI) Total Totals Index (TT) K-Index (KI) SWEAT Index CAPE: Convective Available Potential Energy. (This “index” is most comprehensive since it integrates the amount of buoyancy over the entire vertical extent of the sounding) <1000 J/KG = weak instability 1000-2000 = moderate instability 2000-4000 = strong instability > 4000 = explosive

22. Two examples:

23. A couple more examples:

26. Values of stability indices:

27. Sources for sounding data: weather.uwyo.edu/upperair/sounding.html weather.cod.edu/analysis/analysis.sound.html www.crh.noaa.gov/lot/?n=firewx