Assessing Atmospheric Stability…

…Use of the Skew-T/log P diagram…(what does it all mean?)

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.

Causes of vertical motion:

Forced / mechanical:

fronts

lake breeze

orography

jet streams

short waves

Thermal:differential surface

heatingwarm advection in lower

levels and/or cold advection aloft

cooling on top of moist layers.

Stable: Lifted parcel of air is negatively buoyant.

Unstable: Lifted parcel of air becomes positively

buoyant

Instability = Convection

A convective cell:

pic of cu field

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’

Expansion/Compression example

Moist Ascent: pseudo-adiabatic process

Adiabatic cooling by expansion is partially offset

by release of latent heat of condensation:

γm = -g/Cp + Lv dqv/Cp dZ

The moist (or saturation) lapse rate varies depending on: Dew Point (how much water vapor is present) Buoyancy ( rate of condensation)

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)

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 TpTdp

Parcel can ascend either dry or pseudo adiabatically:

Lift the parcel to a desired altitude:

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

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 C⁰ warmer than the temperature of the environment. It would be positively buoyant, it would be

unstable…

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

Two examples:

A couple more examples:

Values of stability indices:

Sources for sounding data:

• weather.uwyo.edu/upperair/sounding.html• weather.cod.edu/analysis/analysis.sound.html• www.crh.noaa.gov/lot/?n=firewx