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Atmospheric Stabilityand the

Skew-T/Log-P Diagram

Adapted from Material ProducedAt COMET for their ResidenceCourse in Hydrometeorology

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Atmospheric Stability

• Stable versus Unstable

• Dry and Moist Adiabatic Processes

• Skew-T Log-P Diagrams

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Atmospheric Stability (cont.)

• Stable versus Unstable

Stable equilibrium

Unstable equilibrium

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Atmospheric Stability (cont.)

• Adiabatic Processes– Parcel of air expands and cools, or compresses and warms, with

no interchange of heat with the surrounding environment

– An adiabatic process is reversible

• If the parcel doesn’t saturate, then cooling or warming occurs at the dry adiabatic lapse rate

– Constant in our atmosphere 10 oC / km

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Atmospheric Stability (cont.)

• If the parcel does saturate and ascent is occurring...– Condensation (RH = 100%), Latent Heat is released

– Latent Heating offsets some of the cooling

– Cooling at slower rate: moist adiabatic lapse rate

– Not constant, varies with temperature and moisture

Average value ~ 6 oC / km

– Not reversible (heat added, moisture probably removed)

• Pseudo-adiabatic process

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Absolutely Stable

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Absolutely Unstable

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Conditionally Unstable

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Growth of a Thunderstorm

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Effects of Orography

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds• StabilityStability

– Parcel TheoryParcel Theory• CharacteristicsCharacteristics

– Expands and Contracts FreelyExpands and Contracts Freely

– Remains in as a single unitRemains in as a single unit

– No heat exchange with the outside air (Adiabatic)No heat exchange with the outside air (Adiabatic)

• Adiabatic ProcessAdiabatic Process– Expands when lifted due to lessening pressureExpands when lifted due to lessening pressure

» Molecular action slows (decreasing kinetic)Molecular action slows (decreasing kinetic)

» Cools at a steady rateCools at a steady rate

– Compresses when forced downCompresses when forced down

» Molecular action increases (increasing kinetic)Molecular action increases (increasing kinetic)

» Warms at a steady rateWarms at a steady rate

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds• Lapse RateLapse Rate

– Dry Adiabatic Lapse Rate (DALR) – Unsaturated airDry Adiabatic Lapse Rate (DALR) – Unsaturated air

» 10°C per 1000 meters or 10°C per 1000 meters or 5.55.5°F per 1000 ft°F per 1000 ft

– Moist Adiabatic Lapse Rate (MALR or WALR) – SaturatedMoist Adiabatic Lapse Rate (MALR or WALR) – Saturated

» 6°C per 1000 meters or 3.3°F per 1000 ft6°C per 1000 meters or 3.3°F per 1000 ft

– EffectsEffects

» As a dry air parcel rises, it cools at the DALR until it As a dry air parcel rises, it cools at the DALR until it reaches its dewpoint and becomes saturated. If parcel reaches its dewpoint and becomes saturated. If parcel continues to rise, then it will cool at the MALR.continues to rise, then it will cool at the MALR.

» As an air parcel sinks, it warms at the DALR due to As an air parcel sinks, it warms at the DALR due to compressional heating. If the parcel is moist, then it will compressional heating. If the parcel is moist, then it will warm at the MALR for a very short distance and then warm at the MALR for a very short distance and then warm at the DALR for the rest of the sinking moition.warm at the DALR for the rest of the sinking moition.

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds• Stability DeterminationsStability Determinations

– Environmental Lapse Rate (ELR)Environmental Lapse Rate (ELR)• Not constant – takes into account the lack of conduction Not constant – takes into account the lack of conduction

the higher you go upthe higher you go up• Obtained from a weather balloon soundingObtained from a weather balloon sounding

– Absolutely stableAbsolutely stable• Actual sounding temp profile is warmer than the Actual sounding temp profile is warmer than the

parcelparcel’’s temperature.s temperature.• Parcel is cooler than environment, so it sinksParcel is cooler than environment, so it sinks• ELR < MALR…..Why?ELR < MALR…..Why?

– A higher lapse rate yields a cooler temp A higher lapse rate yields a cooler temp – CausesCauses

» Radiational Cooling, Cold Air Advection, Air Crossing a Radiational Cooling, Cold Air Advection, Air Crossing a Cold Surface, Sinking (warming) air at upper levelsCold Surface, Sinking (warming) air at upper levels

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds– Neutral StabilityNeutral Stability

• ELR is equal to the DALR or ELR = MALRELR is equal to the DALR or ELR = MALR• Parcel temp the same as the environmental tempParcel temp the same as the environmental temp

– Absolutely UnstableAbsolutely Unstable• Actual sounding temp cooler than the parcel tempActual sounding temp cooler than the parcel temp• Parcel is warmer than the environment around it so it Parcel is warmer than the environment around it so it

wants to keep risingwants to keep rising• ELR > DALRELR > DALR

– Conditional Stability Conditional Stability • When the environmental temp is between the dry parcel When the environmental temp is between the dry parcel

temp and the moist parcel temptemp and the moist parcel temp• If parcel is unsaturated: then stableIf parcel is unsaturated: then stable• If parcel is saturated: then unstableIf parcel is saturated: then unstable

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds• Causes of InstabilityCauses of Instability

– Daytime heatingDaytime heating– Warm air advectionWarm air advection– Air moving over a warm surface (conduction)Air moving over a warm surface (conduction)– Cold air advection aloftCold air advection aloft– Turbulent mixing of atmospheric layersTurbulent mixing of atmospheric layers

• Can produce an unstable layerCan produce an unstable layer

– Lifting a layer of airLifting a layer of air• Convective lifting causes air to cool at the DALRConvective lifting causes air to cool at the DALR

• Other Lifting mechanismsOther Lifting mechanisms– Fronts (cold air is more dense and forces warm air up)Fronts (cold air is more dense and forces warm air up)

– Mountains (orographic lifting)Mountains (orographic lifting)

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds• Cloud Development

– Mechanisms• Surface Heating – Free Convection

• Topography

• Convergence of Surface Air

• Fronts

– Process• Ground heated by radiation

• Thermal Forms (Convective Updraft)– Air in contact with warmest ground is warmer and

therefore less dense than surrounding air

» Air rises…..it cools as it rises….it expands due to lessening pressure acting on it.

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds– Parcel of air reaches saturation at some height level

» Lifted Condensation Level (LCL)

» Cloud droplet formation

– Air cools enough so that it is cooler than the surrounding air…..air parcel sinks outside of the cloud

» Sets up a Convection Cell or Convective Current

• Assumptions– No Mixing between rising air (air parcel) and surroundings

(environment)

– One thermal updraft = only one cumulus cloud

– Cloud forms at 100% Relative Humidity (saturation)

– Rising air inside the cloud remains saturated

• Entrainment: Mixing of air originally outside of the cloud with saturated air inside the cloud

– If outside air is dry: droplets evaporate….cooling process

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Atmospheric Stability & CloudsAtmospheric Stability & Clouds– Lifting by Elevation: Topographic

• Orographic– Pronounced uplift on the windward side of a mountain

» Responsible for cloudy, wet conditions

– Sinking on the Leeward Side

» Produces a Rain Shadow effect

» Desert-like conditions

– Large-Scale Ascending Air• Usually associated with large cyclonic low pressure

storms

• Cold fronts force warm, moist air to rise over it

– Estimating Cumulus Cloud Heights• Use convective cloud height diagram (p. 177)

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Skew-T Log-P Diagram

• Convenient way to look at the vertical structure of the atmosphere

• Determine unreported meteorological quantities

• Assess parcel stability

• Used to display observations or model output

• Developed by the U.S. Air Force

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Skew-T Log-P Diagram (cont.)

• Basic Definitions

– mixing ratio (w) • mass of vapor to mass of dry air

– saturation mixing ratio (ws)

• maximum for a given T and P

– wet-bulb temperature (Tw)

• equilibrium T when water evaporates from a wetted-bulb thermometer at a rate

where latent heat lost is balanced by flow of heat from surrounding warmer air

– potential temperature ()

• temperature of air if brought dry-adiabatically to 1000 mb

– vapor pressure (e)

• partial pressure of water vapor

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Skew-T Log-P Diagram (cont.)

• Basic Definitions (cont.)

– virtual temperature (Tv) • temperature dry air at pressure P would have so its density equals that of a moist

parcel at T and P

– dew point temperature (Td)

• temperature of a parcel cooled to saturation at constant P

– relative humidity

• 100 x (mixing ratio / saturation mixing ratio)

– specific humidity (q)

• mass of vapor to mass of moist air (nearly the same as mixing ratio)

– equivalent temperature (Te)

• temperature air would have if all of its latent heat were released

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Skew-T Log-P Diagram (cont.)• Basic Definitions (cont.)

– equivalent potential temperature (e)

• temperature of a parcel if all moisture condensed out (latent heat released) then the parcel brought dry-adiabatically to 1000 mb

– Convective condensation level (CCL)

• Height where rising parcel just becomes saturated (condensation starts)

– Convective temperature (Tc)

• T that must be reached for a surface parcel to rise to CCL

– Lifting condensation level (LCL)

• Height where parcel becomes saturated by lifting dry-adiabatically

– Level of free convection (LFC)• Height where parcel lifted dry-adiabatically until saturated, then moist-adiabatically,

first becomes warmer than the surrounding air

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Skew-T Log-P Diagram (cont.)

• Basic Definitions (cont.)

– Positive area (or CAPE) • Area between the sounding and the moist adiabat that intersects the CCL, above the

CCL. Proportional to the amount of energy the parcel gains from the environment.

– Negative area (or CIN)

• Area between the sounding and the dry adiabat that intersects the CCL, below the CCL. Proportional to the energy needed to move the parcel.

– Equilibrium level (EL)

• Height where the temperature of a buoyant parcel again becomes equal to the

temperature of the environment.

– Wet bulb zero

• Height above ground where the wet bulb first reaches zero degrees Celsius. This is

the level where hail will begin to melt.

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Skew-T Diagram

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Skew-T Diagram

Isobars

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Skew-T Diagram

Isotherms

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Skew-T Diagram

Dry Adiabats

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Skew-T Diagram

Moist Adiabats

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Skew-T Diagram

Saturation Mixing Ratio

30Cape Canaveral, FL

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LFC

EL

+

LI

CIN

CAPE

Cape Canaveral, FL

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Brookhaven, NY

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Albany, NY

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Birmingham, AL