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Atmospheric Stability
and the
Skew-T/Log-P Diagram
Adapted from Material Produced
At COMET for their Residence
Course 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 & Clouds
• Stability
– Parcel Theory
• Characteristics
– Expands and Contracts Freely
– Remains in as a single unit
– No heat exchange with the outside air (Adiabatic)
• Adiabatic Process
– Expands when lifted due to lessening pressure
» Molecular action slows (decreasing kinetic)
» Cools at a steady rate
– Compresses when forced down
» Molecular action increases (increasing kinetic)
» Warms at a steady rate
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Atmospheric Stability & Clouds
• Lapse Rate
– Dry Adiabatic Lapse Rate (DALR) – Unsaturated air
» 10°C per 1000 meters or 5.5°F per 1000 ft
– Moist Adiabatic Lapse Rate (MALR or WALR) – Saturated
» 6°C per 1000 meters or 3.3°F per 1000 ft
– Effects
» As a dry air parcel rises, it cools at the DALR until it
reaches its dewpoint and becomes saturated. If parcel
continues to rise, then it will cool at the MALR.
» As an air parcel sinks, it warms at the DALR due to
compressional heating. If the parcel is moist, then it will
warm at the MALR for a very short distance and then
warm at the DALR for the rest of the sinking moition.
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Atmospheric Stability & Clouds
• Stability Determinations
– Environmental Lapse Rate (ELR)
• Not constant – takes into account the lack of conduction
the higher you go up
• Obtained from a weather balloon sounding
– Absolutely stable
• Actual sounding temp profile is warmer than the
parcel’s temperature.
• Parcel is cooler than environment, so it sinks
• ELR < MALR…..Why?
– A higher lapse rate yields a cooler temp
– Causes
» Radiational Cooling, Cold Air Advection, Air Crossing a
Cold Surface, Sinking (warming) air at upper levels 13
Atmospheric Stability & Clouds
– Neutral Stability
• ELR is equal to the DALR or ELR = MALR
• Parcel temp the same as the environmental temp
– Absolutely Unstable
• Actual sounding temp cooler than the parcel temp
• Parcel is warmer than the environment around it so it
wants to keep rising
• ELR > DALR
– Conditional Stability
• When the environmental temp is between the dry parcel
temp and the moist parcel temp
• If parcel is unsaturated: then stable
• If parcel is saturated: then unstable
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Atmospheric Stability & Clouds
• Causes of Instability
–
–
–
–
–
Daytime heating
Warm air advection
Air moving over a warm surface (conduction)
Cold air advection aloft
Turbulent mixing of atmospheric layers
• Can produce an unstable layer
– Lifting a layer of air
• Convective lifting causes air to cool at the DALR
• Other Lifting mechanisms
– Fronts (cold air is more dense and forces warm air up)
– Mountains (orographic lifting)
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Atmospheric 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 & 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 17
Atmospheric 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
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Cape Canaveral, FL
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EL
CAPE
+
LI
LFC
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Cape Canaveral, FL
CIN
32
Brookhaven, NY
33
Albany, NY
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Birmingham, AL