Stability & MovementStability & Movement

Figure 7.1Figure 7.1

A rock, like a parcel of air, that is in stable equilibrium will return A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed.to its original position when pushed.

If the rock instead departs in the direction of the push, it was in If the rock instead departs in the direction of the push, it was in unstable equilibrium.unstable equilibrium.

Movement & TemperatureMovement & TemperatureRising air Rising air expands, using expands, using energy to push out, energy to push out, which slows and which slows and adiabatically cools adiabatically cools the air.the air.

A parcel of air A parcel of air may be forced to may be forced to rise or sink, and rise or sink, and change change temperature temperature relative to relative to environmental air, environmental air, which is sampled which is sampled using radiosonde using radiosonde balloons.balloons.

Figure 7.2Figure 7.2

Normal Atmospheric Conditions…

Temperature Inversion… warmer air slides over the cool air at the surface trapping the surface air beneath a very stable upper air mass

Dry & Moist Adiabatic RatesDry & Moist Adiabatic Rates

Figure 7.3AFigure 7.3A Figure 7.3BFigure 7.3B

A parcel of rising dry air cools at approximately 10° C/km, while A parcel of rising dry air cools at approximately 10° C/km, while latent heat released into the parcel by condensing vapor keeps the latent heat released into the parcel by condensing vapor keeps the moist adiabatic lapse rate nearer to 6° C/km.moist adiabatic lapse rate nearer to 6° C/km.Because the environmental air cools more slowly, the parcel of air Because the environmental air cools more slowly, the parcel of air held by the helicopter is always cooler and sinks back to its held by the helicopter is always cooler and sinks back to its original position.original position.

Stable Atmospheric ConditionsStable Atmospheric Conditions

Cold air below warm air is considered stable because cold air has a Cold air below warm air is considered stable because cold air has a higher density, and when displaced will gravitate back below the higher density, and when displaced will gravitate back below the warm, lighter, air.warm, lighter, air.

A morning atmosphere has this stable vertical profile, keeping fog A morning atmosphere has this stable vertical profile, keeping fog unmixed.unmixed.

Figure 7.4Figure 7.4

Subsidence InversionSubsidence Inversion

Figure 7.5Figure 7.5

Cooler surface air is called a temperature inversion, caused by Cooler surface air is called a temperature inversion, caused by radiational cooling, cold advection, or subsiding air that shrinks in radiational cooling, cold advection, or subsiding air that shrinks in total thickness.total thickness.

As it sinks and shrinks, its top lowers a greater distance and warms As it sinks and shrinks, its top lowers a greater distance and warms more than the air resting at earth's surface.more than the air resting at earth's surface.

The Skew-T

Plot

http://www.rap.ucar.edu/weather/upper/jan.gif

The Environmental Lapse Rate

• The values in the text are average conditions• The dry adiabatic lapse rate averages 10C/1000

meters• The wet adiabatic lapse rate averages 6C/1000

meters• The actual rate of change in the atmosphere is

the ENVIRONMENTAL lapse rate • Radiosonde data is available at• http://weather.uwyo.edu/upperair/sounding.html• http://weather.unisys.com/upper_air/skew/index.

html• http://www.stormchase.us/Upper.html

Absolutely Unstable AtmosphereAbsolutely Unstable Atmosphere

Figure 7.6AFigure 7.6A

Absolutely unstable Absolutely unstable conditions indicate that a conditions indicate that a lifted parcel of air, lifted parcel of air, whether dry or moist, whether dry or moist, will be warmer than the will be warmer than the surrounding surrounding environmental air, and environmental air, and hence continue to rise.hence continue to rise.

Conditionally UnstableConditionally Unstable

Conditions are initially stable Conditions are initially stable when the environmental lapse when the environmental lapse rate (LR) is between the dry rate (LR) is between the dry and moist adiabatic LR.and moist adiabatic LR.

Air that is dry is always cooler Air that is dry is always cooler and sinks, but when the and sinks, but when the parcel's dew point is reached parcel's dew point is reached it cools more slowly, and may it cools more slowly, and may become warmer than the become warmer than the surrounding air, creating surrounding air, creating instability.instability.

Figure 7.7AFigure 7.7A

Stabililty SummaryStabililty Summary

Environmental Environmental temperatures determine temperatures determine stability for rising parcels stability for rising parcels of air.of air.

The atmosphere is The atmosphere is absolutely unstable when absolutely unstable when the environmental lapse the environmental lapse rate (ELR) is steeper than rate (ELR) is steeper than the dry LR, absolutely the dry LR, absolutely stable when the ELR is less stable when the ELR is less steep than the moist LR, steep than the moist LR, and conditionally unstable and conditionally unstable otherwise.otherwise.

Figure 7.8Figure 7.8

Heat as Instability TriggerHeat as Instability Trigger

Figure 7.9Figure 7.9

As the environmental lapse rate As the environmental lapse rate (ELR) steepens, it becomes (ELR) steepens, it becomes more unstable.more unstable.

Heating air below, through Heating air below, through contact with a hot surface or contact with a hot surface or fire, will cause the ELR to fire, will cause the ELR to steepen.steepen.

Cooling air aloft by clouds Cooling air aloft by clouds radiating energy or cold radiating energy or cold advection will also increase advection will also increase instability.instability.

Mixing as Instability TriggerMixing as Instability Trigger

Winds that mix Winds that mix upper and lower upper and lower atmospheric air atmospheric air will steepen the will steepen the environmental environmental lapse rate.lapse rate.

Wind forced Wind forced convection will convection will warm descending warm descending and lower air and lower air while cooling while cooling rising and upper rising and upper air.air.

Figure 7.10Figure 7.10

Layer of Air InstabilityLayer of Air Instability

Figure 7.11Figure 7.11

Lifting a stable layer of air causes it to stretch out vertically, with Lifting a stable layer of air causes it to stretch out vertically, with the top section experiencing a greater elevation change and more the top section experiencing a greater elevation change and more cooling than the lower section, even when both cool at the dry cooling than the lower section, even when both cool at the dry adiabatic rate.adiabatic rate.

Convective Instability & StormsConvective Instability & Storms

Figure 7.12Figure 7.12

Lifting a stable layer of Lifting a stable layer of air that is saturated air that is saturated below and unsaturated below and unsaturated aloft will cause the aloft will cause the moist air to cool more moist air to cool more slowly, and create a slowly, and create a very steep and unstable very steep and unstable environmental lapse environmental lapse rate.rate.

Severe thunderstorms Severe thunderstorms are associated with such are associated with such processes.processes.

Cloud Development TriggersCloud Development Triggers

Figure 7.13AFigure 7.13A

Convection & CloudsConvection & CloudsPockets of warm Pockets of warm air rise as air rise as thermals with thermals with invisible water invisible water vapor, and at the vapor, and at the dew point dew point temperature temperature condensation condensation creates the cloud creates the cloud base.base.

Rising air from Rising air from below is replaced below is replaced by sinking air by sinking air from above, from above, creating areas of creating areas of blue sky.blue sky.

Figure 7.14Figure 7.14

Cumulus Clouds & Clear SkyCumulus Clouds & Clear Sky

Figure 7.15Figure 7.15

Development of Cumulus CloudDevelopment of Cumulus Cloud

The panels reveal vertical trends for temperature and moisture The panels reveal vertical trends for temperature and moisture during cloud development.during cloud development.

The level of free convection is where rising air becomes warmer The level of free convection is where rising air becomes warmer than environmental air.than environmental air.

Figure 7.16Figure 7.16

Stability & Cloud HeightStability & Cloud Height

Figure 7.17AFigure 7.17A

Environmental Environmental lapse rates (ELR) lapse rates (ELR) determine the determine the stability, and as the stability, and as the time of day time of day changes the ELR, changes the ELR, the cloud profile the cloud profile can change.can change.

Cumulus to CumulonimbusCumulus to Cumulonimbus

Figure 7.18Figure 7.18

Topography & CloudsTopography & Clouds

Winds blowing moist air toward a mountain will experience Winds blowing moist air toward a mountain will experience orographic uplift to an elevation where dew point is reached and orographic uplift to an elevation where dew point is reached and clouds are formed.clouds are formed.

When the condensed moisture falls as rainfall, the leeward side of When the condensed moisture falls as rainfall, the leeward side of the mountain is kept in a rain shadow.the mountain is kept in a rain shadow.

Figure 7.19Figure 7.19

Mountain Wave CloudsMountain Wave Clouds

Figure 7.20Figure 7.20

As moist air condenses into clouds on the windward side of the As moist air condenses into clouds on the windward side of the mountain, sinking and swirling winds on the leeward side shape mountain, sinking and swirling winds on the leeward side shape a lenticular mountain wave cloud.a lenticular mountain wave cloud.

Figure 7.21Figure 7.21

Changing Cloud FormsChanging Cloud Forms

Figure 7.22Figure 7.22

Uniform winds may Uniform winds may align a large region align a large region of cirrocumulus and of cirrocumulus and stratocumulus clouds stratocumulus clouds into rows of cloud into rows of cloud streets.streets.

Billow & Castellanus CloudsBillow & Castellanus Clouds

Figure 7.23Figure 7.23

Rapid changes in wind speed with elevation can create Rapid changes in wind speed with elevation can create wavelike billows at the cloud top, while conditionally unstable wavelike billows at the cloud top, while conditionally unstable air above a cloud may cause lifting and condensation in the air above a cloud may cause lifting and condensation in the shape of castles.shape of castles.

Figure 7.24Figure 7.24

Mixing & StratocumulusMixing & Stratocumulus

When air is stable and close to saturation, then forced wind mixing When air is stable and close to saturation, then forced wind mixing of the atmosphere:of the atmosphere:a) steepens the environmental lapse ratea) steepens the environmental lapse rateb) moves moist low-level air aloftb) moves moist low-level air aloft The cooler and moister air aloft then condenses into a deck of The cooler and moister air aloft then condenses into a deck of clouds.clouds.

Figure 7.25Figure 7.25