Atmospheric Stability & Atmospheric Stability & InstabilityInstability

How does is relate to cloudHow does is relate to cloud

development and development and precipitation?precipitation?

Terms & Concepts to Terms & Concepts to KnowKnow

• Evaporation: Evaporation: liquid to gas – Cools the AIRliquid to gas – Cools the AIR

• Condensation: Condensation: gas to liquid – Warms the AIRgas to liquid – Warms the AIR

• Rising air: Rising air: Cools and expandsCools and expands

• Sinking Air:Sinking Air: Warms by compression Warms by compression

• Lapse RateLapse Rate: : Rate of temperature change forRate of temperature change for

a rising parcel of aira rising parcel of air

• Lapse rate for Unsaturated air is 5° /1,000’Lapse rate for Unsaturated air is 5° /1,000’

• Lapse rate for Saturated air is 3° /1,000’Lapse rate for Saturated air is 3° /1,000’

Adiabatic Temperature Adiabatic Temperature ChangesChanges

• An An adiabatic processadiabatic process is one in which is one in which no no

heat is exchangedheat is exchanged between the air between the air

parcel and the surrounding airparcel and the surrounding air

• As air As air Expands – it always coolsExpands – it always cools

• As air is As air is compressed, it always compressed, it always warmswarms

What actually happens withinWhat actually happens withina rising, closed a rising, closed ““air parcelair parcel””

When an air parcel When an air parcel RISES RISES , , the the Temperature Temperature decreases as it decreases as it expands expands

The rate of temperature The rate of temperature decline is 5 ° per 1,000’ decline is 5 ° per 1,000’

But when the air But when the air Temp=dewpoint temp Temp=dewpoint temp , , the rate of decline is only the rate of decline is only 3° /1,000’ 3° /1,000’ because as the because as the water vapor condenses water vapor condenses into droplets, it gives off into droplets, it gives off some heat energy which some heat energy which slows the rate of slows the rate of temperature decline temperature decline

RH: 70%

RH: 85%

RH: 100%

Adiabatic Temperature Adiabatic Temperature ChangesChanges

• Adiabatic cooling and condensationAdiabatic cooling and condensation::– Cooling occurs when air moves up and it Cooling occurs when air moves up and it

expands (and cools)expands (and cools)•Unsaturated air Unsaturated air cools at a rate of cools at a rate of 55°° F/1000’ F/1000’

– this is called the this is called the dry adiabatic ratedry adiabatic rate..

– Condensation is triggered Condensation is triggered when air rises when air rises high enough to become high enough to become saturated saturated (the (the dewpoint temperature) This is called the dewpoint temperature) This is called the Lifting Condensation Level (LCL)Lifting Condensation Level (LCL)•When air rises above the lifting condensation When air rises above the lifting condensation

level, the rate at which it cools is reduced. This level, the rate at which it cools is reduced. This slower rate of cooling is called the slower rate of cooling is called the wet adiabatic wet adiabatic raterate which is about which is about 33°° F/1000’ F/1000’

Stability and InstabilityStability and InstabilityThis refers to the temperature ‘profile’ This refers to the temperature ‘profile’

of the Atmosphere (NOT the Parcel!)of the Atmosphere (NOT the Parcel!)REMEMBER – as you MOVE UP from the surface, the temperature DECREASES in the Troposphere. The Temp. ‘profile’ can however, VARY from day to day! Some days it might only be 2°/1,000’, and other days, might be 6° or more per 1,000’ !

An air parcel that is An air parcel that is Warmer than its Warmer than its surroundings will always surroundings will always Rise…Rise…

As long as the air within As long as the air within the the rising parcel is rising parcel is warmer than its warmer than its surroundings, it will surroundings, it will continue to rise – just continue to rise – just like a hot air balloon. like a hot air balloon. BUT once the air within BUT once the air within the rising (and still the rising (and still cooling) cooling) air parcel gets air parcel gets colder than it colder than it surroundings, it will stop surroundings, it will stop rising, and will want to rising, and will want to sink!sink!

INTSTABILITY…INTSTABILITY…

Instability : Instability : Warm air at the surface Warm air at the surface and cold air and cold air above above allows air parcels that are warm at the allows air parcels that are warm at the

surface to rise…surface to rise…

Stable ConditionsStable ConditionsStability: Stability: Cool air at the surfaceCool air at the surface with with relativelyrelatively warm air aloft warm air aloft will STOP air will STOP air

parcels from rising! parcels from rising!

• InstabilityInstability Exists with warm air at the Exists with warm air at the surface and cold air aloft.surface and cold air aloft. This allows This allows air parcels that are warm at the surface air parcels that are warm at the surface to rise, cool, form clouds and eventually to rise, cool, form clouds and eventually precipitationprecipitation

• Stability:Stability: Exists with cool air at the Exists with cool air at the surface but surface but relativelyrelatively Warmer’ air Warmer’ air aloftaloft. This inhibits surface parcels from . This inhibits surface parcels from rising and can normally force any rising rising and can normally force any rising parcel to return to lower levels – or not parcel to return to lower levels – or not rise at all!rise at all!

IN GENERAL…IN GENERAL…

Adiabatic Temperature Adiabatic Temperature ChangesChanges EXAMPLES… EXAMPLES…

Adiabatic Temperature Adiabatic Temperature ChangesChanges• Adiabatic cooling and condensation:Adiabatic cooling and condensation:

– When air parcels move up, it expands and coolsWhen air parcels move up, it expands and cools

•UnsaturatedUnsaturated air cools at a rate of air cools at a rate of 55°° F/1000’F/1000’

– this is called the this is called the dry adiabatic ratedry adiabatic rate..

– CondensationCondensation is triggered is triggered when air rises high when air rises high enough to reach its enough to reach its saturation point (the saturation point (the dewpoint)dewpoint) and clouds form, this is called the and clouds form, this is called the lifting condensation level (or LCL)lifting condensation level (or LCL) •When air ascends above the lifting When air ascends above the lifting

condensation level, the rate at which it cools condensation level, the rate at which it cools is reduced since condensation warms the air is reduced since condensation warms the air slightly. slightly. The slower rate of cooling The slower rate of cooling is called is called the the wet adiabatic ratewet adiabatic rate (because the (because the air is air is saturatedsaturated), which is about ), which is about 33°° F/1000’ F/1000’

Rising Air Parcels that are Cooler than Rising Air Parcels that are Cooler than Environment Tends Toward StabilityEnvironment Tends Toward Stability

STABLE air does STABLE air does NOT allow warm air NOT allow warm air parcels to rise. parcels to rise. An air parcel that An air parcel that starts to rise (by starts to rise (by whatever mechanism) whatever mechanism) cools at the DRY cools at the DRY ADIABATIC rate that ADIABATIC rate that ends up colder than ends up colder than its surroundings will its surroundings will be forced back downbe forced back down

******

(5° F/ 1000’)(5° F/ 1000’)

WARM SURFACE – COLD AIR ALOFTWARM SURFACE – COLD AIR ALOFTAir Cools at Dry Adiabatic Rate until Reaching Dew Air Cools at Dry Adiabatic Rate until Reaching Dew

Point Point Then Cools at Wet Adiabatic RateThen Cools at Wet Adiabatic Rate

(3° F/ 1000’)

(5° F/ 1000’)(5° F/ 1000’)

Environment -4°C

Environment -12°C

******

Thunderstorm DevelopmentThunderstorm Development

Processes That Lift AirProcesses That Lift Air

• Orographic LiftingOrographic Lifting

• Frontal WedgingFrontal Wedging

• Surface ConvergenceSurface Convergence

• Differential Heating Differential Heating

..

Orographic LiftingOrographic Lifting

Orographic liftingOrographic lifting occurs when occurs when elevated terrains, elevated terrains, such as mountains, such as mountains, act as barriers to act as barriers to the flow of airthe flow of air

Adiabatic cooling can generate clouds and copious cooling can generate clouds and copious precipitation. Many of the precipitation. Many of the wettest places in the world wettest places in the world are located on windward mountain slopesare located on windward mountain slopes

When air reaches the leeward side, much of its When air reaches the leeward side, much of its moisture has been lost – and/or downslope winds moisture has been lost – and/or downslope winds warm and dry the air!warm and dry the air!

Localized Orographic Lifting – KAUAILocalized Orographic Lifting – KAUAI

Kauai has over 450” / year in some of the high Kauai has over 450” / year in some of the high mountains on Windward (East) side of the mountains on Windward (East) side of the mountain drops off to less than 10” on west mountain drops off to less than 10” on west side!side!

Winter Storms can often bring Winter Storms can often bring 5 to 10 5 to 10 feet of snow at a time feet of snow at a time to the Sierras, to the Sierras, while very little falls in Nevada!while very little falls in Nevada!

Strong Orographic Strong Orographic Lifting Lifting

during the Winter during the Winter SeasonSeason

SFO

Strong Orographic Strong Orographic Lifting Lifting

during the Winter during the Winter SeasonSeason

Death ValleyDeath Valley

Just as astonishing is how Temperaturesare forced to rise on the Leeward side of the southern Sierras east of Los Angeles. The hottest temperature in the US are typically found in Death Valley where less than 3” of rains during an entire year – some years there is less than 1”!

Frontal WedgingFrontal Wedging

Warm Front Warm Front

When masses of warm and cold air collide, When masses of warm and cold air collide, producing producing fronts, fronts, cooler, less denser air acts cooler, less denser air acts as a barrier over which the warmer, less as a barrier over which the warmer, less dense air risesdense air rises

This process is called This process is called frontal wedgingfrontal wedging..

Shallow gradual slope

Shallow gradual slope

Frontal WedgingFrontal Wedging

Cold Front Cold Front

Sharp st

eep slope

Sharp st

eep slope

Cold air pushing into a warm Air mass pushes against Cold air pushing into a warm Air mass pushes against the the

warm air - sometimes at a powerful, violent rate warm air - sometimes at a powerful, violent rate triggering triggering

strong Thunderstorms strong Thunderstorms

Uplifting by Uplifting by ConvergenceConvergence

Convergence Convergence over Floridaover Florida

Clouds, showers & Clouds, showers & thunderstorms thunderstorms forming inland as forming inland as afternoon sea breeze afternoon sea breeze from Atlantic and from Atlantic and Gulf of Mexico collide Gulf of Mexico collide over the interior of over the interior of the statethe state

MIAMIA

TPATPA

JAXJAX

FMYFMY

Uplifting by Uplifting by ConvergenceConvergence

MIAMIA

TPATPA

WarmWarm moist

Colliding air Colliding air forced upwardforced upward

Warm moistWarm moist

Converging horizontal air flow results in Converging horizontal air flow results in upward movement. Convergence can come in upward movement. Convergence can come in many other situations as well! many other situations as well!

Localized Convective Localized Convective Lifting Lifting Differential Differential

HeatingHeatingUnequal surface heating causes localized Unequal surface heating causes localized ‘pockets’ of air to rise‘pockets’ of air to rise

Differences in Terrain & type of ground cover & even cloud cover results in ‘bubbles’ of warmer air near the surface

These ‘bubbles’ of warm air – called thermals – begin to rise

The Critical Weathermaker: The Critical Weathermaker: Atmospheric Stabilit Atmospheric Stabilityy

• Stable air Stable air resists vertical movementresists vertical movement

• Unstable air Unstable air rises due to buoyancy rises due to buoyancy

• Environmental lapse rate Environmental lapse rate is the actual is the actual temperature of the ‘fee’ atmosphere and temperature of the ‘fee’ atmosphere and is measured at various heights in the is measured at various heights in the atmosphere (and it surrounds our ‘parcel’)atmosphere (and it surrounds our ‘parcel’)

• Very cold aloft with warm air at the Very cold aloft with warm air at the surface is VERY Unstable The bigger this surface is VERY Unstable The bigger this differential – the more unstable!differential – the more unstable!

Stability and Daily Stability and Daily WeatherWeather

• How stability changes:How stability changes:– Instability is enhanced Instability is enhanced by the following:by the following:

• Intense warming of the lowest layer of the Intense warming of the lowest layer of the atmosphere atmosphere

•Heating of an air mass from belowHeating of an air mass from below

•General General upward movement upward movement of the air caused of the air caused by by orographic lifting, orographic lifting, frontal wedgingfrontal wedging, and / , and / or convergenceor convergence

•Radiation cooling of the cloud tops Radiation cooling of the cloud tops (frequently occurs over the tropical ocean in (frequently occurs over the tropical ocean in the summer)the summer)

Stability and Daily Stability and Daily WeatherWeather

• How stability changes:How stability changes:– Stability is increased Stability is increased by the following:by the following:

•Radiation cooling of Earth’s Radiation cooling of Earth’s surfacesurface after sunset after sunset

•Cooling of an air mass from below as it traverses Cooling of an air mass from below as it traverses a cold surface (ex: air moving over colder water)a cold surface (ex: air moving over colder water)

•General subsidence (sinking) is a general, General subsidence (sinking) is a general, downward airflow from aloft and results in downward airflow from aloft and results in increased stability with clear, blue, cloudless increased stability with clear, blue, cloudless skiesskies

Stability and Daily Stability and Daily WeatherWeather

• Temperature changes and stability:Temperature changes and stability:– When otherwise very stable but somewhat When otherwise very stable but somewhat

moist air manages to cool by radiational moist air manages to cool by radiational cooling of the layer close to the ground, cooling of the layer close to the ground, widespread, ‘radiation fog’ often formswidespread, ‘radiation fog’ often forms

– In winter, air can become sufficiently In winter, air can become sufficiently unstable when cold, dry air passes over a unstable when cold, dry air passes over a warm, wet surface to produce showers. warm, wet surface to produce showers. For example, very cold air passing over For example, very cold air passing over the relatively warmer Great Lakes, can the relatively warmer Great Lakes, can often produce lake effect snow – especially often produce lake effect snow – especially in early winterin early winter