Atmospheric stability
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Atmospheric Stability. Stability of Atmosphere is affected by. Temperature of the surrounding environment Temperature of the air parcel. Altitude. 0. 0. Temperature. Environmental Lapse Rate (ELR). 6 o C/1,000m. Altitude. 0. 0. Temperature. Isothermal Lapse Rate. Altitude (km). 0.

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Atmospheric Stability

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Atmospheric stability

Atmospheric Stability


Stability of atmosphere is affected by

Stability of Atmosphere is affected by

  • Temperature of the surrounding environment

  • Temperature of the air parcel


Environmental lapse rate elr

Altitude

0

0

Temperature

Environmental Lapse Rate (ELR)

6oC/1,000m


Isothermal lapse rate

Altitude

0

0

Temperature

Isothermal Lapse Rate


Temperature inversion

Altitude (km)

0

0

Temperature

Temperature Inversion

2

1

10

20


Conditions for radiation inversion

Conditions for Radiation Inversion

  • At night

  • Clear sky

  • Calm condition

  • Relatively dry air

  • Snow cover

Radiation loss

Ground


Air drainage inversion

Cold air of higher density

Cold air of higher density

Warm air

Air drainage inversion

  • In a valley

  • At night


Frontal inversion

Warm air mass

INVERSION

Cold air mass

Frontal Inversion

Ground


Advection of warm air over a cold surface

Advection of warm air over a cold surface


Adiabatic lapse rate

Adiabatic lapse rate

  • Adiabatic cooling

  • Decrease in temperature without heat exchange with surrounding air

  • When an air parcel is forced to rise,

  • It expands due to the lower air pressure of surrounding air

  • Increase of volume leads to decrease in temperature


Dry adiabatic lapse rate

Dry adiabatic lapse rate

  • DALR

  • Temperature change of dry air mass

  • 1oC/100 m


Saturated adiabatic lapse rate

Saturated adiabatic lapse rate

  • SALR - Temperature change of air mass when condensation occurs

  • 6oC/1,000 m in usual cases

  • SALR depends on

  • Moisture content & temperature

  • Higher moisture content leads to smaller lapse rate

  • SALR is smaller when temperature is lower


Absolutely unstable air

Altitude

DALR

SALR

ELR

0

0

Temperature

Absolutely Unstable Air

ELR% > DALR > SALR


Absolutely unstable air1

Altitude

ELR

0

0

Temperature

Absolutely Unstable Air

SALR

ELR% > DALR > SALR

Condensation level

DALR


Absolutely unstable air2

Absolutely Unstable Air

  • ELR% > DALR > SALR

  • Therefore, the rising air parcel is always hotter than the surrounding air

  • The density is lower

  • Further upwards displacement is resulted

  • For a sinking air parcel, further downwards displacement is resulted


Conditionally stable unstable air

Altitude

SALR

DALR

0

0

Temperature

Conditionally Stable / Unstable Air

ELR

DALR> ELR > SALR


Conditionally stable unstable air1

Conditionally Stable / Unstable Air

  • DALR > ELR

  • Means the air parcel is cooler than surrounding air

  • It will not rise if the external force discontinues

  • However, when condensation takes place

  • Latent heat is released

  • The air parcel cools more slowly : ELR>SALR

  • The air parcel will become warmer than surrounding air - Unstable


Absolutely stable air

Altitude

SALR

DALR

0

0

Temperature

Absolutely Stable Air

ELR% < SALR < DALR

ELR

Air parcel tends to return to its original position


Stability and cloud formation

Stability and Cloud formation

  • Instability leads to continuous rise of air parcel

  • Thick clouds such as cumulus and cumulonimbus will be formed

  • Stability will make rising air parcel go back to original position

  • Only thin clouds such as stratus


Atmospheric stability and cloud development

Altitude

Cloud top

SALR

Condensation level

DALR

0

0

Temperature

Atmospheric stability and cloud development

ELR

Air parcel tends to rise because of higher temperature


Reference website

Reference Website

www.piercecollege.com/offices/weather/stability.html


Lapse rate exercise

Lapse Rate Exercise


Describe the change of elr

Describe the change of ELR

  • Below 250m, temperature increases with height causing a temperature inversion

  • From 250m upward, temperature generally decreased with increasing altitude at a rate about 12.4℃/1,000m

  • The ELR below 1,750m is greater than above, i.e. the temperature decreases more rapidly from 250m to 1,750m and less rapidly above 1,750m.


Atmospheric stability

Dew point=5℃

DALR=10℃/1km

SALR=5℃

Condensation level


Dew point

Dew point

  • Dew-point is reached at 1,500m

  • Condensation process takes place with the changing of water vapour into water droplets.

  • The development of rain clouds with a thickness of 2,100m

  • Freezing process ocurs with formation of ice crystals if freezing temperature is reached.


Dew point1

Dew point

  • The coalescence of water droplets and ice crystals may induce precipitation


Stability of air

Stability of air

  • Below 1,700m, the rising air parcel is colder, and hence denser, than its surrounding air. There is a tendency for the rising air to sink back to ground level. The air parcel is said to be stable.

  • The temperature inversion reinforces the stable condition.


Stability of air1

Stability of air

  • As condensation process occurs at 1,500m with the release of latent heat, the rising air parcel cools less rapidly at the SALR.

  • From 1,700m upwards, the air parcel becomes warmer, and hence lighter, than its surrounding environment. It will become unstable and will continue to rise its own buoyancy


Stability of air2

Stability of air

  • As a general, the rising air parcel is said to be conditionally unstable since it is stable when it is unsaturated but becomes unstable when saturated.


Natural mechanism lead to uplift

Natural mechanism lead to uplift

  • Air movement encountering mountian barrier

  • Meeting of air masses with different properties along frontal surface

  • Convection uplift due to local heating

  • Convergence of air masses at a lower pressure zone


Uplift in a highly urbanized environment

Uplift in a highly urbanized environment

  • Urban heat island promotes convective turbulence and the upward movement of air

  • Surface roughness due to the presence of high buildings enhances vertical air motion


Atmospheric stability

22℃

24℃

28℃

26℃

Urban Heat Island


Atmospheric stability

  • Urban heat island encourages condensation and cloud formation

  • However, high pressure in surrounding rural areas leads to closed circulation

  • Pollutants in urban can hardly diffused

  • Pollutants together with fog and cloud leads to the formation of smog

  • Pollution induced temperature inversion create stable condition which hinders diffusion of pollutants


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