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


Stability of Atmosphere is affected by

  • Temperature of the surrounding environment

  • Temperature of the air parcel


Altitude

0

0

Temperature

Environmental Lapse Rate (ELR)

6oC/1,000m


Altitude

0

0

Temperature

Isothermal Lapse Rate


Altitude (km)

0

0

Temperature

Temperature Inversion

2

1

10

20


Conditions for Radiation Inversion

  • At night

  • Clear sky

  • Calm condition

  • Relatively dry air

  • Snow cover

Radiation loss

Ground


Cold air of higher density

Cold air of higher density

Warm air

Air drainage inversion

  • In a valley

  • At night


Warm air mass

INVERSION

Cold air mass

Frontal Inversion

Ground


Advection of warm air over a cold surface


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

  • DALR

  • Temperature change of dry air mass

  • 1oC/100 m


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


Altitude

DALR

SALR

ELR

0

0

Temperature

Absolutely Unstable Air

ELR% > DALR > SALR


Altitude

ELR

0

0

Temperature

Absolutely Unstable Air

SALR

ELR% > DALR > SALR

Condensation level

DALR


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


Altitude

SALR

DALR

0

0

Temperature

Conditionally Stable / Unstable Air

ELR

DALR> ELR > SALR


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


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

  • 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


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

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


Lapse Rate Exercise


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.


Dew point=5℃

DALR=10℃/1km

SALR=5℃

Condensation level


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 point

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


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 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 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

  • 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

  • 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


22℃

24℃

28℃

26℃

Urban Heat Island


  • 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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