Atmospheric Stability

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Atmospheric Stability
Stability of Atmosphere is affected by
 Temperature of the surrounding
environment
 Temperature of the air parcel
Environmental Lapse Rate (ELR)
Altitude
6oC/1,000m
0
0
Temperature
Isothermal Lapse Rate
Altitude
0
0
Temperature
Temperature Inversion
Altitude (km)
2
1
0
0
10Temperature 20
Conditions for Radiation Inversion
 At night
 Clear sky
 Calm condition
 Relatively dry air
 Snow cover
Radiation loss
Ground
Air drainage inversion
•In a valley
•At night
Cold air of
higher density
Cold air of
higher density
Warm air
Frontal Inversion
Warm air mass
INVERSION
Cold air mass
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
Absolutely Unstable Air
Altitude
DALR
SALR
ELR% > DALR > SALR
ELR
0
0
Temperature
Absolutely Unstable Air
Altitude
SALR
ELR% > DALR > SALR
ELR
Condensation level
DALR
0
0
Temperature
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
ELR
SALR
DALR> ELR > SALR
DALR
0
0
Temperature
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
ELR% < SALR < DALR
Altitude
SALR
ELR
Air parcel tends to return
to its original position
DALR
0
0
Temperature
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
ELR
Cloud top
SALR
Air parcel tends to rise
because of higher temperature
Condensation level
DALR
0
0
Temperature
Reference Website
www.piercecollege.com/offices/weather/s
tability.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
Urban Heat Island
22℃
24℃
26℃
28℃
 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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