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Importance of Clouds
Release heat to atmosphere
Help regulate energy balance
Indicate physical processes
Atmospheric Stability
Clouds form as air rises and cools
Adiabatic processes: change in
temperature without giving or
removing energy
Dry rate = 10°C/1000m
Moist rate = 6°C/1000m
Stability is a state of equilibrium in
terms atmospheric movement; no
vertical movement occurs
Determining Stability
Warm air rises or is unstable
Cool air sinks or is stable
Compare air parcel lapse rate to
environmental lapse rate
Stepped Art
Fig. 6-2, p. 143
Adiabatic Processes
No net exchange of energy
• First Law of Thermodynamics
• Through the expansion of rising air
Dry adiabatic lapse rate
• -1oC/100m (-5.5oF/1000ft)
• Sinking parcels experience compression
warming
Saturated (wet) adiabatic lapse rate
• Occurs in saturated air
• ~-0.5oC/100m (-3.3oF/1000ft)
The Environmental Lapse Rate
Overall decrease in air temperature with height
Changes diurnally from place to place
• Air aloft is located farther from surface
heating
A comparison of
adiabatic and
environmental
cooling rates
Lapse rates
Change of temperature with height
Dry adiabatic (DALR)
Wet adiabatic (SALR)
Environmental (ELR)
Lifting condensation level (LCL)
Transition from dry to wet
Altitude at which saturation occurs
Buoyancy and Lapse Rates
Determining Stability
Stable environment
Environmental lapse rate less than moist
lapse rate
If an air parcel is forced it will spread
horizontally and form stratus clouds
Usually a cool surface (radiation,
advection)
Inversion: warm over cool.
Absolute stability
Stable mornings
Determining Stability
An Unstable Atmosphere
Environmental lapse rate greater than the
dry adiabatic lapse rate
As air parcel rises it forms a vertical cloud
Convection, thunderstorms, severe weather
Absolute instability
Determining Stability
A Conditionally Unstable Atmosphere
Moist adiabatic lapse rate is less than the
environmental lapse rate which is less than
the dry adiabatic lapse rate
Stable below cloud unstable above cloud
base
Atmosphere usually in this state
Conditional instability
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Unstable
afternoons
Extremely Stable Air
Inversions
• Radiation cooling
• Frontal interactions
• Subsidence
Profile of a frontal inversion
Determining Stability
Special Topic: Subsidence Inversions
High pressure system
Strong subsidence exacerbates air pollution
due to the lack of vertical motion.
Pollution is not diluted.
Subsidence inversion
Fig. 6-6, p. 145
Profile of a subsidence inversion
Subsidence inversion
Fig. 1, p. 150
Instability
Causes of Instability
Cool air aloft (advection, radiation cooling
in clouds)
Warming of surface (insolation, advection,
warm surface)
Factors Influencing the ELR
Heating/cooling the lower atmosphere
A diurnal profile of the ELR
Factors Influencing the ELR
Advection of Cold/Warm Air at Different Levels
Factors Influencing the ELR
Advection of an Air Mass with a Different ELR
Lifting Æ instability
Fig. 6-13, p. 149
Convective instability
Bottom wet
Top dry
Forced lifting
___________
Severe storm
Fig. 6-14, p. 149
Cloud Development
Clouds develop as an air parcel rises
and cools below the dew point.
Usually a trigger or process is need to
initiate the rise of an air parcel.
Thermal
Topographic
Convergence
Front
Cloud Development
Convection
Differential land surface heating creates
areas of high surface temperature
Air above warm land surface heats,
forming a ‘bubble’ of warm air that rises or
convection
Level of free convection: lifted air becomes
warmer than environment
Cloud base forms at lifting condensation
level
Stepped Art
Fig. 6-16, p. 152
Mixing cold dry air with warmer water
Fig. 6-21, p. 154
LCL
Fig. 2, p. 155
Limitations on the Lifting of Unstable Air
A layer of stable air
Entrainment
Ambient air intrusions into parcels
• Mixes cold air with warm air to decrease buoyancy
Limitations on the Lifting of Unstable Air
Cloud Development
Topography
Orographic uplift
• Orographic clouds
• Windward, leeward, rain shadow
Wave clouds
• Lenticular clouds
Orographic uplift
Orographic uplift
Orographic uplift (right)
and orographically
induced clouds
(below)
Wave clouds
Wave clouds
Lenticular (wave) clouds
Mechanisms that Lift Air
Convergence lifting
• Similar air masses come together
– Low pressure system
– Lee of mountain range (Seattle)
– Intertropical convergence zone
• Warm/moist air rises to form clouds
Mechanisms that Lift Air
Frontal lifting
• Boundaries between unlike air masses
• Warm/moist air rises to form clouds
• Cold (a) and warm (b) fronts occur