Condensation

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Condensation Clouds

• Condensation Processes
• Stability Conditions
• ELR Variations and Stability
• Climatic Significance of Stability
• Chapters 4.3 4.5

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Condensation Processes (3)
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• Cooling with vertical motions (convection)
• Forced (mechanical) convection
• Orographic uplift (stable air)
• Frontal uplift (stable air)
• Convergence (conditionally unstable)
• Free (thermal) convection
• Heating of surface by sun (unstable)
• Cold air over a warm surface (unstable)

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Orographic Uplift (Stable)
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• Orographic lifting occurs when elevated terrain
  acts as barriers to the flow of air
• Therefore, air ascends a mountain slope and cools
  adiabatically, often generating clouds and
  precipitation
• As air descends (subsidence) on the other side of
  the mountain it warms adiabatically, suppressing
  cloud formation and resulting in warmer
  temperatures (Rainshadow Effect)

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Orographic Uplift
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Not in Book
5
Orographic Uplift
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Not in Book
6
Frontal Uplift (stable)
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• Frontal uplifting occurs when two air masses come
  into contact
• The warmer, less dense, and moist air overrides
  the colder, more dense air
• The warmer air is forced to ascend and clouds are
  often created

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Frontal Uplift
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Not in Book
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Convergence (conditionally unstable)
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• Convergence occurs whenever air flows together
• Results in a general upward movement of the air
  and then condensation

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Convergence
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Not in Book
Converging Winds and Forceful Lifting
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Free (thermal) Convection (Unstable)
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• Thermal convection occurs when air rises because
  it is warmer, and therefore less dense, than the
  surrounding air

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Free Convection
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12
Free Convection
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13
ELR Variations and Stability
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• While the ELR averages a decrease of 6.4 C per
  1000 m, the ELR can be almost anything
• Varies diurnally and spatially from place to
  place
• ELR variations can affect the stability of the
  location

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Diurnal Variations in ELR
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Figure 4.10
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Spatial Variations in ELR
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• Spatial variations in the ELR can be similar to
  diurnal variations
• There can be a wide range of ELRs from place to
  place

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ELR Variations and Stability
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• Consider two extreme ELRs that can occur due to
  diurnal or spatial variations

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ELR Variations and Stability
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• Inversions
• An ELR profile where temperature increases with
  height
• Results in stable conditions
• Absolute Stability

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Inversion
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Inversions
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• Due to anything that causes an increase in
  temperature with height
• Cold ocean currents
• Location in polar regions
• Warm air advection over a cold surface
• Radiative cooling of the surface

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ELR Variations and Stability
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• Superadiabat
• An ELR profile where temperature sharply
  decreases with height
• Results in unstable conditions
• Absolute Instability

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Superadiabat
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Superadiabat
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• Due to anything that causes a dramatic decrease
  in temperature with height
• Warm ocean current
• Location in a desert region
• Cool air advection over a warm surface
• Strong radiative heating

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Climatic Significance of Stability
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• Smog in urban areas
• Air mass types
• West versus east coasts of continents
• Rainshadow effect
• Cloud/Precipitation type
• Unstable (cumuliform)
• Stable (stratiform)

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