Condensation

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Condensation Clouds

• Condensation
• Condensation Processes
• Lapse Rates
• Stability Conditions
• Chapters 4.3 4.5

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Condensation
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• Conversion of water vapor to water
• A reverse of evaporation
• Results in a release of latent heat
• A warming process
• Saturation is achieved
• Clouds

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Condensation Processes
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• Three main condensation processes

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Condensation Processes (1)
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• Air mass mixing
• An air mass is a large body of air having little
  horizontal variation in temperature and moisture
• Air mass mixing occurs when two air masses with
  different temperature and moisture contents
  converge and mix
• The mixture may be saturated with respect to the
  new temperature

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Condensation Processes (2)
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• Cooling without vertical motion
• Radiation fog
• A low level cloud formed when the atmosphere
  loses heat through radiation loss
• Advection fog
• Fog formed when warm, moist air is blown
  (advection) over a cool surface and chilled below
  the dew point temperature

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Condensation Processes (2)
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Not in Book
Aguado and Burt Understanding Weather and
Climate
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Condensation Processes (3)
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• Cooling with vertical motions (convection)
• Forced (mechanical) convection
• Orographic uplift
• Frontal uplift
• Convergence
• Free (thermal) convection
• Heating of surface by sun
• Cold air over a warm surface

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Condensation Processes (3)
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• The cooling with an increase in height is called
  an adiabatic process
• Adiabatic process
• The process in which a rising parcel of air cools
  through expansion or a sinking parcel warms
  through compressionbut with no exchange of heat
  between the parcel and its surrounding environment

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Adiabatic Processes
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Adiabatic Processes
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• Why do we have adiabatic processes?
• Air is a poor thermal conductor
• Air parcels move quicker than heat can be
  exchanged with the environment

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Lapse Rates
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• Environmental Lapse Rate (ELR)
• The observed temperature distribution with height
• Variable in time and space
• Can be anything
• Is not adiabatic

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Lapse Rates
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• Normal Environmental Lapse Rate
• The ELR averaged over the globe
• 6.4 C/1000 m

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Normal Environmental Lapse Rate
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Not in Book
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Lapse Rates
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• Dry Adiabatic Lapse Rate (DALR)
• The rate at which an unsaturated parcel of air
  cools or warms
• DALR 10 C/1000m
• Can be a decrease in temperature (rising air) or
  an increase in temperature (sinking air)

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DALR (cooling)
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DALR (warming)
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Lapse Rates
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• Saturated Adiabatic Lapse Rate (SALR)
• Also called the Wet Adiabatic Lapse Rate (WALR)
• The rate at which a saturated parcel of air cools
• SALR 0.6 C/1000 m (slightly variable)
• Lower rate of cooling because of heat released
  during condensation

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SALR (cooling)
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Relates to Figure 4.6
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Stability Conditions
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• Three types of Stability Conditions
• Stable
• Unstable
• Conditional Instability

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Stability Conditions
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• When an air parcel is warmer than the surrounding
  environment, the parcel will be less dense than
  its surroundings and will rise through the
  atmosphere
• When an air parcel is cooler, it will be more
  dense and sink through the atmosphere

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Stable Conditions
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• The parcel of air is cooler than its surrounding
  environment
• The parcel will sink once the forcing mechanism
  is removed

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Stable Conditions
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Figure 4.7
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Unstable Conditions
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• The parcel of air is warmer than its surrounding
  environment
• The parcel will continue to rise once the
  initially forcing mechanism is removed

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Unstable Conditions
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Figure 4.7
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Conditional Instability
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• The parcel of air is stable when it begins
  rising, but becomes unstable during ascent
• Occurs when the ELR is between the DALR and the
  SALR

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Conditional Instability
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Figure 4.7
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Conditional Instability
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• The air parcel starts rising and is stable
• Parcel cools at DALR until it becomes saturated
  at the Lifting Condensation Level
• Clouds start to form (Condensation Level)

Figure 4.7
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Conditional Instability
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• The air parcel starts to cool at the SALR
• The parcel eventually is warmer than the
  environment at the Level of Free Convection
• The air parcel is unstable

Figure 4.7
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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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