NATS 101 Lecture 10 Condensation: Fog and Cloud Formation

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NATS 101Lecture 10Condensation Fog and Cloud
Formation

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Summary Moisture

• Air has a saturation level for water vapor,
  beyond no additional vapor can be added
• Saturation level depends on air temperature
• Actual water vapor content of air can be
  quantified by several measures
• Vapor Pressure, Saturation Vapor Pressure,
  Relative Humity, Dew Point, Wet-Bulb

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Summary Moisture Measures

• Saturation Vapor Pressure (SVP) - highest
  possible partial pressure for water vapor
• Vapor Pressure - actual partial pressure of water
  vapor
• Relative Humidity - ratio of actual water vapor
  content over the saturation value

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Summary Moisture Measures

• Dew Point (DP) - temperature to which air must be
  cooled at constant pressure to become saturated.
  Plotted on WX maps.
• Wet Bulb Temperature - lowest temperature to
  which air can be cooled by evaporation of water.
  Higher than DP since water vapor is added to air.
• Temp ? Wet Bulb Temp ? Dew Point

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Cloud Condensation Nuclei

• Small, airborne particles are necessary on which
  water vapor can condense to produce cloud
  droplets
• Without such particles, RHgt100 would be needed
  to produce clouds
• Such surfaces are called Cloud Condensation
  Nuclei (CCN)
• CCN are light and stay suspended for days

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Cloud Condensation Nuclei

• Sources
• Dust, volcanic ash, smoke, soot, salt, sulfate
  particles
• Concentrations
• 1,000-10,000 per cc Highest over cities Highest
  at surface

Ahrens, Meteorology Today, 5th Ed.
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Cloud Condensation Nuclei
Ahrens, Meteorology Today, 5th Ed.
Water-Seeking Salt Sulfuric acid Nitric acid
Water-Repelling Oils Gasoline Waxes
Condensation can occur on hygroscopic CCN for
RHlt100 Accounts for Haze when the RH approaches
60-80
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Haze over Melting Snow
Ahrens, Meteorology Today, 5th Ed.
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Condensation

• When air becomes supersaturated from either the
  temperature cooling or the addition of water
  vapor, water condenses onto CCN as small cloud
  droplets
• Diameters of droplets are 2-20 microns, (10-100
  times smaller than human hair)
• Concentrations are 50-1,000 droplets per cc

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Size of Cloud Droplets
1000 times
100 times
Williams, The Weather Book, p73
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What is Fog?

• Fog - a cloud at the ground
• There are four basic types of fog
• Radiation Fog
• Advection Fog
• Upslope Fog
• Steam Fog
• Combinations exist (Radiation-Advection)

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Radiation or Ground Fog
Williams, The Weather Book, p68
On clear nights with light winds (lt 5 mph), the
ground can cool rapidly by loss of IR radiation
and absence of mixing Fog forms as air near
surface cools to its dew point The morning sun
raises temperatures above dew point, and the fog
burns off Strong winds and clouds prevent fog
formation
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Valley Fog
Williams, The Weather Book, p68
Valley fog is a deep (1,000 ft thick), long-lived
radiation fog that occurs under persistent high
pressure Sun is too weak to evaporate fog
completely during day Fog persists until a winter
storm with strong winds literally scours out the
cold air that is trapped in the valley Occurs in
Central Valley of California, Salt Lake City
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Radiation Fog in Valley
Ahrens 4th Ed., Fig 4.14
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Fog Dissipation in the Morning
Ahrens, Meteorology Today, 5th Ed.
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Advection Fog
Williams, The Weather Book, p68
Warm, humid air flows over colder ground. Air
cools to its dew and fog forms. Occurs over
central US in winter when air from Gulf of Mexico
flows inland, especially over snow covered
ground. Occurs along West Coast in summer when
air from mid-Pacific flows over cold, upwelling
water along coast.
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Advection Fog at Golden Gate
Ahrens 4th Ed., Fig 4.15
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Upslope Fog
Williams, The Weather Book, p68
Occurs along windward slopes of mountain
ranges Widespread fog common on upper Great
Plains Air cools to its dew point as it rises
under easterly flow Fog drifts up the slope.
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Steam Fog or Sea Smoke
Williams, The Weather Book, p68
Occurs when cold air flows over much warmer
water Cold air mixes with warm, moist air at
waters surface Mixture condenses into fog, a
couple of feet above surface Common off of East
Coast of US during winter Over lakes, ponds and
rivers during fall
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Why You See Your Breath on a Cold Winter
Night

• Condensation can occur if moist, warm air mixes
  with cold, dry air.
• Temp RH SVP VP
• Air 0oC 20 6 mb 1 mb
• (clear)
• Breath 36oC 80 63 mb 55 mb
• (clear)
• 50-50 18oC 140 20 mb 28 mb (cloud)
• 90-10 4oC 90 8 mb 6 mb
• (clear)
• The 50-50 mix condenses into a cloud that quickly
  evaporates as your exhalation is diluted further

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Steam Fog over Thermal Pool
Ahrens, Fig 4.16
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Precipitation or Frontal Fog
Williams, The Weather Book, p68
A type of Steam Fog Occurs when warm rain falls
into colder unsaturated air Warmer, rain drops
evaporate as they fall Added water vapor
increases dew point to saturate the air Most
common in the shallow layer of cold air, just
ahead of warm fronts
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Number of Days with Fog
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Summary Condensation

• Condensation
• Can occur by cooling or moistening of air
• CCN permit condensation at RH near 100
• Small (lt0.2 to 1 microns) airborne particles
• Responsible for Haze formation at RH lt 100

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Summary Fog

• Fog - a cloud at the ground
• Composed of small (20 micron) water drops
• Four primary types of Fog
• Radiation-Advection-Upslope-Steam
• Occur under distinct weather conditions

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Next Lecture Assignments

• Topic - Cloud Types
• Reading - Ahrens pp 96-108
• Problems - 4.16