Condensation, Drops, and Fog

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Condensation, Drops, and Fog

• WX201
• Henry Robinson

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Temperatures Colder than Boiling Temperature

• Water exists in the atmosphere as a solid liquid
  and gas (vapor). Even near DB in Summer
• Some water molecules are far enough apart that
  they dont feel the attraction of other water
  molecules so they can remain a gas even though
  the temperature is below the boiling point.

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Water Vapor

• As the number of water vapor molecules in the air
  increase in number, they will have more chance of
  being close to another molecule and being
  attracted to each other.
• After there are a certain number of molecules in
  the air, any more molecules will cause the
  molecules to clump together forming a liquid.
• This point is the saturation point.

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Saturation Point

• Saturation point dependent upon
• temperature (higher temperature, faster
  molecules get away from one another)
• pressure (number of molecules present)

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Evaporation/Condensation

• Liquid-gas interface (surface of water) will
  exchange molecules.
• Some molecules go from liquid to gas, some go
  from gas to liquid. When air is at saturation
  the number of molecules going equal the number of
  molecules coming.

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Aitken Nuclei

• Air has large numbers of microscopic bits of
  dust, smoke, salt, and other particles.
• Particles are small, generally less than .2
  microns.
• Typically 1000 particles per cubic cm.
• Number density variable.
• Frequently called Aitken nuclei

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Condensation Nuclei

• Condensation nuclei are important for several
  reasons
• Hygroscopic (water seeking) nuclei attract water
  vapor molecules and help form water droplets.
• Nuclei are very important in the formation of
  precipitation.

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Haze

• Haze is a restriction to visibility caused by
  fine particles in the air.
• Aitken particles are smaller than visible light,
  so have limited light scatter- typically they
  scatter blue light.

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Blue haze caused by very small particles (around
.2 micrometers)
Blue Ridge Mountains blue comes from small
hydrocarbon particles (terpenes) released by
vegetation. As particles get larger the haze
becomes more white.
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LA Smog As particles get larger the color shifts
from blue to brown to red to white
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Wet Haze

• Hydroscopic (water seeking) particles will start
  to condense liquid on them.
• As humidity reaches 100 (temperature reaches dew
  point), many droplets will form causing fog.
• Density of fog droplets determines visibility.

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IFR/MVFR/VFR

• VFR- Visible Flight Rules Pilot must be able to
  see the ground at all times.
• IFR Instrument Flight Rules Pilot has special
  training and equipment to fly in clouds.
• MVFR Marginal VFR conditions. Still legally VFR
  but pilots should be aware of conditions that may
  exceed their capabilities
• LIFR Low IFR.

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Fog IFR/MVFR/VFR

• VFR Visibility greater than 5 miles.
• MVFR Visibility 3-5 miles.
• IFR Visibility 1-3 miles.
• LIFR Visibility less than 1 mile.

Red IFR Magenta LIFR Blue MVFR
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Fog Climatology
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Fogy Weather

• Number of days with dense fog conditions

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D. C. Pearson, 2002
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Forecasting Dense Fog is Difficult
LIFRLow IFR
FARFalse Alarm Rate It was forecast but did not
occur.
PODProbability of Detection It happened Was it
forecast?
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Types of Fog Radiation Fog

• Radiation at night cools ground. Air temperature
  cools below dew point temperature. Fog forms
  near ground.

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Nighttime Radiation Fog Formation
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Radiative Cooling
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Radiative Cooling Causes Surface Temperature
Inversion
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Cooling Causes Droplets to Form
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Fog Deepens. Radiative Cooling Primarily from Top
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Weak Heat Flux from Ground Can Cause Base to Lift
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Variations in Ground Composition Can Cause
Variations in Initial Fog Formation.
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Radiation Fog Near Ground in Valley
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Satellite Visible of Fog in Valley
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Dissipation of Fog

• Sunlight warms ground, which warms air, which
  lowers relative humidity, which causes fog
  droplets to evaporate.
• Thick fog reflects most sunlight which limits how
  quickly fog will dissipate.
• Fog generally dissipates (burns off) first near
  edges of foggy area because fog is thinnest there
  as well as dryer air mixes with foggy air.

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Fog Dissipation When Air Warms
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Advection Fog

• Warm, moist air moving (advecting) over a cold
  surface.
• Cold surface lowers the temperature to dew point,
  forming clouds.
• Daytime fogs are generally advective fogs.
• Frequent over Pacific coast over fishing areas
  north of Gulf Stream in Atlantic snowy areas
  with warm winds blowing over them.

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Advection Fog over San Francisco
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Fog Formation over San Francisco
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SST off West Coast

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Air picks up moisture and cools as it flows
across the colder water

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Resulting in fog near sea surface
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Radiative Cooling of Fog Top will Thicken Fog
Layer
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On Shore Winds Will Advect Fog Inland

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Upslope Fog

• Air is lifted by moving up to higher ground.

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Upslope Fog Example
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Steam Fog

• Cold air over warm water. Evaporation/mixing near
  water surface.

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Areas of Sea Smoke Fog Formation
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Evaporative/Mixing Fogs

• Sea Smoke over oceans caused by cold artic air.
• Rain fog- warm rain falling through layer of cold
  air.
• Precipitation can lower visibility to below IFR
  criteria, such as snow or heavy rain conditions.

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Reduced visibility during precipitation event due
to both falling precipitation and to cooling of
air to dew point temperature
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Summary 1

• Saturation point condensation will occur.
• Nuclei can help condensation start as well as
  causing haze.
• Fog will limit visibility causing flight
  restrictions IFR conditions.
• IFR-visibility 1-3 miles LIFR visibility less
  than 1 mile.
• Fog common along west coast, NE US in winter, and
  Gulf coast in winter.
• IFR conditions most frequent factor in weather
  related aviation accidents.
• Radiation fog cooling of surface/fog top by
  long wave radiation loss until fog forms.
• Dissipation by sun warming ground frequently fog
  burns off from edges.

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Summary2

• Advection Fog warm moist air moving over colder
  surface.
• Advection fog frequent over Pacific coast, over
  North Atlantic fishing grounds, and over the NE
  US with snow cover.
• Upslope fog terrain forces air upward until
  condensation occurs.
• Upslope fog frequent in western Kansas and
  eastern Colorado
• Steam fog cold air over warmer water.
• Mixing fogs- occurs during precipitation