Lecture 5.1

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Lecture 5.1 Fog

• Chapter 6 of Ahrens

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Condensation

• To get condensation in the atmosphere it is not
  sufficient to achieve saturation
• You need a nucleus for the water to condense onto
• These are cloud condensation nuclei CCN
• Or surfaces at the surface

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Typical CCN

• Salt
• Soot
• Dust
• Other stuff

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If there are no CCN

• You can achieve RH gt 100, this is known as
  supersaturation
• Eventually you can achieve condensation without
  CCN, but this is rare
• Condensation will occur in a clean atmosphere if
  you cool or supersaturate the parcel enough

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Hygroscopic nuclei

• These attract water vapor (actually reduce the
  pressure needed for vapor molecules to stick)
• With these present we can start condensation at
  relative humidities lower than 100 (sometimes as
  low as 80)

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Fog

• Fog is a cloud that is in contact with the
  ground.
• Fog is defined for visibilities less than 1000
  meters.
• To create a fog we need all of the ingredients
  required for cloud formation
• Condensation Nuclei (CCN)-- A place for water to
  condense. Well revisit these later.
• Supersaturation
• A way to obtain and maintain supersaturation.

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FOG

• Fog formation occurs in a number of different
  ways
• These different formation mechanisms demonstrate
  the various ways of achieving saturation in the
  atmosphere

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Fog

• Methods to obtain supersaturation
• Radiational cooling
• Advection
• Upslope cooling
• Mixing
• Many of these are also applicable to clouds!

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Radiation Fog
On clear nights with light winds the earth
radiates energy to space. This energy is lost
from the surface and the surface cools.
The earth radiates.
The air above the surface will cool both by
radiation and by conduction
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Radiation fog

• If there is a source of moisture nearby (lake,
  marsh, wet grass,etc.) then the temperature of
  the air can fall to the dew point. Once this
  occurs, fog will form on the plentiful CCN.

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Valley Fog
The earth radiates.
We can take this concept to the hills.
Cool air, at the same pressure, is more
dense than warm air. As a result the cool air
sinks and drains into the valley. If the
temperature of the air at the bottom of the
valley is cool enough and there is sufficient
moisture, a fog may form at the bottom of the
valley. This can be a very persistent form of
Valley Fog.
Cool Air
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Radiation Fog
Radiation fog typically burns off in a few
hours after sunrise.
Sun incident on a thick layer. Little heating of
the surface.
Fog
Sun incident on a thin layer. There is heating
of the surface. This causes convection and mixing
with dry air. The fog dissipates from the
outside in.
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Radiation Fog
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Advection Fog
The cooling of the surface air to saturation may
be accomplished by moving warm, moist air over a
much cooler surface.
Warm, Moist
Cold Land
Fog can roll in off the ocean (e.g. San
Francisco). If the fog is carried over warmer
land, the land will raise the air temperature
above the dew point and the fog will
dissipate. Fog provides moisture to plants
during the dry season on the west coast.
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Advection Fog
Figure 6.5
Warm moist air that moves, or advects, above a
cold surface may become cooled to its dew point
temperature, creating an advection fog. This fog
often forms above the ocean due to mixing
currents, or when warm ocean air rolls into the
cooler waters at the Pacific Coastline.
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Upslope Fog
Warm, Moist
Warm, moist air flows up the hill. As it does it
cools. If it cools to the dew point, then a fog
will form on the side of the hill.
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Fog by Mixing

• When a dry parcel of air moves into a more moist
  area (one with a source of water vapor), for
  example from land to sea, then moisture can be
  added to the air causing it to become saturated.
• This mixing of dry cold air with warmer moist air
  can cause fog

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Mixing
Steam Fog
Cold Air
Warm Water
The clouds that form resemble the steam that
rises from your tea cup.
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Evaporation or Mixing Fog
Fog can form by mixing warm unsaturated air with
cool unsaturated air, which can occur during
evaporation. Steam fog is an example of this
mixing process and occurs when warm pools of
water are a source for vapor that condense into
the cooler air above.
Figure 6.7
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Homework

• Chapter 5 Page 128, questions for thought 2 and
  6.
• Chapter 6 Page 162, question for thought 3
• Homework dew Friday 26th September 2003 (start
  of class)