Dew, Frost and Fog

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Dew, Frost and Fog
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RECAP

• Hydrological cycle transport of water and
  energy.
• Humidity absolute humidity, specific humidity,
  water mixing ratio, relative humidity.
• Saturation pressure the maximum vapor pressure
  at a given T.
• Dew point the temperature to which air would
  have to be cooled in order to get saturated.
• The effect of large water bodies on the humidity
  of a climate (Florida/California).
• The effect of cooling/heating systems on the
  humidity in the house.
• Heat index what the air temperature feels like
  to the average person for different combinations
  of temperature and relative humidity

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Condensation

• As air cools, it becomes first saturated and then
  supersaturated -gt condensation/deposition
• Condensation requires some initiator
• surface (dew, frost)
• nuclei (haze, fog, clouds)
• Classification of condensation nuclei according
  to
• size (not important)
• properties
• hygroscopic water-seeking, condensation at
  RHlt100
• hydrophobic water-repelling, condensation at
  RHgt100.

saturated
Super-saturated
unsaturated
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Dew

• Condensation of water vapor on cool surfaces.
• Usually forms at ground level (air is coldest
  there).
• Frozen dew air temperature drops below zero
  after dew has formed
• Atmospheric conditions clear, calm nights.
• Clear air the ground emits IR radiation to
  space. There are no clouds to absorb and
  reradiate the IR energy.
• Calm air (no winds) The wind enhances
  evaporation and inhibits condensation. It also
  enhances mixing in the air, so in the absence of
  wind, the ground gets colder (recall radiation
  inversion).

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Frost

• Deposition of water vapor on cool surfaces.
• Atmospheric conditions cold, clear, calm nights.
• Clear air the ground emits IR radiation to
  space. There are no clouds to reabsorb IR energy.
• Cold the temperature of the ground is below
  freezing.
• Calm air no winds- wind enhances evaporation and
  inhibits condensation. It also enhances mixing in
  the air, so if there is no wind, the ground gets
  colder (recall radiation inversion).

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Frost on Windows

• Cold, outside temperatures cool the window. The
  water vapor in the room is cooled down to below
  freezing temperatures and is deposited on the
  inside of the window, forming delicate ice
  crystals.

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Haze

• A layer of small nuclei (dust, aerosols, salt
  particles).
• Dry haze at low relative humidity
• Wet haze at relative humidity 75-100. The
  presence of hygroscopic (water-seeking) nuclei
  facilitates condensation at RHlt100.
• Haze is more visible in the morning and in the
  evening (higher relative humidity) because the
  wet particles are bigger.
• Book sez Dry haze appears bluish on dark
  background and yellowish on light background
  (why? Wait for the answer until Chapter 19).

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Haze on Titan
Cassini spacecraft imaging system
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Fog

• Wet haze near RH100 is called fog.
• Fog is a cloud resting near the ground.
• Wet haze or fog? International definition fog,
  if visibility lt 1km. The individual water
  particles are bigger than the haze particles and
  they become visible to the eye.
• Fog scatters light at all wavelengths (geometric
  scattering) and as a result it appears
  white/gray.
• Fog is most likely to form in the morning or
  evening when the RH is highest.

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Making Fog
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Radiation (Ground) Fog

• Preconditions local radiative processes
• dominate.
• Formation.
• The ground cools down due to
• thermal (IR) radiation
• The layer next to the ground cools down
• The RH is increasing and the air saturates
  forming fog close to the ground.
• When does it occur?
• Late night hours when the ground temperature is
  minimum.
• Late fall and winter (long nights)
• When does it dissipate?
• As the sun comes up it heats up the ground. The
  ground air is heated by conduction and the fog
  droplets evaporate.
• The fog dissipates from the ground up.

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Advection Fog

• Preconditions.
• Air mass movements (advection,
• weak wind) over a moisture source.
• Temperature difference between the
• water surface and the land.
• Formation.
• Warm land and cold water warm air mass moves
  over to the water. It cools down, the RH rises,
  the air saturates
• Cold land and warm water cold air mass moves
  over the water. It mixes up with warmer moist air
  from above the water surface. The resulting mixed
  air is saturated.
• Cold water and warm water two ocean currents
  with different temperatures flow next to each
  other. Warm air moving over the cold current
  produces fog.

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Upslope Fog

• Preconditions.
• Weak winds
• Presence of topography
• Formation.
• Moist air moved up the slope of a hill
• The air mass cools down and the RH increases
• The saturated air forms
• fog

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Evaporation Fog

• Preconditions
• Cool air over a body of warm
• water (no need for advection).
• Formation
• Water evaporates from the warm lake (pool).
• The cool air moistens and saturates.
• It is a local effect.
• When does it occur?
• Often during the fall and early winter (water is
  warmer than the land)
• After a rainshower as the sun heats up the ground
  

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Foggy weather in the US
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The nuisance of foggy weather

• The limited visibility leads to
• Poor driving conditions
• Use fog lights
• Do not use high beams
• Poor sailing conditions
• Lighthouses
• Poor flying conditions
• Airport closure
• Railways also are affected