Places Where Condensation Occurs

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Places Where Condensation Occurs
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Places Where Condensation Occurs

• Generally we subdivided the places where
  condensation occurs into three locations
• on the ground (dew)
• in the air right next to the ground (fog)
• in the air away from the ground (clouds).

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Condensation on the Ground

• If the temperature of the ground decreases to the
  dew point temperature, then the layer of air
  molecules right at the surface will be saturated
  and water will condense on the surface forming
  dew.

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Dew Formation
Dew forms as liquid water condenses on the
surface.
Ground cools to the dew point
temperature
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Dew Formation (Cont.)

• Dew is most likely to form on clear, calm nights.
• A clear night allows for the maximum emission of
  terrestrial radiation from the surface to space.
  This produces the biggest decrease in
  temperature.
• Calm winds prevent drier air from above the
  surface mixing with the air at the surface and
  allow the air to become saturated.

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Frost Formation

• If the ground cools to a dew point temperature
  that is below freezing, then water vapor is
  deposited on the surface as ice crystals and
  frost forms.

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Condensation in the Air Next to the Ground (Fog)

• (1) evaporation fog usually forms after rain or
  snow has fallen. The precipitation leaves some
  water on the surface. Either water evaporates
  from puddles from rain or melting snow, or water
  sublimates into water vapor from the snow. If
  enough water vapor enters the air next to the
  ground, then it becomes saturated and fog forms.

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Evaporation Fog
Air becomes saturated and fog forms
water evaporates
liquid water
liquid water
Ground
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Fog Formation (Cont.)

• (2) steam fog forms when cooler drier air moves
  over a warmer, water surface. Water evaporates
  into the drier air until it becomes saturated,
  then water vapor condenses and fog forms.
  Internal energy is also transferred to the cooler
  air, increasing its temperature and it starts to
  rise like steam, which is how this fog gets its
  name.

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Steam Fog
the warming air starts to rise and the fog looks
like steam rising over the water.
the air becomes saturated and fog forms
transfer of internal energy increases the
temperature
water evaporates into the drier air
cooler, drier air
cooler, drier air
warmer liquid water surface (lake, river, etc.)
land
land
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Steam Fog (Cont.)

• Steam fog is common in Ohio during the autumn
  when the temperature of lakes and rivers is still
  relatively warm. On clear, relatively calm
  nights, cooler drier air will drain down the
  sides of valleys and move over the water. If
  enough water evaporates into the air, then steam
  fog forms.

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Fog Formation (Cont.)

• (3) radiation fog forms at night when the
  emission of terrestrial radiation from the ground
  causes the temperature of the ground and the air
  next to the ground to decrease. If the air next
  to the ground cools to the dew point temperature,
  then it becomes saturated and fog forms.

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Radiation Fog
terrestrial radiation is emitted by the surface
at night
if the temperature of the air cools to the dew
point temperature then the air is saturated and
fog forms
cooler, denser air flows into valleys making fog
thicker
temperature of the ground and the air next to the
ground decreases
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Fog Formation (Cont.)

• (4) advection fog forms when warm moist air moves
  over a cooler surface. The cooler surface may be
  either land or water. The warm moist air
  transfers internal energy to the cooler surface.
  If the temperature decreases to the dew point
  temperature, then the air becomes saturated and
  fog forms. This fog gets its name because
  advection means the horizontal movement of air.

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Advection Fog
loses internal energy to the surface
warm moist air
temperature decreases to dew point temperature,
air becomes saturated and fog forms
cooler surface can be either cool land or cool
water
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Advection Fog (Cont.)

• Advection fog is most common in the eastern U.S.
  in the late autumn, winter and spring, when warm,
  moist air from the Gulf of Mexico moves inland
  over the cooler ground.
• Advection fog can also occur over the oceans when
  air flows over a warm current and then passes
  over a cold current.

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Fog Formation (Cont.)

• (5) upslope fog forms when warm moist air flows
  uphill. As the air flows up the slope, it
  encounters lower pressure, expands, does work and
  the temperature decreases. If the temperature
  decreases until the air becomes saturated then
  fog forms.

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Upslope Fog
if it becomes saturated fog forms
air expands and cools
warm moist air
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Condensation in the Air Away from the Ground
(Clouds)

• Clouds form when rising air parcels cool until
  the become saturated.
• The level at which saturation first occurs is
  called the Lifting Condensation Level (LCL).
• We see the LCL as the bottom of the clouds.

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Computing the Lifting Condensation Level (LCL)

• Suppose an air parcel is at a height of zero
  meters and it has a temperature of 14C and a dew
  point temperature of 2C. If the dew point
  temperature remains constant, what would the
  Lifting Condensation Level be?

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Computing the LCL (Cont.)

• T Td 14C - 2C 12C
• So, the air parcel would have to cool 12C.
• Unsaturated air parcels cool at the DALR.
• So, they cool 1C for every 100 meters they rise.

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Computing the LCL (Cont.)

• In order to cool 12C, the parcel would have to
  rise 1200 m.

At 1200 m T 2C, Td 2C and the air is
saturated. Thus, the LCL 1200 m.
At 0 meters T 14C and Td 2C