Atmospheric Moisture Continued

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Atmospheric Moisture Continued

• EAS 211
• Spring 2005
• 01/31/05

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Methods of Achieving Saturation

• Adding Water Vapor to the parcel
• Evaporation from falling raindrops can raise the
  T d in the air beneath the cloud from which the
  rain is falling.
• If enough vapor is added to the air to saturate a
  light fog forms beneath the cloud
• Mixing cold air with warm, moist air
• When warm water evaporates into cold airwe get
  steam and fog.
• If cold air blows over a warm lake, it mixes with
  the moist air just above the lakes surface to
  form a thin layer of fog.

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Methods cont.

• Lowering the T to the Td
• Air temperature changes can occur in two ways
• Diabatic Processes
• Involves the input or removal of energy (heat)
• Example Air passing over a cold surface..cools
  by a process called conduction (Nighttime
  Cooling)
• Obeys the Second Law of Thermodynamics which
  states that heat transfer is always from hot to
  cold.
• Adiabatic Processes
• Thoses processes in which T changes but NO heat
  is added or removed.
• Obeys the First Law of Thermodynamics which
  states that if no heat is added or removed from
  a system, work performed by the air (expansion)
  causes a decrease in internal energy (cooling)
  work performed on the gas (compression) leads to
  an increase in internal energy (warming).

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First Law of Thermodynamics

• Mathematically
• For an adiabatic process
• So
• Where
• ?H is the change in energy
• P is pressure
• a called the specific volume and is simply 1/?
• Cv is the specific heat of air at a constant
  volume
• ?T is change in temperature
• Ex In the atmosphere, suppose a parcel of air is
  displaced upwardP is lower at higher altitudes,
  so the air expands. The expansion causes the
  parcel to cool since
• The rate at which an unsaturated parcel cools is
  called the Dry Adiabatic Lapse Rate (DALR) and is
  10 ºC/km or 5.5 ºF/1000ft
• So if a parcel is rising it cools at this rate
  and if it is descending it warms at this rate.

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Adiabatic Temperature Changes
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• NOTE If a parcel has an RH 99.999999999 it is
  still unsaturated.
• Has to have TTd (RH100) to considered
  saturated.
• As a parcel rises, it cools adiabatically coming
  closer and closer to saturation. If the parcel
  rises sufficiently long enough it can reach
  saturation. The point which a parcel reaches
  saturation is called the
• Lifted Condensation Level (LCL)The level to
  which a parcel must ascend or be lifted to in
  order to reach saturation. If the parcel
  continues to rise above the LCL, it still
  coolsBUT some of the cooling is offset by the
  warming from condensation. The cooling rate is
  slower 5-6 ºC/km or 3.3 ºF/100ft called the
  Moist Adiabatic Lapse Rate (MALR). MALR not
  constant but varies slightly with temperature.

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Dry Wet Adiabatic Rates
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• Environmental Lapse Ratethe vertical change in T
  through the still air (Recall )
• Now we are not dealing with an isolated parcel as
  with the DALR and MALR we are looking at a
  column in the troposphere.
• Usually T decreases w/ z, but the change varies
  significantly with time, location and even in
  different levels of the atmosphere.
• AdiabaticNo heat or mass is exchanged b/w a
  parcel and the surrounding environment. Here are
  some examples of adiabatic processes.
• Vertical displacementlifted parcel expands and
  cools a sinking parcel compresses and warms
• Release of LHlifted to saturation further
  ascent causes condensationthe parcel releases
  heat, thus cooling at a slower rate (MALR)
• Absorption of LHLH has been released in a parcel
  (condensation has occurred) and there is liquid
  water present if a parcel sinks again, it warms.
  Therefore, as it warms, some of the water will
  evaporate. The water molecules absorb some heat,
  and the parcel warms at the MALR.
• NOTE The last two (above) occur during
  saturation therefore they are considered moist
  adiabatic processes

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Psuedo-Adiabatic Processes

• When condensation occurs in a rising parcel, the
  water droplets usually do not remain in the
  parcel, but fall out as rain or snow.
• This causes an exchange in mass with the
  surrounding environment so that the process can
  no longer be accurately described as adiabatic it
  now considered pseudo-adiabatic
• When the parcel sinks again, the water droplets
  arent there (they fell out). Therefore, little
  or no absorption of LH occurs (evap.). Now, as
  the parcel sinks, it warms at a faster rate than
  when the water vapor had been present.

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Forms of condensation

• Dewliquid condensation on a sfc.
• At night, the sfc cools diabatically due to loss
  of long-wave radiation if T drops to the Td
  condensation occurs.
• Water droplets form on sfs
• Typically forms on clear, windless nights
• Lack of clouds allows more long-wave radiation to
  escape
• Lack of winds precludes mixing of warmer air from
  above.
• Frost
• Ice crystals on sfs when the T drops to the frost
  point (a Td below freezing)
• Forms a treelike branching pattern
• Ice crystals are deposited onto sfcs (deposition)
• Frozen Dew
• Condensation occurs at T slightly above freezing
  but the air continues to cool and drops below
  freezing.
• The dew freezes, forming a thin continuous layer
  of ice.
• Fog
• A cloud with its base at the sfc
• Obstructs visibility to lt 1 km
• Forms by
• Cooling air to saturation
• Adding water vapor

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Radiation Fog

• Forms on clear nights, when loss of long-wave
  radiation leads to the T cooling to the Td
• Forms by cooling to the Td (diabatic)
• Shallow moist layer at the sfc is too shallow to
  absorb much LW radiation
• Form on clear nights with calm (light) sfc winds
  ( lt 5 kts)
• Most commonly in the early morning hours and in
  low-lying areas.
• Shallow only a few meters deep
• During the morning hours, the sun heats the sfc,
  causing the fog to droplets to evaporate form the
  sfc upward.

Diagram courtesy of COMET
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• Satellite image of dense fog in Californias San
  Joaquin Valley on Nov. 20, 2002. This early
  morning radiation fog was responsible for several
  car accidents in the region, including a 14-car
  pile-up.

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• Moist air is advected (moves across) a relatively
  colder sfc so that the air cools to saturation by
  conduction and forms fog.
• Forms by cooling to saturation (diabatic)
• Mod.-strong winds (10-30 km/h) to sustain
  advectionif winds are strong enough, they can
  advect the fog well downwind of where it formed.
• Stronger winds can cause the fog to exceed depths
  of 500 meters.
• Ex. San Francisco summerswarm Pacific air flows
  eastward and passes over a narro, cold ocean
  current off the coast of CA. The air cools to
  saturation, and fog forms. The strong winds
  advect the fog inland, where it can remain for
  days.

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• Moist air rises along a gently sloping sfc.
• The air expands and saturates as it rises
  (adiabatic)
• Often occurs on the eastern slopes of the Rockies
  and the western slopes of the Cascades

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Also called Mountain Fog
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Evaporation Fog (Steam Fog)

• Forms when cold air blows over a warm body of
  water.
• Forms by adding water vapor to the air (diabatic)
• Water evaporates from the water sfc increases
  water content in the air above the water body
• Td increases in the cold air above the sfc until
  saturation occurs, causing a fog to form over the
  water
• Ex. Lakes and rivers, wet roadways (after it
  rains), when we breathe on a cold day.

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• Forms when warm rain falls into a layer of colder
  air
• Formed by adding water vapor to the air
  (diabatic)
• Some or all of the rain evaporates into the air,
  increasing the water content of the air until
  saturation
• A fog forms below the cloud.

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Frontal Fog or Virga
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Condensation Nuclei

• Condensation Nucleimicroscopic particles which
  serve as sfcs on which water vapor may condense
  to form raindrops.
• 1 cubic cm of air may contain up to 10,000
  condensation nuclei
• With the presence of these nuclei, RH of gt200
  would be required to form water droplets.
• Cloud Condensation Nuclei (CCN)
• Radius gt 0.1 µm
• Density of nuclei is about 100-1000 nuclei per
  cubic cm
• Ex. Dust, smoke, volcanic ash, sea salt
• They are very light (lt1 trillionth of a gram)
• Cities have higher density of CCNs than rural
  areas (pollutants)
• Oceans and coastal regions have more CCN due to
  sea spray
• Hygroscopic nuclei-water attracting particles
  (sulfuric and nitric acids (acid rain), salt)
• Hydrophobic nucleiwater repelling particles (oil
  and gasoline)

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