Formation of Clouds and Precipitation

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Formation of Clouds and Precipitation
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Condensation and the Formation of Clouds

• Cloud droplets form when water vapor condenses on
  small particles called condensation nuclei.
• Dust particles, salt crystals, volcanic ash,
  particles from combustion, etc. may act as
  condensation nuclei.

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Condensation and the Formation of Clouds (Cont.)

• A droplet will grow larger if more water vapor
  joins the drop as a result of condensation than
  the leaves the drop as a result of evaporation.
• Two processes affect the growth of these small
  droplets.

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The Curvature Effect

• The curvature effect is the term used to account
  for the fact that cloud droplets are
  approximately spherical in shape, while the
  formal definition of saturation refers to a plane
  (flat) surface of water.

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The Curvature Effect (Cont.)
The spherical droplet has a much larger surface
area from which water molecules can evaporate.
The plane surface has less area for water
molecules to evaporate and so fewer molecules
need to re-enter the surface in order for the
saturation equilibrium to occur.
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The Curvature Effect (Cont.)

• The curvature effect inhibits the growth of small
  droplets because it requires a higher vapor
  pressure to maintain the saturation equilibrium.
• As the droplets get larger, then the importance
  of the curvature effect is reduced.

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The Solute Effect

• The solute effect occurs when a condensation
  nucleus dissolves in the droplet of water and
  some of the molecules replace water molecules at
  the surface of the droplet.

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The Solute Effect (Cont.)
Some of the surface of the drop is now occupied
by molecules from the condensation nucleus.
Water cannot evaporate from those locations.
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The Solute Effect (Cont.)

• The solute effect makes it easier for droplets to
  grow, because it reduces the evaporation of water
  molecules from the droplet.
• As droplets get larger the solute effect is
  reduced, because the dissolved molecules occupy a
  smaller percentage of the surface area of the
  droplet.

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Cloud Droplets

• The droplets that form as a result of
  condensation are too small to fall to Earth
  against the rising air that generated the cloud.

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Cloud Droplets (Cont.)

• Laboratory experiments show that is would take a
  very long time (i.e. many hours or days) to
  produce drops big enough to fall to Earth as a
  result of condensation alone.
• Some other process must occur in order to produce
  drops or ice crystals that are large enough to
  fall as precipitation.

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Formation of Precipitation

• Much of the time precipitation forms as a result
  of the Bergeron (also called the ice crystal or
  three phase) process.

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Temperatures Inside Clouds

• In portions of clouds where the temperature is
  above freezing (0C), there are liquid drops but
  no ice crystals.
• In the parts of clouds where the temperatures are
  between -10C and -40C, there is a
  mixture of liquid drops and ice crystals.

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Temperature Inside Clouds (Cont.)

• As the temperature inside a cloud gets colder,
  the percentage of ice crystals increases and the
  percentage of liquid drops decreases.
• In the portion of a cloud where the temperatures
  are colder than -40C, there are ice crystals but
  few liquid drops.

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Temperatures Inside Clouds (Cont.)
ice crystals
- 40C
ice crystals and liquid drops
-10C
liquid drops
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Key Fact

• The saturation vapor pressure around an ice
  crystal is lower than the saturation vapor
  pressure around a liquid drop.

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Effect of Saturation Vapor Pressure Difference

• This means that it takes less water vapor to
  maintain the saturation equilibrium around an ice
  crystal than it does around a liquid drop.
• Thus, in the same environment an ice crystal will
  grow more than a liquid drop.

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Effect of Saturation Vapor Pressure Difference
(Cont.)
The ice crystals grow faster than
the water drops.
liquid drop
ice crystal
water vapor molecule
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Effect of Saturation Vapor Pressure Difference
(Cont.)

• As the water vapor molecules deposit on the
  growing ice crystals, evaporation may cause the
  liquid droplets to get smaller.
• The molecules that evaporate from the liquid
  drops may also deposit on the ice crystals
  causing them to grow even larger.

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Precipitation

• When the ice crystals grow large enough, the
  force of gravity begins to pull them toward the
  surface of the Earth against the rising air in
  the updraft.

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Collision-Coalescence

• If the ice crystals, or subsequent rain drops,
  collide as they fall, then they may combine
  (coalesce).
• This is called the collision-coalescence process.

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Types of Precipitation

• Snow the air is below freezing all the way down
  to the surface and the ice crystals reach the
  surface without melting.
• Rain there is a sufficiently deep layer of air
  with the temperature above freezing and the ice
  crystal melts into a liquid drop and reaches the
  ground as rain.

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Types of Precipitation (Cont.)

• (3) Sleet there is a layer with the temperature
  above freezing, that allows the ice crystals to
  melt. However, between that layer and the
  surface, there is another layer with the
  temperature below freezing and the liquid drop
  freezes before it reaches the surface as an ice
  pellet.

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Types of Precipitation (Cont.)

• (4) Freezing rain occurs when the temperature
  of the air warms to above freezing and remains
  above freezing, except for the surface, or a very
  thin layer above there surface where the
  temperature is below freezing. The ice crystals
  melt in the warmer air and then freeze on contact
  with the surface.

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Types of Precipitation (Cont.)
Air temperature below freezing
Air temperature above freezing
Freezing rain
Rain ..
Snow
Sleet

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Virga

• Virga is rain that falls into very dry air and
  evaporates before it reaches the surface.

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Virga (Cont.)
The rain drops evaporate completely before
reaching the surface.