Precipitation Chapter 7

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PrecipitationChapter 7
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Objectives

• Differentiate several ways that precipitation is
  formed
• Distinguish between several types of precipitation

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General concept

• There are several forces acting on a water
  droplet or ice crystal in a cloud
• Winds
• Atmospheric stability
• Gravity
• Drag (friction)

• When a droplet reaches a certain critical mass
  the force of gravity will exceed the other
  forces and precipitation will fal
• Rain drops are 100X larger than cloud droplets

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Formation of precipitation

• Condensation and deposition
• Collision and coalescence
• The Bergeron process

Warm air
Cold air
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Condensation and deposition

• As long as the rate of condensation exceeds the
  rate of evaporation water will be deposited and
  accumulate on condensation nuclei
• Droplets grow very slowly
• Rarely produces rain drops

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Collision and coalescence

• As water droplets fall through a cloud, the
  larger droplets fall faster than the smaller
  ones, thus they collide
• If the droplets coalesce a larger drop is formed
• If it gets big enough rain will fall

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Collision efficiency

• A drop must be larger than the other drops to be
  an efficient collider
• If the drop is too big it will be less efficient,
  because it creates high pressure that pushes
  small drops out of the way

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Cold and cool clouds

• Cold

• Cool Part is
• Part is 00C

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The Bergeron process

• Vapor pressure over ice is less than vapor
  pressure over water of the same temperature
• Thus, water molecules move from water to ice and
  freeze on the ice
• If the crystals get large enough they will fall
• If they fall through cold air they will stay
  frozen and it will snow
• If they fall through warm air they will melt and
  it will rain

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Growth of ice crystals

• Riming super-cooled water freezes onto ice
  crystals

• Aggregation Ice crystals join together to form
  large snow flakes. Happens more readily if ice
  crystals have a thin coating of liquid

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Types of precipitation
rain
sleet
graupel
hail
freezing rain
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Rain

• Can originate as liquid drops or ice. Ice melts
  as it falls through warmer air
• Shape of rain drops

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Snow

• Ice crystals grow in a cloud. Variety of shapes
  depending on temperature

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Graupel

• Ice crystals grow by riming
• Contain air pockets
• Has a spongy texture

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Sleet

• Ice crystals melt as they fall through warn air
  aloft. Then water drops freeze as they fall
  through cold air below and hit the surface as ice
  pellets

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Freezing Rain

• Ice crystals fall through warm air aloft and
  melt, Then liquid water falls through air at
  nearly 00C and freezes when it hits the cold
  (00C)ground

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Hail

• Strong updrafts in a cloud force ice particles
  upward
• As they move up and down in the cloud hail stones
  accumulate layers of ice
• Eventually they get too heavy and fall

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Distribution of precipitation in the US
Why?
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Distribution of snowfall in the US

• Why?

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Any questions?
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Have a great day!