Precipitation I

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Precipitation I
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RECAP

• Moisture in the air (different types of
  humidity).
• Condensation and evaporation in the air (dew
  point).
• Stability of the atmosphere determines the type
  of clouds
• Cloud formation moist air rises up in the
  atmosphere where it expands adiabatically, cools
  down, saturates and forms clouds.
• Chapter 7 precipitation. Any form of water
  (liquid or solid) that falls from a cloud and
  reaches the ground.

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From droplets to raindrops

• Typical sizes (diameter)
• Condensation nuclei 0.2 mm
• Cloud droplet 20 mm
• Raindrop 2000 mm
• The cloud droplets need to grow
• in order to become raindrops!
• Growth is determined by the balance of
  condensation (C) and evaporation (E)
• CgtE the droplet grows
• CltE the droplet gets smaller
• CE the droplet stays the same
• (in equilibrium), hence
• Saturation (equilibrium) vapor pressure

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The growth of cloud droplets

• Curvature effect The saturation water vapor
  pressure depends on the curvature of the water
  surface.
• The larger the curvature the easier it is for the
  water molecules to leave the surface of the
  liquid water.
• The saturation vapor pressure for small droplets
  is higher therefore they require more vapor to
  keep their size
• Bottom line the smaller the droplet, the more
  difficult it is to grow. Small droplets dont
  make it as raindrops.

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The growth of cloud droplets

• If small droplets were to survive and grow, they
  require supersaturation of the air and the help
  of condensation nuclei
• Condensation nuclei jump start the formation of
  the cloud droplets.
• Hygroscopic nuclei condensation begins at
  RHlt100.
• Decrease the starting curvature of the initial
  droplet.
• Solute effect salt particles partially dissolve
  in the water and decrease the saturation vapor
  pressure.

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From droplets to raindrops

• Condensation by itself is a very slow process,
  cannot produce raindrops, only cloud droplets
  (20 mm).
• The cloud droplets may stay suspended in the air
  by air currents for a long time
• Those which happen to descend below the cloud,
  evaporate and dont make it to the ground
• There must be other ways to grow raindrops
• Collision-coalescence process first must discuss
  how droplets fall.
• Ice-crystal (Bergeron) process

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Terminal Velocity

• Gravity makes things fall, constant force -gt
  acceleration
• Air-drag force increases with the speed of the
  falling object
• Eventually gravity and air-drag become equal -gt
  no net force -gt the body falls at constant speed.
• This velocity is called terminal velocity.
• The terminal velocity depends on
• Shape of the body
• Size of the body
• Mass of the body
• Air properties (density).

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Terminal Velocity-Examples
Falling object Mass Area Terminal velocity
Skydiver 75 kg 0.7 m2 60 m/s 134
mi/hr Baseball (3.66 cm) 145 gm 42 cm2 33
m/s 74 mi/hr Golf ball (2 cm) 46 gm 14
cm2 32 m/s 72 mi/hr Hail stone (0.5 cm
radius) .48 gm .79 cm2 14 m/s 31 mi/hr
Raindrop (0.2 cm radius) .034 gm .13 cm2
9 m/s 20 mi/hr
m mass, g gravity acceleration, C-shape
coefficient of proportionality, r-air density, A
- area of the cross section of the body, v-
velocity
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Collision and Coalescence Processes

• Larger drops fall faster, overtake and absorb
  smaller drops
• coalescence
• Larger drops grow faster than smaller drops.
• Larger drops are the first to hit the ground,
  the first raindrops are very large and heavy.

• Smaller drops evaporate before they reach the
  ground.
• Smaller drops are absorbed on the way down

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Droplet growth in warm clouds

• Warm clouds above freezing temperature
• Thick clouds are associated with strong updraft
  winds.
• This maximizes the time the droplet spends in the
  cloud -gt more time for coalescence to take place
• Thick clouds produce large rain drops.
• Thin clouds produce at most a drizzle.

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Cold clouds

• The temperature of a cold cloud drops below the
  water freezing point.
• Below 0 deg C the cloud water droplets are
  supercooled.
• The smaller the droplet, the lower the
  temperature at which it will freeze.
• Below -40 deg C almost all droplets freeze and
  form ice crystals.
• Small particles in the air serve as ice nuclei
• deposition nuclei
• freezing nuclei
• contact nuclei

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Ice Crystal Particles

• There are many more condensation nuclei than ice
  nuclei -gt there are many more water droplets than
  ice crystals in the cloud
• How does snow form?

• The saturation vapor pressure above a water
  surface is larger than the saturation vapor
  pressure above an ice surface.
• Water molecules evaporate more easily than ice
  molecules

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Ice-crystal (Bergeron) process

• Water vapor molecules migrate towards the ice
  crystals.
• Cloud ice crystals grow at the expense of the
  water droplets.

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Ice crystals in the clouds
Accretion
Fracture
Aggregation
Skip the sections Cloud seeding and
precipitation and Precipitation in clouds