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Fig. 7-CO, p. 162 Precipitation Processes SIZES OF: NUCLEI , WATER DROPLETS , and WATER DROPS Factors of 100 X Condensing Nuclei 0.2 m Cloud Droplet 20 m Raindrop 2,000 m Fig. 7-1, p. 164 Precipitation Processes

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Fig. 7-CO, p. 162

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precipitation processes
Precipitation Processes
  • Factors of 100 X
  • Condensing Nuclei 0.2 m
  • Cloud Droplet 20 m
  • Raindrop 2,000 m
precipitation processes4
Precipitation Processes
  • Cloud Droplets -- Form from a condensing nucleus. Droplets form at relative humidity well below 100%, e.g., around 78%. Because many nuclei are hygroscopic (e.g., salt nuclei) there is a reduction of the vapor pressure because of the molecular bond with the water molecule. This reduces the vapor pressure and is called the solute effect.
precipitation processes5
Precipitation Processes
  • Cloud droplets are in equilibrium with their environment. There are more molecules surrounding the curved surface because that surface has less surface bonding than a flat surface. Hence the cloud droplet has a higher equilibrium vapor pressure. This is the curvature effect.
precipitation processes7
Precipitation Processes
  • The region around a cloud droplet is supersaturated so it is above 100% RH.
  • If the moisture continues (water supply) after condensation the droplet increases, if not it decreases.
  • Over water (many nuclei) thousands of droplets / cm3
  • Over land (fewer nuclei) hundred droplets/cm3
precipitation processes9
Precipitation Processes
  • Now if the RH increases, the droplets grow because evaporation from the droplet is less than the condensation.
  • If the air temp cools, then the humidity increases and the droplet grows further.
precipitation processes10
Precipitation Processes
  • Falling drop has a terminal velocity
  • v = 2ga2/(9η)
  • where a is the droplet diameter, η is the viscosity of air, g = acceleration of gravity
  • (Above applies to only droplets)
  • Volume/ air resistance area ratio = 4a/3
  • So larger radii drops will fall faster
collision and coalescence
Collision and Coalescence
  • In warm clouds (T > -15oC) Collision and Coalescence plays a major role in producing rain drops from cloud droplets.
  • Ingredients: liquid water content
  • range of droplet sizes
  • updrafts of the cloud
  • electric charge of the droplets
  • and cloud electric field.
bergeron process
Bergeron Process
  • Cold Clouds (T < 15oC) ice-crystal process is the significant process in producing precipitation.
  • Water droplets are super-cooled and exist down to T = -39oC
  • At T = -20oC there are more super-cooled water droplets than ice crystals
  • Nuclei - kaolinite, bacteria (deposition nuclei) and ice crystals (feezing nuclei)