Fig. 7-CO, p. 162
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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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Precipitation Processes



  • Factors of 100 X

  • Condensing Nuclei 0.2 m

  • Cloud Droplet 20 m

  • Raindrop 2,000 m

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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.

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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.

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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

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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.

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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

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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.

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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)