Chapter Eight Notes - Chapter 8 Precipitation I Cloud...

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Chapter 8: Precipitation I) Cloud droplets grow larger A) Ordinary cloud droplet has a diameter of 20 µ m 1) 100x smaller than a typical raindrop 2) When in equilibrium size of droplet does not change a) Total number of vapor molecules around the droplet remains fairly constant b) At saturation vapor pressure or equilibrium vapor pressure c) Cloud droplet has higher equilibrium vapor pressure than flat surface water  because water molecules are less strongly attached to a curved water surface  and evaporate more readily d) Curvature effect: more curvature=greater rate of evaporation i) To keep droplet in equilibrium more vapor molecules are needed to replace  molecules that are evaporating ii) Smaller cloud droplets have greater curvature, require greater vapor pressure  to keep them from evaporating away e) To stay in equilibrium air must be supersaturated (relative humidity greater than  100%) 3) Not in equilibrium size will increase/decrease a) Relative humidities less than saturation vapor pressure cause drops to shrink by  evaporation b) Relative humidities greater than saturation vapor pressure cause drops to grow  by condensation B) Relative humidities are rarely greater than 100% so something else causes droplets to  grow -- Solute effect 1)  when a solute is added to water the equilibrium vapor pressure is reduced 2) Example: a) Salt ions in solution bind closely with water molecules  b) Makes it more difficult for water molecules to evaporate C) As air cools relative humidity increases, which is countered by growing droplets that  remove more water vapor 1) When droplets are small they cant condense water vapor easily so relative humidity  increases 2) Larger droplets are more efficient in removing water vapor unit rate at which water  vapor is removed from the air equals the rate at which it is made available for  condensation, relative humidity decreases D) Over land masses there are large concentrations of nuclei competing for available  supply of water vapor, over oceans the concentration of nuclei is less and there are  fewer but larger cloud droplets
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E) Cloud droplets require only slight upward air currents to keep them suspended 1) Droplets that do fall descend slowly and evaporate in drier air beneath the cloud 2) Most clouds cant produce precipitation 3) Takes about 1 million average size cloud droplets to make an average size raindrop F) Rain drops are produced in two ways: 1)
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