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Fig. 7-CO, p. 162

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  1. Fig. 7-CO, p. 162

  2. 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

  3. Fig. 7-1, p. 164

  4. 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.

  5. 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.

  6. Fig. 7-2, p. 165

  7. 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

  8. Fig. 7-3, p. 165

  9. 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.

  10. 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

  11. Table 7-1, p. 166

  12. 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.

  13. Fig. 7-4, p. 166

  14. Fig. 7-5, p. 167

  15. 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)

  16. Fig. 7-6, p. 168

  17. Fig. 1, p. 169

  18. Fig. 7-7, p. 169

  19. Fig. 7-8, p. 170

  20. Fig. 7-9, p. 170

  21. Fig. 7-10, p. 171

  22. Fig. 7-11, p. 172

  23. Fig. 7-12, p. 173

  24. Fig. 7-13, p. 173

  25. Fig. 7-14, p. 174

  26. Fig. 2, p. 175

  27. Table 7-2, p. 175

  28. Fig. 7-15, p. 176

  29. Fig. 7-16, p. 176

  30. Table 7-3, p. 176

  31. Fig. 3, p. 177

  32. Fig. 7-17, p. 178

  33. Table 7-4, p. 178

  34. Fig. 4, p. 179

  35. Fig. 7-18, p. 179

  36. Fig. 7-19, p. 180

  37. Fig. 7-20, p. 180

  38. Fig. 7-21, p. 180

  39. Fig. 5, p. 181

  40. Fig. 7-22, p. 181

  41. Fig. 7-23, p. 182

  42. Fig. 7-23a, p. 182

  43. Fig. 7-23b, p. 182

  44. Fig. 7-23c, p. 182

  45. Fig. 7-23d, p. 182

  46. Fig. 7-24, p. 182

  47. Fig. 7-25, p. 182

  48. Fig. 7-26, p. 183

  49. Fig. 7-27, p. 183

  50. Fig. 7-28a, p. 184