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Formation of Cloud Droplets

.3. Pure Water. Supersaturation (%). .2. .1. 100. 95. 10 -15 g NaCl. Relative Humidity (%). 90. 85. 80. 10. .1. 1. .01. Droplet Radius ( m m). Formation of Cloud Droplets. Reading. Wallace & Hobbs pp 209 – 215 Bohren & Albrecht pp 252 – 256. Objectives.

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Formation of Cloud Droplets

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  1. .3 Pure Water Supersaturation (%) .2 .1 100 95 10-15 g NaCl Relative Humidity (%) 90 85 80 10 .1 1 .01 Droplet Radius (mm) Formation of Cloud Droplets

  2. Reading • Wallace & Hobbs • pp 209 – 215 • Bohren & Albrecht • pp 252 – 256

  3. Objectives • Be able to identify the factor that determines the rate of evaporation from a water surface • Be able to identify the factor that determines the rate of condensation of water molecules on a water surface • Be able to draw a curve that shows the relationship between temperature and water vapor pressure at equilibrium for a flat water surface

  4. Objectives • Be able to show supersaturated and subsaturated conditions on an equilibrium curve • Be able to draw a balance of force diagram for a water droplet • Be able to calculate the equilibrium water vapor pressure for a flat water surface • Be able to calculate the equilibrium water vapor pressure for a curved water surface

  5. Objectives • Be able to define saturation ratio and supersaturation • Be able tocalculate saturation ratio and supersaturationof the air • Be able to calculate the critical size of a droplet given a saturation ratio • Be able to distinguish between heterogeneous and homogeneous nucleation

  6. Objectives • Be able to list the three different types of aerosols that may act as cloud nuclei • Be able to describe the characteristics of each type of aerosol that may act as a cloud nuclei • Be able to describe the change in saturation vapor pressure as a result of solute effect

  7. Objectives • Be able to calculate the fractional change in saturation vapor pressure using Raoult’s formula • Be able to pat your head and tummy simultaneously while whistling “Livin’ La Vida Loca” • Be able to identify areas on the Kohler curve that are influenced by solute and curvature effect

  8. Objectives • Be able to define deliquesce • Be able to determine critical radius on a Kohler curve • Be able to determine critical supersaturation on a Kohler curve • Be able to state the condition of a water droplet based on supersaturation on a Kohler curve

  9. Objectives • Be able to describe the operation of a thermal diffusion chamber • Be able to compare CCN spectra for maritime and continental locations • Be able to list the sources for CCN

  10. Formation of Cloud Droplets • Nucleation • Homogeneous Nucleation • Heterogeneous Nucleation

  11. Homogeneous Nucleation • The formation of droplets from vapor in a pure environment

  12. Homogeneous Nucleation • Chance collisions of water molecule • Ability to remain together • Depends on supersaturation

  13. Thermodynamics Reveiw • Molecules in liquid water attract each other • Like to be in between other water molecules

  14. Thermodynamics Reveiw • Molecules at surface have more energy • Don’t need to be surrounded by other molecules

  15. Thermodynamics Reveiw • Molecules are In motion

  16. Thermodynamics Reveiw • Collisions • Molecules near surface gain velocity by collisions

  17. Thermodynamics Reveiw • Fast moving molecules leave the surface • Evaporation

  18. Thermodynamics Reveiw • Soon, there are many water molecules in the air

  19. Thermodynamics Reveiw • Slower molecules return to water surface • Condensation

  20. Thermodynamics Reveiw • Net Evaporation • Number leaving water surface is greater than the number returning

  21. Thermodynamics Reveiw • Net Evaporation • Evaporation greater than condensation • Air is subsaturated

  22. Thermodynamics Reveiw • Molecules leave the water surface at a constant rate • Depends on temperature of liquid

  23. Thermodynamics Reveiw • Molecules return to the surface at a variable rate • Depends on mass of water molecules in air

  24. Thermodynamics Reveiw • Rate at which molecule return increases with time • Evaporation continues to pump moisture into air • Water vapor increases with time

  25. Thermodynamics Reveiw • Eventually, equal rates of condensation and evaporation • Air is saturated • Equilibrium

  26. Thermodynamics Reveiw • Equilibrium • Tair = Twater

  27. Thermodynamics Review • What if? • Cool the temperature of liquid water • Fewer molecules leave the water surface

  28. Thermodynamics Review • Net Condensation • More molecules returning to the water surface than leaving • Air is supersaturated

  29. Water at Equilibrium • Equilibrium Curve Rate of Condendation = Rate of Evaporation es Equilibrium es = water vapor pressure at equilibrium (saturation) Pressure Temperature

  30. Supersaturation • Water Vapor Pressure > Equilibrium es e >es e Pressure Temperature

  31. Supersaturation • Water Vapor Pressure > Equilibrium Net Condensation es e>es e Pressure Temperature

  32. Equilibrium • Water Vapor Pressure = Equilibrium Condensation = Evaporation es e=es Pressure e Temperature

  33. Subsaturation • Water Vapor Pressure < Equilibrium es Net Evaporation Pressure e< es e Temperature

  34. Subsaturation • Water Vapor Pressure < Equilibrium es Net Evaporation Pressure e< es e Temperature

  35. Equilibrium • Water Vapor Pressure = Equilibrium Condensation = Evaporation es e=es Pressure e Temperature

  36. Equilibrium Curve • Assumed for flat water surface es Equilibrium Pressure Temperature

  37. Equilibrium Curve • Different for a water sphere

  38. Water Sphere • Water molecules at surface have higher potential energy • Molecular attraction is pulling them to center

  39. Surface Tension (s) • The surface potential energy per unit area of surface

  40. Surface Tension (s) • The surface energy is contained in a layer a few molecules deep

  41. Surface Tension (s) • Pressure inside the drop is greater than the pressure outside (due to surface tension) Po P

  42. Surface Tension (s) • Let’s derive an expression for the difference in pressure bewteen inside & outside! Po P

  43. Surface Tension (s) • Cut the drop in half!

  44. Surface Tension (s) • Determine the balance of force for the drop

  45. Surface Tension (s) • Force acting to the right • Outside Pressure • Force per unit area • Acts as if force is applied to circle area Po

  46. Surface Tension (s) • Force acting to the right • Outside Pressure Po

  47. Surface Tension (s) • Force acting to the right • Surface Tension • At periphery • Energy per area, or • Force per length

  48. Surface Tension (s) • Force acting to the right • Surface Tension

  49. Surface Tension (s) • Forces acting to the left • Internal Pressure Po P

  50. Surface Tension (s) • Balance of Forces • Outside Pressure • Surface Tension • Internal Pressure Po P

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