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• 1 Precipitation

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• What are the requirements for cloud formation?

  1. Sufficient water vapour
  2. condensation (supersaturation of air)
  3. aerosols (which act as cloud condensation nuclei (CCNs)
  • dust, smoke, sea salt
    

• Cloud formation - condensation, sequence

  • Pressure decreases with height
  • rising air parcels expand
  • costs energy
  • lifting by, e.g.
  • Presence of front,
  • convection,
  • orography
  • slow lifting, large areas
  • stratiform cloud formation
  • fast lifting, small areas
  • convective cloud formation
  • causes cooling of the air with increasing height
  • air becomes saturated
  • vapour pressure becomes larger than the saturated vapour pressure
  • results: condensation, cloud droplets
    

• Clausius - Clapeyron (CC) relation

  • Saturated vapour pressure is a strongly nonlinear function of temperature
  • when air reaches saturation vapour pressure, the water vapour in it, will condense
    

• On which depends the evaporation rate?

  The evaporation rate depends on the curvature effect of the droplet. Small droplet have a high curvature, and a larger chance to evaporate. The bigger the droplet, the smaller the curvature and thus the better the droplet will survive and won't evaporate.

• What is stated in Raoult's law

  
  The saturated vapour pressure Pa of the solvent A over a solution is always lower than that over the pure solvent.
  
  With the addition of a solute, the proportion of the surface area occupied by water molecules is reduced, so that the number of water molecules escaping from the surface, and therefore the equilibrium vapour pressure of the solution, should be reduced relative to that of pure water.
  
  
  Smaller cloud droplets can remain stable due to physical mechanism as described by Raoult's law

• How does the collision-coalescence process works?

  Large (faster) collector drops collide and coalesce with smaller (slower) cloud droplets

• What is an important factor with the collision-coalescence process?

  The time needed by a collector drop (while colliding and coalescing with smaller droplets) to fall through the cloud.

• What cloud do you get with limited time available to collect smaller droplets with the collison-coalescence process?

  Relatively thin stratiform clouds: limited time available to collect smaller droplets, this results in smaller raindrops.

• Which cloud type forms when there is more time available to collect droplets with the collision-coalescence process?

  Vertically extended convective cloud (with updraft): more time available to grow; result: larger raindrops.

• Wegener-Bergeron-Findeisen process (principle)

  
  Principle of ice growth through vapour deposition on ice crystals at the expense of water.
  
  Wegener: theory that if crystals in clouds grew large enough to fall out, this could be a viable precipitation mechanism.