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Class #25: Friday, March 6

Class #25: Friday, March 6. Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone. Ice crystal formation. The ice crystal process begins with the formation of ice crystals At temperatures below -40ºC, ice crystals can form spontaneously (deposition)

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Class #25: Friday, March 6

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  1. Class #25: Friday, March 6 Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone Class #25: Friday, March 6, 2009

  2. Ice crystal formation • The ice crystal process begins with the formation of ice crystals • At temperatures below -40ºC, ice crystals can form spontaneously (deposition) • At higher temperatures, small particles called ice nuclei form surfaces for water vapor to freeze. • There are lots less ice nuclei than CCN Class #25: Friday, March 6, 2009

  3. Mixed clouds have water droplets and ice crystals • At temperatures just below freezing, few substances can act as ice nuclei • At lower temperatures (higher in the cloud) more substances can act as ice nuclei • Ice nuclei have molecular structures similar to the ice crystal • Condensation of supercooled water occurs for T<0º without an ice nucleus Class #25: Friday, March 6, 2009

  4. Ice crystals can also act as ice nuclei Class #25: Friday, March 6, 2009

  5. Ice crystal types depend on temperature Class #25: Friday, March 6, 2009

  6. The ice crystal process in a cloud of mixed ice and liquid • The ice crystal process takes place when a few ice crystals are present with supercooled liquid cloud droplets • The saturation vapor pressure is lower for ice than for water • Water vapor diffuses to deposit on the supersaturated crystals and evaporate from the subsaturated drops. Class #25: Friday, March 6, 2009

  7. Saturation vapor pressure favors growth of ice crystals Class #25: Friday, March 6, 2009

  8. Saturation vapor pressure over ice < liquid water Class #25: Friday, March 6, 2009

  9. A few ice crystals grow at the expense of many droplets Class #25: Friday, March 6, 2009

  10. Aggregation: the process of collision and collection of ice Class #25: Friday, March 6, 2009

  11. Overview of the precipitation-formation process Class #25: Friday, March 6, 2009

  12. Different precipitation types: they depend on the sounding Class #25: Friday, March 6, 2009

  13. Even in summer clouds have snow and ice Class #25: Friday, March 6, 2009

  14. Snowfall with subfreezing temperatures below cloud base Class #25: Friday, March 6, 2009

  15. A shallow sub-freezing layer at the ground and freezing rain Class #25: Friday, March 6, 2009

  16. Sleet (ice pellets) with a deeper surface-based inversion Class #25: Friday, March 6, 2009

  17. Hours of freezing rain per year on average varies greatly Class #25: Friday, March 6, 2009

  18. Mean snowfall amounts over the continental U.S. Class #25: Friday, March 6, 2009

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