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