Class #25: Friday, March 6 Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone Class #25: Friday, March 6, 2009
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
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
Ice crystals can also act as ice nuclei Class #25: Friday, March 6, 2009
Ice crystal types depend on temperature Class #25: Friday, March 6, 2009
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
Saturation vapor pressure favors growth of ice crystals Class #25: Friday, March 6, 2009
Saturation vapor pressure over ice < liquid water Class #25: Friday, March 6, 2009
A few ice crystals grow at the expense of many droplets Class #25: Friday, March 6, 2009
Aggregation: the process of collision and collection of ice Class #25: Friday, March 6, 2009
Overview of the precipitation-formation process Class #25: Friday, March 6, 2009
Different precipitation types: they depend on the sounding Class #25: Friday, March 6, 2009
Even in summer clouds have snow and ice Class #25: Friday, March 6, 2009
Snowfall with subfreezing temperatures below cloud base Class #25: Friday, March 6, 2009
A shallow sub-freezing layer at the ground and freezing rain Class #25: Friday, March 6, 2009
Sleet (ice pellets) with a deeper surface-based inversion Class #25: Friday, March 6, 2009
Hours of freezing rain per year on average varies greatly Class #25: Friday, March 6, 2009
Mean snowfall amounts over the continental U.S. Class #25: Friday, March 6, 2009