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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

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
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
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
Ice crystals can also act as ice nuclei

Class #25: Friday, March 6, 2009


Ice crystal types depend on temperature
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 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 over ice liquid water
Saturation vapor pressure over ice < liquid water

Class #25: Friday, March 6, 2009




Overview of the precipitation formation process
Overview of the precipitation-formation process

Class #25: Friday, March 6, 2009



Even in summer clouds have snow and ice
Even in summer clouds have snow and ice

Class #25: Friday, March 6, 2009






Mean snowfall amounts over the continental u s
Mean snowfall amounts over the continental U.S.

Class #25: Friday, March 6, 2009


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