class #25: friday, march 6

Class #25: Friday, March 6, 2009 1

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)

• At higher temperatures, small particles called ice nuclei form surfaces for water vapor to freeze.

• There are lots less ice nuclei than CCN


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


Ice crystals can also act as ice nuclei


Ice crystal types depend on temperature


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.


Saturation vapor pressure favors growth of ice crystals


Saturation vapor pressure over ice < liquid water


A few ice crystals grow at the expense of many droplets


Aggregation: the process of collision and collection of ice


Overview of the precipitation-formation process


Different precipitation types: they depend on the sounding


Even in summer clouds have snow and ice


Snowfall with subfreezing temperatures below cloud base


A shallow sub-freezing layer at the ground and freezing rain


Sleet (ice pellets) with a deeper surface-based inversion


Hours of freezing rain per year on average varies greatly


Mean snowfall amounts over the continental U.S.


class #25: friday, march 6

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