Precipitation Chapter 7

Objectives Differentiate several ways that precipitation is formed Distinguish between several types of precipitation

General concept There are several forces acting on a water droplet or ice crystal in a cloud Winds Atmospheric stability Gravity Drag (friction) When a droplet reaches a certain critical mass the force of gravity will exceed the other forces and precipitation will fall Rain drops are 100X larger than cloud droplets

Formation of precipitation Condensation and deposition Collision and coalescence The Bergeron process Warm air Cold air

Condensation and deposition As long as the rate of condensation exceeds the rate of evaporation water will be deposited and accumulate on condensation nuclei Droplets grow very slowly Rarely produces rain drops

Collision and coalescence As water droplets fall through a cloud, the larger droplets fall faster than the smaller ones, thus they collide If the droplets coalesce a larger drop is formed If it gets big enough rain will fall

Collision efficiency A drop must be larger than the other drops to be an efficient collider If the drop is too big it will be less efficient, because it creates high pressure that pushes small drops out of the way

Cold and cool clouds Cold < 00C throughout Cool Part is < 00C

The Bergeron process Vapor pressure over ice is less than vapor pressure over water of the same temperature Thus, water molecules move from water to ice and freeze on the ice If the crystals get large enough they will fall If they fall through cold air they will stay frozen and it will snow If they fall through warm air they will melt and it will rain

Growth of ice crystals Riming – super-cooled water freezes onto ice crystals Aggregation – Ice crystals join together to form large snow flakes. Happens more readily if ice crystals have a thin coating of liquid

Types of precipitation snow rain sleet graupel hail freezing rain

Rain Can originate as liquid drops or ice. Ice melts as it falls through warmer air Shape of rain drops

Snow Ice crystals grow in a cloud. Variety of shapes depending on temperature

Graupel Ice crystals grow by riming Contain air pockets Has a spongy texture

Sleet Ice crystals melt as they fall through warn air aloft. Then water drops freeze as they fall through cold air below and hit the surface as ice pellets

Freezing Rain Ice crystals fall through warm air aloft and melt, Then liquid water falls through air at nearly 00C and freezes when it hits the cold (00C)ground

Hail Strong updrafts in a cloud force ice particles upward As they move up and down in the cloud hail stones accumulate layers of ice Eventually they get too heavy and fall

Distribution of precipitation in the US Why?

Distribution of snowfall in the US Why?

Any questions?

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