Precipitation Chapter 7 : Precipitation Chapter 7
Objectives : Objectives Differentiate several ways that precipitation is formed
Distinguish between several types of precipitation
General concept : 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 fal
Rain drops are 100X larger than cloud droplets
Formation of precipitation : Formation of precipitation Condensation and deposition
Collision and coalescence
The Bergeron process Warm air Cold air
Condensation and deposition : 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 : 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 : 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 and cool clouds Cold < 00C throughout Cool Part is < 00C
Part is > 00C
The Bergeron process : 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 : 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 : Types of precipitation rain graupel sleet freezing rain hail
Rain : Rain Can originate as liquid drops or ice. Ice melts as it falls through warmer air
Shape of rain drops
Snow : Snow Ice crystals grow in a cloud. Variety of shapes depending on temperature
Graupel : Graupel Ice crystals grow by riming
Contain air pockets
Has a spongy texture
Sleet : 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 : 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 : 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 : Distribution of precipitation in the US Why?
Distribution of snowfall in the US : Distribution of snowfall in the US Why?
Any questions? : Any questions?
Have a great day! : Have a great day!