1. Precipitation Chapter 7

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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. Types of precipitation
snow
rain
sleet
graupel
hail
freezing rain

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

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

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

15. 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

16. 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

17. 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

18. Distribution of precipitation in the US
Why?

19. Distribution of snowfall in the US
Why?

20. Any questions?

21. Have a great day!