1. Bellwork 4/3 Welcome Back 

2. The Plan: Today: Precipitation formation & types
Wednesday: Intro to Air Pressure & Factors that Affect Wind
Friday: “Quest” on Clouds & Precipitation
Humidity/Dew Point
Air stability
Cloud Formation & Types
Precipitation Formation & Types

3. Precipitation Cloud droplets
Less than 20 micrometers (0.02 millimeter) in diameter
Fall incredibly slow

4. Particle sizes involved in condensation and precipitation

5. Precipitation Formation 2 Processes
Bergeron process
Relies on Supercooling & Supersaturation
Supercooling: water at 0⁰C, but in the liquid state
Supersaturation: air is at over 100% relative humidity relative to ice crystals

6. How does the Bergeron Process work?
Ice crystals collect water vapor
Large snowflakes form and fall to the ground or melt during descent and fall as rain
Main source of precipitation in Middle Latitudes
Why?
How does snow form?

7. The Bergeron process

9. Precipitation Formation 2 Processes
Collision-coalescence process
Warm clouds
Large hygroscopic condensation nuclei
Large droplets form
Droplets collide with other droplets during their descent
Common in the tropics
Why?

10. The collision-coalescence process

11. Precipitation Types: Rain and drizzle
Rain – droplets have at least a 0.5 mm diameter
Drizzle – droplets have less than a 0.5 mm diameter

12. Precipitation Types Snow – ice crystals, or aggregates of ice crystals
“powdery” or “compact” snow depends on air temperature
Lower than -5⁰C = fluffy, light snow; low moisture content
Higher than -5⁰C = heavy composites of snowflakes; high moisture content

13. Precipitation Types Sleet and glaze Sleet Occurs when
Warmer air overlies colder air
Rain freezes as it falls
Glaze (or freezing rain)
impact with a solid causes freezing

14. Precipitation Types Rime Forms on cold surfaces
Freezing of Supercooled fog or Cloud droplets

15. Precipitation Types Hail Hard rounded pellets Concentric shells of ice
Most diameters range from 1 to 5 cm
Formation
Occurs in large cumulonimbus clouds with violent up- and down drafts
Layers of freezing rain are caught in up- and down drafts in the cloud
Pellets fall to the ground when they become too heavy