Bellwork 4/3 Welcome Back 

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

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

Particle sizes involved in condensation and precipitation

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

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?

The Bergeron process

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?

The collision-coalescence process

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

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

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

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

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