14. percipitation process

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Simplified precipitation process

H2OV ------> liquid water (Clouds)--------> precipitation

cloud droplet size
compared to raindrop?

-avg. size is about 20 microns in diameter & fall very slowly, 1000 m/48 hrs; usually evaporate
-1 raindrop = about 1 million cloud droplets

how do cloud droplets avoid evaporation?

they must coalesce (join together to form a raindrop)

2 mechanisms to explain precipitation formation:

1) Bergeron Process: or Ice-Crystal Process
2) Collision-Coalescence Process

Bergeron Process

-discovered by a Swedish meteorologist, Tor Bergeron in early 1930's
-primarily process for forming rain in the mid and high latitudes

2 properties of water that the Bergeron Process uses

1. pure water in the air doesn't freeze until -40 degrees C
2. saturation vapor pressure over ice crystals is much lower than over suspercooled(water<0 degree C) liquid water

other conditions needed for Bergeron Process

-need freezing nuclei to initiate deposition of water vapor
-need T°'s below -10°C; at T°'s between -10° & -20°C will have both liquid drops & ice crystals
-difference in vapor pressure allows for supersaturation to exist; RH's > 100%

what happens under supersaturation conditions?

-ice crystals collect more water vapor than they lose & thus grow
- as they grow, may break-up & these pieces act as freezing nuclei to make more ice crystals
-these join to make snow crystals & then snowflakes

Collision-Coalescence Process

-primary process in tropics for rain drop formation
-need larger cloud droplets to form "large" condensation nuclei or hydroscopic nuclei
-small droplets don't coalesce & collide by themselves very well
-max size of a raindrop =5mm; if it gets bigger then it gets pulled apart by friction
-this process is the idea behind cloud seeding.

Fall or Terminal Velocities of Rain

Type Diameter Velocity
(mm) (kph) (mph)
Typical cloud
droplet .02 04 .03
Drizzle .5 7 4
Raindrop 2 - 5 23 - 33 14 - 20

precipitation types

-freezing rain


at least .5 mm to 5 mm in size; from nimbostratus and cumulonimbus clouds

following Bergeron Process
frozen(cloud)-------melts ---------> liquid (before it hits ground)

following collision coalescence process
frozen(cloud)--------------> liquid (as it hits ground)


1 - 2 mm in size; water vapor deposited as ice crystals that stay frozen

Bergeron Process only

frozen(cloud)----------> frozen (as it hits ground)

equivalent of snow to rain

10 inches of snow =1 inch of rainfall
(does vary with heaviness of snow)


0.5 - 5 mm in size; freezes as it falls and is a frozen raindrop before it hits the ground

frozen(cloud)---melts----->liquid(in atmo) ------>freezes (refreezes before it hits the ground)

frozen(cloud)---------> liquid(in atmo) ------->freezes (before it hits the ground)

freezing rain

0.5 - 5 mm in size; supercooled raindrops which
freeze on contact with solid objects/surfaces

frozen(cloud)------>liquid (liquid as it hits the it then refreezes)-------->liquid/frozen (melts in atmo as it falls)


liquid(cloud)---------> liquid/frozen (liquid as it hits the ground then refreezes)


5 mm - 10 cm+; hard, rounded pellets or lumps of ice; only produced in large cumulonimbus clouds/ thunderstorms
- largest reported hailstone = 1.67 lbs. & over 5.5" in diameter
-have a series/network of updrafts and down drafts within the thunderstorm that move hail up and down in clouds

things that affect precipitation
Idealized Continent for precipitation:

Pressure systems, winds, seasonality, and Landmasses & Ocean

Pressure systems

-the areas of uplift (ITCZ) and subsidence (STH) affect precipitation patterns


-don't blow in nice straight paths and where they meet, like in the polar front, are turbulent


these pressure belts and winds shift from season to season

Landmasses & Ocean

-mountains get in the way of winds and moisture
-differential heating

Some exceptions to the Idealized pattern which give us a truer picture of precipitation patterns:

1. Subtropical high pressure cells (STHs)
2.Rainshadow deserts
3. Monsoon

Subtropical high pressure cells (STHs)

-don't have the same characteristics on both the east and west side

Eastside of STH:

-subsidence, temp increase, and upwelling of cold ocean currents lead to stable, dry condition

EX: Subtropical deserts; Sahara of N. Africa, deserts of Baja California

Westside of STH:

--little subsidence, more uplifting, convergence, and warm ocean currents lead to greater instability, and wet conditions
EX: Southeastern U.S., esp. Florida

Rainshadow deserts

-due to mountain barriers and orographic effects
-leeward side often much drier than windward side
-in the Gobi desert in China

EX: Nevada deserts of western U.S., Patagonia in southern South America


-an annual cycle of dryness and wetness with seasonally, shifting winds produced by changing atmospheric pressure systems

EX: southern Asia(India); also the southwestern U.S.

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