Environmental Science 102
Precipitation Processes

Precipitation Definitions



Cloud droplets start at ~0.02 mm in size.
Typical raindrop is ~2 mm in diameter.
1 million time larger than a cloud droplet.
Condensation alone is insufficient to create large enough drops to precipitate.

Bergeron Process

Two important Properties of Water
1) Cloud droplets do not freeze at 0°C.
2) Saturation vapor pressure above ice crystals is lower than above supercooled liquid droplets.(this means than water escapes more readily from liquid droplets than from the solid ice)

Bergeron Process

Both ice and liquid are present.
Air around the ice crystal is super-saturated.
Water will deposit on the ice faster than it will sublimate.
Water evaporates off droplets providing water for ice growth.
Ice will grow large enough to fall (falls as rain, if the ice melts).


Warm clouds – clouds located below the freezing level
Large droplets form (>0.02 mm)
These large droplets fall, colliding with smaller droplets growing larger.
Commonly occurs:
Most efficient:

Forms of Precipitation

Mist: 0.005 – 0.05 mm, liquid
Drizzle: less than 0.05 mm, liquid
Rain: 0.5 to 5 mm, liquid
Virga: 0.5 to 5 mm, liquid
Sleet: 0.5 to 5 mm, solid
Glaze (Ice Storms); layers of ice 1 mm – 2 cm thick, solid
Rime: solid coating
Snow: 1 mm – 2 cm, solid
Graupel: 2 – 5 mm, solid
Hail: 5 mm – 10 cm, solid

Precipitation Measurement

Rain gauge measurement: Units of depth of fall per unit time.

Snow Fall

Measurement of Snow fall?

Precipitation Processes

Three processes of precipitation
1) Orographic precipitation
2) Convectional precipitation
3) Cyclonic precipitation

Orographic Precipitation

Moist air is forced up and over a mountain range.

Convectional Precipitation

Moist air rises due to solar heating.