Physical Geography 101


Definition of precipitate:

Formation of Precipitation

Condensation results in droplets - 0.05-0.1 mm in size
Collisions of these droplets increases droplet size to 0.5 mm - drizzle
Continued collisions increase size to 1-2 mm - rain
Ice crystals also form in clouds - supercooled droplets of water collide with a particle, causing the water to crystallize.
Large crystals will fall and be deposited as snow (rain if ice melts before reaching surface).

Snow Flakes and Other Forms of Precipitation

Crystals of water are minerals.
Different snow crystals form at different temperatures/humidity.

Morphology diagram showing air temperature, water saturation and ice crystal formation
Image created and copyright by Kenneth G. Libbrecht, Caltech (

Sleet - raindrops falling through cold air become partially or completely frozen.
Ice Storms - supercooled raindrops contact below 0° surfaces and freeze.
Hail - ice crystals formed in convection cells in thunderstorms.

Measurement of Precipitation

Rain gauge:
Units of depth of fall per unit time.
cm/day cm/month cm/year
One cm of rainfall would cover ground with one cm deep water if it did not run off, evaporate or soak into ground.
Measurement of Snow fall?

Precipitation Processes

Three processes of precipitation

Orographic Precipitation

1) Moist air is forced up and over a mountain range.
- As the air rises it cools adiabatically.
2) Condensation begins once air mass has reached dew point temperature.
- As air mass continues to rise, precipitation will occur if there is sufficient moisture in the air.
- Air mass continues to cool, along Wet Adiabatic Lapse Rate.
3) Air mass reaches summit, begins descending lee side of range.
- Air becomes compressed as it descends, warming along Dry Adiabatic Lapse Rate.
4) Air mass is now warmer and drier than when it started.

Convectional Precipitation


Strongest form of convectional precipitation.
Iintense local storms associated with tall, dense, cumulonimbus clouds with strong updrafts and downdrafts.
Consist of multiple convection cells.
Heights of clouds can be 6-12 kilometers.
Intense precipitation, hail and lightning are common.