Environmental Science 102
Precipitation Processes


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
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 from droplets providing water for ice growth.
Ice will grow large enough to fall.
During the fall, ice grows larger as it intercepts droplets
If ice breaks up during the fall, each small crystal becomes and additional freezing nuclei to continue this process.
Ice will fall as rain, if the ice melts before reaching the surface.


Warm clouds - clouds located below the freezing level
Large droplets form (>0.02 mm)
These large droplets fall colliding with smaller droplets
Then they fall faster as they grow larger, and increase their chance for more collisions.
Clouds with thick vertical development or strong updrafts have a better chance of producing precipitation in this manner.
Most efficient over tropical oceans (tends to produce large droplets due to fewer condensation nuclei).

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: rain which evaporates before reaching the ground
Sleet: 0.5 to 5 mm, solid: forms as rain falls through a cold layer of air and partially or completely freezes
Glaze (Ice Storms); layers of ice 1 mm - 2 cm thick, solid: super-cooled raindrops come into contact with surfaces that are below 0°C and freeze on that surface.
Rime: deposit of ice formed by fog or cloud droplets freezing on cold surfaces.
Snow: 1 mm - 2 cm, solid: Crystals of water are minerals. Different snow crystals form at different temperatures.
Graupel: 2 - 5 mm; rime encrusted snow
Hail: 5 mm - 10 cm, solid: hard, rounded pellets or irregular lumps of ice. Concentric shells which form as the stones travel on strong updrafts and downdrafts in cumulonimbus clouds

Dew and Frost

Dew is condensation on objects that have been cooled below the dew point temperature of the surrounding air.
Hoar Frost (white frost) forms when the dew point is below freezing. Water vapor in the air changes directly to solid ice (deposition) upon contact with the cold surface.

Precipitation Measurement

Rain gauge measurement: Units of depth of fall per unit time.
One cm of rainfall would cover the ground with one cm deep water if it did not run off, evaporate or soak into the ground.
Tipping-bucket Gauge
Weighing Gauge

Snow Fall

Measurement of Snow fall?
Water Equivalent - snow is melted to give a rain fall equivalent.

Measurement Errors

Rain and snow fall measurement tend to under-record the actual fall amount.
Turbulence, splash, surface tension and evaporation can all lead to lower recorded amounts in a rain gauge.
Under-recording in U.S. ranges from 7 to 20%; high latitude stations as high as 80%.

Radar Measurements

Weather Radar measures the intensity, rate, and movement of precipitation.