Precipitation

Cloud particles are so small that they are easily kept afloat by even the

weakest updrafts.

This is why most clouds do not produce rain.

For precipitation to occur, cloud particles must grow in size!

Cloud Constituents

• Condensation nuceli, r=0.1µm .

• Typical Cloud Droplet, r=10µm

• Large Cloud Droplet, r=50µm

• Typical Rain Droplets, r=1000µm (1mm)

Rain drops large enough to fall must grow 100 times greater than typical Cloud droplets.

How do rain drops grow?• Growth by condensation alone does not produce droplets large

enough to fall as rain.

• Collision and Coalescence– Warm clouds (up to 30o N and S of the equator)– Collector droplet over takes smaller droplets. They collide and join

together.

• Bergeron Process – Saturation vapor pressure for ice is less than that for supersaturated

liquids, at the same temperature.

• Rimming and Aggregation– Aides the Bergeron Process

Bergeron Process1. Ice crystals and supercooled liquids co-exist in clouds.2. The vapor pressure of the air is equal to the saturation

vapor pressure. The air is saturated with respect to the liquid droplets (balance between evaporation and condensation)

3. For ice crystals the vapor pressure exceeds the saturation vapor pressure and ice crystals begin to grow.

4. As the ice crystals grow water vapor is depleted in the air and the air becomes undersaturated for water droplets. The liquid water evaporates.

5. Ice crystals continue to grow at the expensive of liquid water.

How do ice crystals grow larger?

• Rimming and Aggregation– Rimming - ice crystals act as nuclei and may collide

with supercooled liquids in clouds and the liquid may freeze onto them.

– Aggregation - two or more ice crystals may collide and stick together.

Forms of Precipitation

• 1) Snow - frozen, crystalline precipitation that forms and remains in the ice stage as it descends

• Results from the growth of ice crystals through the Bergeron process and rimming and aggregation.

• Occurs as six-sided crystals of a variety of forms– Depends on temperature, cooling rate and moisture content.

• Sizes of crystals are 50 µm to 5 mm

• Snow pack - warmer moister

• Powder - colder, drier

Snow Crystals

Snow Crystals

Snow Crystals

Snow Crystals

Snow Crystals

Lake Effect• Snowfall is enhanced downwind of the great lakes or

any large body of water.

1. Lakes or other bodies of water remain relatively warm into earlier winter. (water has a higher heat capacity)

2. As cold air from the north flows over lakes, heat and water vapor are transferred upward and the air becomes moist and unstable.

3. As the air passes over the shore, the effects of topography and vegetation slow the wind. The decrease in wind speed results in (speed) convergence, which is a mechanism for uplift.

Therefore passage of cold air over a lake provides three mechanisms for cloud formation and precipitation.

1) Uplift2) Unstable air3) Moisture

Forms of Precipitation• 2) rain - precipitation arriving at the surface in the

form of liquid droplets, usually between 5µm and 5mm in size.

• In the mid-latitudes( 30 - 60o) most precipitation is initiated through the Bergeron process.

• 3) graupel - ice crystals that grow by rimming sometimes have a porous with bubbles and a spongey texture and milky appearance. The original snow crystals are no longer identifiable

• 4) hail - layered ice crystals.

• 5) Sleet - precipitation in the form of ice pellets, resulting when raindrops freeze before reaching the surface.

• Requires– Temperature inversion– Usually a warm front– Precipitation must fall through a thick section of cold air.

• 6) Freezing rain - rain droplets that freeze as they hit the surface forming a layer of ice on the surface..

Measuring Precipitation

• Raingauge - instrument used to measure amount of rainfall– Collecting surface with 8 inch diameter.– Precipitation funnels into a tube with 1/10 the

diameter of the collecting surface.– The 10X amplification increases the precision

of the measurement.

Measuring Precipitation

• Tipping bucket - instrument used to measure amount of rainfall– Water is funneled (like a raingage) into one of two pivoting

buckets.– One bucket is always upright and the other tipped– Whent the bucket collects rain to a certain depth, it tips over

and the other bucket is brought upright.– The tipped bucket triggers an electric current to a computer

and the time and amount of rain of recorded.– Gives a measure of both amount and intensity of rainfall.

Measuring Precipitation

• Weighing Bucket - similar to tipping bucket but the weight of the water is converted to a depth of rainfall.

Measuring Precipitation• Problems with raingages

1. Point measurements - they represent rainfall at a given point.

2. Splash - water may splash off of the collecting surface.

3. Wind may deflect precipitation.

4. Water may bounce from the ground into the collecting surface.

5. Measurements at sea.Surface is not levelOcean water splash

Measuring Precipitation

• Weather Radar - intensity of precipitation is measured by emitting microwave radiation with = several centimeters.

– Precipitation scatters the emitted radiation back to a receiver which records the intensity of backscattered radiation.

Measuring Precipitation

• Snow Measurement– Raingages are not reliable for snowfall measurement.

– Depth of accumulated snow is measured directly and converted to water equivalent (10:1)

– Snow pillow - large mattress filled with anti-freeze and connected to a pressure gauge.

Cloud Seeding• Injection of materials into non-precipitating clouds

1. Dry Ice (frozen CO2) - dry ice lowers the temperature enough to initiate homogeneous nucleation of ice.

2. Silver iodide - acts as nuclei to initiate the Bergeron process.

1. Problems1. Not cost effective2. Drains moisture from the air which needs to be replenished, affecting

areas downwind3. Poor results

Cloud Coverage

Term Coverage (%)Overcast >90Broken 60 - 90 Scattered 10-60Clear <10