Chapter 7

Precipitation Processes Precipitation is any form of water that

falls from a cloud and reaches the ground.

How do cloud drops grow?When air is saturated with respect to a flat

surface it is unsaturated with respect to a curved droplet of water.○ Super saturated

Precipitation Processes Collision & Coalescence

Droplets of different sizes collide and coalesce into larger droplets; warm cloud process

Ice-Crystal Process○ Cold clouds a mixture of ice & water○ Ice crystals grow at expense of surrounding

water droplets○ Saturation vapor pressure greater over water

as compared to ice.

Stepped ArtFig. 7-5, p. 169

Precipitation Processes Topic: Freezing of Cloud Droplets

Spontaneous or homogeneous freezingIce embryo

Precipitation Processes Cloud Seeding

Inject cloud with small particles that act as condensation nuclei, starting the precipitation process.

NEED CLOUDS: seeding does not generate clouds

Cold clouds with a low seed ration bestDry ice, silver iodide

Stepped ArtFig. 7-12, p. 174

Precipitation in Clouds Starts quickly Most Precipitation formed through

accretion Many times rain starts as ice

Precipitation Types Rain: falling drop of liquid water

Drizzle less than 0.5 mmVirgaCloudburst

Snow: frozen water falling from sky (crystal or flake)Most precipitation starts as snowFreezing level, snow & cloud appearance, fall

streaks, drifting snow, blizzardA blanket of snow is a good insulator

Precipitation Types Topic: Tear Drops

Raindrops not tear shapedShape is size dependent

○ Less than 2 mm = sphere○ Greater than 2 mm = flattened sphere

Precipitation Types Topics: Sounds and snow

A blanket of snow will act like an acoustic tile and absorb sound waves.

Topics: Snow with Temperature above FreezingUnsaturated wet bulb temperature below or

equal to 0°C, rain cooled by evaporation forms snow despite environmental temperature above freezing.

Precipitation Types Sleet: air below freezing, then travels

through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface.

Freezing Rain: ground surface is freezing as rain hits the surface it freezes.

Precipitation Types Observation: Aircraft Icing

Aviation hazard is created by the increase in weight as ice forms on the body of the airplane.

Spray plane with anti-freeze.

Precipitation Types Snow Grains: solid equivalent of drizzle,

no bounce or shatter Snow Pellets: larger than grains,

bounce, break, crunch underfoot Graupel: ice particle accumulation with

rime Hail: graupel act as embryo in intense

thunderstorm, grow through aggregation as pushed up by updraft.

Stepped ArtFig. 7-29, p. 185

Measuring Precipitation Instruments

Rain gauge: standard, tipping bucket, weighing○ Snow: average depth at 3 locations, 10:1 water

equivalent Doppler Radar

Transmitter generates energy toward target, returned energy measured and displayed○ Brightness of echo = amount/intensity of rain

Doppler: measures speed of horizontal rain

Stepped ArtFig. 7-33, p. 188

Measuring Precipitation Measuring from space

Specific satellites designed to assess clouds, atmospheric moisture, and rain○ TRMM○ CloudSat