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 Art Fig. 7-5, p. 169

Precipitation Processes  Topic: Freezing of Cloud Droplets Spontaneous or homogeneous freezing Ice 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 best Dry ice, silver iodide

Stepped Art Fig. 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 mm Virga Cloudburst  Snow: frozen water falling from sky (crystal or flake) Most precipitation starts as snow Freezing level, snow & cloud appearance, fall streaks, drifting snow, blizzard A blanket of snow is a good insulator

Precipitation Types  Topic: Tear Drops Raindrops not tear shaped Shape 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 Freezing Unsaturated 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 Art Fig. 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 Art Fig. 7-33, p. 188

Measuring Precipitation  Measuring from space Specific satellites designed to assess clouds, atmospheric moisture, and rain ○ TRMM ○ CloudSat