Precipitation

1. Chapter 7 Precipitation

2. 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

3. 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.

6. Stepped Art Fig. 7-5, p. 169

11. Precipitation Processes • Topic: Freezing of Cloud Droplets • Spontaneous or homogeneous freezing • Ice embryo

12. 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

15. Stepped Art Fig. 7-12, p. 174

16. Precipitation in Clouds • Starts quickly • Most Precipitation formed through accretion • Many times rain starts as ice

18. 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

25. Precipitation Types • Topic: Tear Drops • Raindrops not tear shaped • Shape is size dependent • Less than 2 mm = sphere • Greater than 2 mm = flattened sphere

26. 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.

27. 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.

33. 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.

34. 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.

40. Stepped Art Fig. 7-29, p. 185

41. 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

45. Stepped Art Fig. 7-33, p. 188

48. Measuring Precipitation • Measuring from space • Specific satellites designed to assess clouds, atmospheric moisture, and rain • TRMM • CloudSat