Precipitation Chapter 7 Part 1 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 – Curvature effect Super saturation needed for droplets to survive in pure water vapor environment Droplet Growth • With cloud condensation nuclei (CCN) we can have growth below supersaturation • Solute effect: impurity replaces water molecules causing the equilibrium vapor pressure surrounding the droplet to be lowered – NCl(NHNaCl, (NH4)2SO4, NH4NO3, etc. • Droplet growth can occur at RH ≈ 78% or more Typical Aerosol Particles http://alg.umbc.edu/usaq/archives/2005_06.html Precipitation Processes • Collision & Coalescence – Droplets of different sizes collide and coalesce into larger droplets; warm cloud process (T > -15°C) Precipitation Types • Drop Shapes – Raindrops not tear shaped – Shape is size dependent • Less than 2 mm = sphere • Greater than 2 mm = flattened sphere 3 mm droplet R. Jaenicke, Institute for Atmospheric Physics (IPA), Johannes Gutenberg University at Mainz, 2002 http://www.mpch-mainz.mpg.de/wolkengalerie/Precipitation/FallingDrop/droplet.htm Cloud droplet rising in updraft, growing by collision/coalescence and falling as large raindrop Precipitation Processes • Ice-Crystal Process (Bergeron) – Cold clouds a mixture of ice & water – Important in middle and high latitudes – Initiate on ice forming nuclei • Clay particles (e. g. kaolinite) are good nuclei – Ice crystals grow at expense of surrounding water droplets – Saturation vapor pressure greater over water as compared to ice Note that there are still liquid water droplets at -20°C – “supercooled” es is greater over water than ice at saturation More vappqppor molecules above liquid, difference in vapor pressure causes water vapor to diffuse toward ice crystal es ((qliquid ) – es(()ice) Summary of Ice Crystal Bergeron process Ice crystal growth at the expense of liquid water droplets Accretion, Aggregation & Snow 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 Cloud Seeding: Methods • Ground based seeding • Burn a solution of silver iodide, sodium iodide and salt in acetone to release silver chloro-iodide particles which create additional ice crystals Cloud Seeding: Methods • A seeding aircraft is used to augment ground seeding operations. The aircraft releases AgI from pyrotechnic flares or solution burners AgI solution burners AgI Flares in Aircraft Flare Rack Precipitation through ice crystal growth Precipitation Types • Rain: falling drop of liquid water > 0.5 mm – Drizzle less than 0.5 mm –Virga – Cloudburst – Trace < 0.01 inches – Light rain 0.01-0.10 inches/hour – Moderate rain 0.10 -0.30 inches/hour – Heavy rain >0.30 inches/hour • Rainfall scavenges (removes) pollutants Virga • Falling droplets that evaporate before reaching the ground Precipitation Types • Snow: frozen water falling from sky as a crystal or flake – Most precipitation starts as snow – A blanket of snow is a good insulator – Without snow cover, ground freezes deeper and take longer to thaw in spring- delaying cultivation – Snow flurry- short duration, often from cumulus clouds – SllSnow squall- bibrie f, heavy snow Observations of melting point Darker above melting point Fallstreaks • Falling ice crystals that evaporate before falli ng to the groun d Ice crystals More Fallstreaks Effects on Visibility INTENSITY CRITERIA • Light Visibility > 1/2 mile • Moderate Visibility >1/4 mile and <1/2 mile • Heavy Visibility 1/4 mile or less • Blizzard ≡ falling or blowing snow with visibility <1/4mile and wind >35 mph for at least 3 hours.