Precipitation Processes • SIZES OF: NUCLEI, WATER DROPLETS, • and WATER DROPS • Factors of 100 X • Condensing Nuclei 0.2 m • Cloud Droplet 20 m • Raindrop 2,000 m
Precipitation Processes • Cloud Droplets -- Form from a condensing nucleus. Droplets form at relative humidity well below 100%, e.g., around 78%. Because many nuclei are hygroscopic (e.g., salt nuclei) there is a reduction of the vapor pressure because of the molecular bond with the water molecule. This reduces the vapor pressure and is called the solute effect.
Precipitation Processes • Cloud droplets are in equilibrium with their environment. There are more molecules surrounding the curved surface because that surface has less surface bonding than a flat surface. Hence the cloud droplet has a higher equilibrium vapor pressure. This is the curvature effect.
Precipitation Processes • The region around a cloud droplet is supersaturated so it is above 100% RH. • If the moisture continues (water supply) after condensation the droplet increases, if not it decreases. • Over water (many nuclei) thousands of droplets / cm3 • Over land (fewer nuclei) hundred droplets/cm3
Precipitation Processes • Now if the RH increases, the droplets grow because evaporation from the droplet is less than the condensation. • If the air temp cools, then the humidity increases and the droplet grows further.
Precipitation Processes • Falling drop has a terminal velocity • v = 2ga2/(9η) • where a is the droplet diameter, η is the viscosity of air, g = acceleration of gravity • (Above applies to only droplets) • Volume/ air resistance area ratio = 4a/3 • So larger radii drops will fall faster
Collision and Coalescence • In warm clouds (T > -15oC) Collision and Coalescence plays a major role in producing rain drops from cloud droplets. • Ingredients: liquid water content • range of droplet sizes • updrafts of the cloud • electric charge of the droplets • and cloud electric field.
Bergeron Process • Cold Clouds (T < 15oC) ice-crystal process is the significant process in producing precipitation. • Water droplets are super-cooled and exist down to T = -39oC • At T = -20oC there are more super-cooled water droplets than ice crystals • Nuclei - kaolinite, bacteria (deposition nuclei) and ice crystals (feezing nuclei)