Fig. 7-CO, p. 162

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

Fig. 7-1, p. 164

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.

•

Fig. 7-2, p. 165

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

Fig. 7-3, p. 165

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

Table 7-1, p. 166

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.

Fig. 7-4, p. 166

Fig. 7-5, p. 167

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)

Fig. 7-6, p. 168

Fig. 1, p. 169

Fig. 7-7, p. 169

Fig. 7-8, p. 170

Fig. 7-9, p. 170

Fig. 7-10, p. 171

Fig. 7-11, p. 172

Fig. 7-12, p. 173

Fig. 7-13, p. 173

Fig. 7-14, p. 174

Fig. 2, p. 175

Table 7-2, p. 175

Fig. 7-15, p. 176

Fig. 7-16, p. 176

Table 7-3, p. 176

Fig. 3, p. 177

Fig. 7-17, p. 178

Table 7-4, p. 178

Fig. 4, p. 179

Fig. 7-18, p. 179

Fig. 7-19, p. 180

Fig. 7-20, p. 180

Fig. 7-21, p. 180

Fig. 5, p. 181

Fig. 7-22, p. 181

Fig. 7-23, p. 182

Fig. 7-23a, p. 182

Fig. 7-23b, p. 182

Fig. 7-23c, p. 182

Fig. 7-23d, p. 182

Fig. 7-24, p. 182

Fig. 7-25, p. 182

Fig. 7-26, p. 183

Fig. 7-27, p. 183

Fig. 7-28a, p. 184

Fig. 7-28b, p. 184

Fig. 7-29, p. 184

Table 7-5, p. 185

Fig. 7-30, p. 185

Fig. 7-31a, p. 187

Fig. 7-31b, p. 187