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