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
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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.
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.