10 12. Precipitation Development a. Warm Cloud Process i. Clouds form at temperatures above freezing, but the entire cloud is not necessarily above freezing. However, all droplets remain liquid water. ii. Collision and coalescence occurs here – droplets continue to collide until they get big enough to fall to the ground as rain. 1. Cloud droplets of different sizes are necessary for collisions. Droplets must grow to a certain size before falling or they will evaporate before reaching the surface. 2. Larger raindrops have smaller ratio of surface to weight and thus have a higher terminal velocity, that is, they fall faster. b. Cold cloud process i. Cloud formation takes place below freezing ii. Most of cloud will be below freezing iii. Cloud tops are only ice (called glaciation) iv. Saturation pressure over a water surface is greater than that over an ice surface at the same subfreezing temperature. This establishes a force that causes water vapor molecules to move from liquid droplets to ice crystals.
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