Small Raindrops


In this chapter we will investigate the steady-state behavior of small raindrops or drizzles. We will neglect the mist effect and the wind effect, and consider only raindrops that are spherical in shape. The spherical shape of a small raindrop with diameter less than 2 mm is caused by surface tension. Cohesive forces among water molecules at the surface of a small raindrop cause surface tension. These surface molecules have a tendency to contract to form a spherical shape in order to minimize their excess surface energy. As the raindrops fall through the atmosphere, they are fighting against the drag forces at their bottom surface. These drag forces tend to flatten the spherical raindrop’s bottom and change the shape of the raindrop. At the same time they collide and combine with other raindrops to increase their size, while taking on irregular shapes.

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