|MadSci Network: Earth Sciences|
Two forces act on a falling raindrop. The Earth's gravitational force is a downward acting force and the force exerted by the air (frictional resistance) is an upward acting force. Earth's gravity pulls objects downward at a constant rate, 9.8 meters per second each second ( 9.8 meters per second squared). The raindrop's fall will accelerate (or speed up) until the gravitational force is equal to the force of the air pushing up against it. The net force on the raindrop is then zero and according to Newton's second law, the drop stops accelerating and continues to fall at a constant speed. This speed is called the terminal velocity. The terminal velocity depends on the size of the raindrop. Smaller droplets have lower terminal velocities. (The smallest drops, cloud droplets, are suspended in the atmosphere and do not fall.) The upward force exerted on the raindrop is proportional (directly related) to the cross-sectional area of the raindrop. If the raindrop is round, then its area is given by the area of a sphere, ( (pi) times (the radius of the raindrop squared) ). The downward force is proprotional to the volume of the sphere, ( (4/3 pi) times (radius of the raindrop cubed) ). For larger drops, the downward force changes more, at a cubic rate, while the upward force changes less, at a squared rate. The downward force on the raindrop is greater with larger raindrops. In reality, raindrops are not round. The smallest cloud droplets are indeed spheres. Larger drops are distorted by the upward force and are essentially squashed as they fall, becoming flat on the bottom. A great little book about raindrops and other stuff is From Raindrops to Volcanoes: Adventures in Sea Surface Meteorology by Duncan Blanchard. It is out of print but can be found in libraries and used book stores.
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