Terminal Velocity and The Collision/Coalescence Process
"The particular falling speed, for any given object moving through a fluid medium of specified physical properties, at which the drag forces and buoyant forces exerted by the fluid on the object just equal the gravitational force acting on the object; after which it falls as constant speed, unless it moves into air layers of different physical properties."
- Terminal Velocity is defined by AMS Glossary of Weather and Climate as:
"The speed at which precipitation elements are falling through the adjacent air, up to 9 meters per second for the largest raindrops and much greater for hail."
- For droplets, this terminal velocity is also called, fall speed and is defined as:
In other words, as cloud droplets fall, air resists their downward motion and the amount of this air resistance is based on the size of the drop and its rate of fall, or velocity. The speed of the falling cloud droplet increases until the air resistance equals the downward pull of gravity and then begins to fall at a constant speed, no longer accelerating downward. This constant speed that is finally reached is called the drop's terminal velocity, or fall speed.
- Raindrops usually start out close to spherical in shape but as they begin to fall faster the drag from surrounding air tends to change their shape. In fact, if a droplet has grown large enough through collision and coalescence, it may break apart and form several smaller droplets that will begin the collision and coalescence process all over again.
Several droplets have now taken the place of one larger drop that was too large to remain intact with the force of the air pushing upward on it.
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