The term terminal velocity is used to describe the falling motion that results from the balance between gravity and air resistance (friction). Without the atmosphere, objects would continue to accelerate to faster and faster speeds until striking the surface. This occurs on planets and moons with little to no atmosphere. Even very small objects can accelerate to enormous speeds before striking the ground. One advantage to having an atmosphere like Earth is that it acts as a filter to removing small objects that fall into the atmosphere. The friction with the air is so high that is causes fast falling objects to burn up before striking the ground surface. The atmosphere also slows the rate of objects that enter the atmosphere. The terminal velocity is a balance between gravity which wants to continue to accelerate the object to faster and faster speeds and air friction which wants to slow the object down. The terminal velocity impacts falling precipitation particles. Precipitation is pushed toward the ground by gravity, but is slowed by air resistance and wind. Small precipitation particles can take a long time to reach the ground due to air currents keeping them suspended aloft. As precipitation particles get larger they are more likely to reach the ground faster and are more likely to have a higher terminal velocity. The diagram below shows two of the forces acting on a falling particle of rain. Gravity pushes it down while air resistance slows the motion. Note that air resistance will not accelerate the raindrop upward but it can slow the downward motion. Thus, air resistance increases as falling motion increases. Eventually the raindrop will no longer be able to accelerate and it will still fall but at a steady speed instead of an accelerated speed. Wind motions though will also play a significant role in the motion of the raindrop also. These additional forces such as updrafts, downdrafts, and horizontal winds will impact the motion.  |