Any thing moving through the atmosphere experiences the force
of gravity and a drag force due to the friction of the air
as it moves through it. When the object is falling, the force
of gravity is directed towards the ground and the drag force
directed upwards. As the body moves faster, the drag force
becomes larger and larger until it equals the force of gravity
and the object then falls at a constant velocity. This velocity
is called the terminal velocity. For a person jumping out
of an airplane, in skydiving, it is roughly 120-150 miles
per hour. The terminal velocity of a water drop is much smaller.
The following experiment allows you to select a water drop
size and see the terminal velocity of it. Click and move the
slider to select a drop diameter. The drop diameters are very
small. The diameter units here are in inches. The diameter
of your hair is 0.004 of an inch.
What you find: The fall velocity of a typical rain drop
is such that it does not hurt us as it lands on us in a rain
storm. The fall velocity of cloud particles are very small.
You can see some of these size particles in the mist about
a lawn sprinkler. They sort of just float in the air. The
small updrafts in the sky near clouds cause the small water
droplets to just stay there and not to fall toward the earth.
That is why clouds do not fall out of the sky.
The fall velocity of a rain drop still allows it to have
a fair amount of energy that it gives up when it hits the
earth. This energy can go into throwing soil particles out
from the site of the raindrop impact. This ejection of soil
is one of the main processes in soil erosion.
