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Pothole
Eddy
Pothole
 Pothole—The process of forming a pothole as stones spin around in the turbulent water.
Fast-flowing river
Trapped pebbles
Solid rock of river bed
Pothole
 Pothole—River bed pothole with some of the pebbles that scoured it.
The biggest droplets (and partly frozen droplets called hailstones) come from giant thunderstorms, since they have the strongest updrafts.
Some forms of precipitation
do not come from water or snow falling from clouds. Dew forms directly on a cold surface when air near the ground cools. If the air is very cold, a thin sheet of ice (called black ice) or a layer of ice crystals (called frost) can form.
A hole drilled in the bed of a fast-flowing river. This can
only happen in a river that has a rocky bed and is carrying pebbles or larger materials. As the water swirls around, it forms natural whirlpools or eddies in which pebbles become trapped. As they bump against the rocky bed, they rub it away, forming a depression that tends to make the whirlpool more permanent. Eventually, deep potholes can be drilled this way.
Pothole is also used as a term for a hollow where a stream disappears into a permeable rock, such as limestone, although it should correctly be called a “swallow hole.”
Precipitation
Any liquid or solid water that comes from the air and wets the ground. The forms of precipitation include dew, drizzle, hail, ice, rain, sleet, and snow.
Precipitation begins when air cools to a point at which it can no
longer hold water
as vapor. At this point condensation occurs, and
liquid water droplets form. If
the air is very cold, ice crystals form instead of water droplets. The droplets or ice crystals that form
in the air are minute and mainly grow around small particles of dust. When large numbers of these droplets form, they partly obscure the sky and are seen as cloud.
The formation of droplets or
ice crystals in the air does not usually lead to precipitation. For that to happen, the droplets or ice crystals have to come together and form large masses that can fall out against the natural updraft of air in a cloud.
The stronger the updraft,
the bigger the water droplet or snowflake has to be before it falls.
32
R
Rain, raindrop, rainfall
Droplets of moisture that have become big enough to fall out of clouds. It is one form of precipitation. (See also: Water cycle.)
 Rain—Torrential rainfall during an Indian monsoon.