The next time you see a raindrop splash onto the ground or into
a puddle, stop for a moment to ponder its origin and fate.  Most
of Earth's rain originates from water vapor evaporated from the
oceans. When sunlight warms the oceans, evaporation increases
and the adjacent air is also warmed. The warm air then rises
along with its cargo of water vapor.  The moist air gradually
cools as it rises. Cool air can contain less water vapor than
warm air, so the rising air might become so saturated with water
vapor that it condenses into tiny droplets around microscopic
particles of salt, dust, smoke or even bacteria.  As the
formerly invisible moisture continues to condense, it makes its
presence known as a visible cloud. While a cloud may contain
many tons of liquid water, it floats in the sky because it
weighs slightly less than an equal volume of dry air.  A drop of
rain is formed when many thousands of microscopic droplets
merge. When a drop reaches a diameter of around 2 millimeters (a
little thicker than a nicke