of water vapor in the air are so small you cannot see them. For
a person to be able to see an object, light must strike the object,
which then reflects, or bends, the light back toward a person's
eyes. Water molecules are so small that they do not bend back enough
light for us to see. As water molecules combine, they form water
droplets. For a water droplet to be visible it must contain about
10 billion molecules of water.
water vapor rises into the atmosphere high above the earth's surface
and meets cooler air, it can change in two ways. One way the vapor
may change is by turning into water droplets. But if the air temperature
is very cold, deposition-the change of water vapor directly into
solid ice crystals-may occur. Deposition also takes place on the
ground. Frost is an example of water vapor that has gone through
deposition, changing from a vapor into a solid without first becoming
warm months, the sun heats the air. But the high air temperatures
often fall at night after the sun goes down. In the early morning,
blades of grass may be coated with water droplets, or dew, the result
of cooler night air temperatures. (Remember, cool air holds less
water than warm air.) When water vapor condenses, dew forms on surfaces.
As the air cools, it reaches a point, called the dew point, at which
it is saturated with water vapor. Condensation begins at the dew
point because the air is saturated and cannot hold any more water.
As water vapor high in the atmosphere cools to the dew point, it
condenses around dust particles, forming droplets. The droplets
come together in larger and larger clumps until they are big enough
to reflect light for us to see. These visible clumps of water droplets
and if the air temperature is cold enough, ice crystals-are called
Millions of water droplets and ice crystals combine to form the
large clouds we see in the sky. The cottony clouds that look so
fluffy and light actually contain enough water droplets and ice
crystals to weigh half a million tons. The enormous black clouds
of a thunderstorm may weigh several million tons! When water droplets
and ice crystals become too heavy for air currents to hold them
suspended in the atmosphere, they fall toward the earth as precipitation.
clouds can contain enough water to weigh millions of tons, the total
amount of water contained in all the clouds at any one time is actually
less than .001 of I percent of the earth's water supply. If suddenly
all the water droplets, ice crystals, and vapor contained in clouds
and the atmosphere were to condense and fall evenly over the earth's
surface, the total amount would only measure about 1 inch (2.54
cm) of rain.