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When
millions of vapor particles unite, they form droplets of
moisture. As these increase in size, they finally become
heavy enough to fall to earth as precipitation in such
varied forms as rain, snow, sleet, hail and dew.
It
is estimated that 16 million tons of precipitation in
any of these forms falls earthward each second. Through
the process of evaporation it is drawn back into the
atmosphere.
In
nature's balanced operations, evaporation equals
precipitation.
As
water falls to earth in this never-ceasing moisture
circulating system, it serves to cleanse both the air
and the ground.
No
doubt you have many times noted the fresh clean smell of
the air after a heavy rain. This is because the rain has
absorbed suspended solid matter (dust, dirt and soot),
gases, odors, and other impurities, polluting the air
over the area. While precipitation may remove large
quantities of impurities, it never succeeds in wholly
eliminating them.
In
falling, moisture absorbs amounts of the atmospheric
gases because these are partially soluble in water. For
example, water dissolves and collects carbon dioxide to
produce carbonic acid.
The
amount of free carbon dioxide in rain varies from 2 to 6
parts per million. Any amount of free carbon dioxide
above 1 to 2 ppm comes not from the atmosphere itself
but from other sources such as chimneys or industrial
fumes.
Rain
water also encounters sulfuric acid from the gases in
burning coal over cities. In addition it may pick up
bacteria and the spores in microorganisms.
When
precipitation continues for some time, the first amounts
to fall are apt to contain a great deal more suspended
solids that that which falls later.
Of
all forms of precipitation the snow falling high in the
mountains contains the least amount of mineral content.
This is due to the smaller amount of dust in the
atmosphere at high altitudes. As a result many mountain
streams deriving their water from high fallen snow have
extremely low dissolved mineral content.
Normally
when such water reaches the earth, it is slightly acid,
corrosive and relatively soft (though not as soft as man
can make it through his skill in treatment of water)
After
water reaches the ground, it may pick up additional
amounts of carbon dioxide from decaying vegetable
matter. Equipped with this booster action, it reacquires
even grater potential for dissolving minerals and other
impurities on or below the surface.
The
30 percent of precipitation which is not quickly
evaporated either seeps deep into the soil or finds its
way into lakes and rivers and eventually flows into the
oceans.
A
certain percentage of precipitation becomes surface
run-off. In this process it acquires further amounts of
hardness minerals in additional to quantities of clay,
silt, decayed animal and vegetable matter.
Not
all run-off is violent. Where heavy vegetation and
gently sloping grades permit, water enjoys an almost
imperceptible rate of flow, Under such conditions it
absorbs much objectionable taste, odor and color from
available decaying plant and animal life.
Only
a portion of the total precipitation seeps into the
soil. Curiously, when water percolates into the ground,
it loses some of the impurities it absorbed from the air
and on the ground. But while the soil structure filters
out certain impurities, it provides ample opportunity
for water to dissolve large amounts of earth minerals.
These, of course, increase its hardness and iron content
among other things
As
water seeps into the soil, it begins a journey that may
carry it for quite some distance through underground
crevices and fissures. Further, the journey may require
many years before this water is pumped to the surface.
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