WMOD00544

• Project Name: Project Skywater
• Title: Project Skywater - An Introduction to Rivers in the Sky
• Date: 12/1/1973
• Weather Modification - Doc Type: Report

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<:~O~~(~,AR.E MADE. up of bill~ons of
tiny ice. crystal's. or -water dI'oplets.A>r a
combination of both, which form around
microscopic particles - soil, dust, smoke,
salt crystals, and other materials that are
ever present in the atmosphere envelop-
ing the earth. Scientists classify some of
these microscopic particles as condensa-
tion nuclei on which water vapor condenses
to form cloud droplets, and a few as ice
nuclei on which condensed water freezes,
or ice crystals form directly from water
vapor. As a general rule there is an abun-
dance of condensation nuclei, but usually
a scarcity of ice nuclei available in the air.

The sizes, types, and concentrations of
nuclei present in the atmosphere play an
important role in determining the effi-
ciency with which a cloud system precipi-
tates. Tons upon tons of water flow above
the United States in these "rivers in the
sky," precipitating little or not at all for
want of certain required conditions. Of
prime importance for both initiation and

19

amount of precipitation from a cloud sys-
tem are (1) vertical &I1d_horizontal dimen-
sions of cloud, (2) lifetime of cloud, and (3)
sizes and concentration of cloud droplets
and ice particles. Under proper conditions,
one or more of these three factors can be
favorably modified through seeding the
cloud with appropriate nuclei.

Basically there are two mechanisms by
which precipitation forms in clouds. They
are sometimes called the "warm rain" and
the "cold rain" processes. The term "warm
rain" was derived after scientists noticed
that rain in tropical regions often fell from
clouds whose temperature throughout was
warmer than 320 F. Rain is formed in
these "warm" clouds when larger droplets
collide with and absorb smaller cloud
droplets in a process known as coales-
cence. The term "warm rain" is a mis-
nomer in that the coalescence mechanism
is operative in cold clouds as well. The
cold rain process occurs in clouds whose
temperature, in all or part, is colder than
320 F. The supercooled (below 320 F)
region of the cloud is generally comprised
of a combination of water droplets and
ice crystals, and oftentimes only the for-
mer. Ice crystals that form in the super-
cooled region grow rapidly, drawing
moisture from the surrounding cloud, until
their weight causes them to fall. In falling,
they coalesce with smaller droplets and
fall from the cloud as rain or snow.

THE ATMOSPHERIC NUCLEI present
during cloud formation exert a strong in-
fluence on the efficiency with which the
warm and cold rain processes operate.
For instance, giant condensation nuclei are
prevalent in the oceanic regions which
allows for larger cloud droplets to form