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<br />, <br />. <br /> <br />I I . THE PHYSICAL BASIS FOR CLOUD SEEDING <br /> <br />" <br /> <br />" <br /> <br />During cloud development, there is some mechanism producing vertical <br /> <br />movement of water vapor. This causes it to cool and eventually reach a <br /> <br />temperature at which condensation occurs and forms water droplets. <br /> <br />Such mechanisms are: (a) the rising air from the ground produced by <br /> <br />intense surface heating as d\lrin'g the summertime; (b) cold frontal systems which <br /> <br />intrude and force air upward over the frontal boundary; (cl when air in the <br /> <br />upper atmosphere sinks because it is relatively colder than the air it is <br /> <br />displacing. The sinking air causes air to rise elsewhere ahead of it over <br /> <br />large areas. <br /> <br />Water droplets themselves are formed by water vapor condensing upon <br /> <br />microscopic particles called condensation nuclei. Such microscopic part- <br /> <br />icles are present in the atmosphere worldwide - including dust, smoke and <br /> <br />salt crystals. <br /> <br />Also, there are a few kinds of special particles known as ice nuclei <br /> <br />particles upon which condensed water freezes, or upon which ice crystals <br /> <br />form directly from water vapor. Usually there is an abundance of conden- <br /> <br />sation nuclei found in the atmosphere, but a relative scarcity of ice nuclei. <br /> <br />Hence, clouds can be made up of billions of small water droplets or ice <br /> <br />crystals, or a combination of both. <br /> <br />Sizes, concentrations, and chemical properties of all nuclei present <br />;' <br />in the atmosphere combine to play an important role in determining the <br /> <br />efficiency with which a particular cloud system can produce precipitation. <br /> <br />At any given time there are massive amounts of water in the atmosphere in <br /> <br />the form of water vapor; oftentimes, there occurs no precipitation because <br /> <br />certain required conditions for the formation of precipitation are absent. <br /> <br />There are two natural cloud processes which are responsible for creat- <br /> <br />ing precipitation, the ~warm cloud" process, and the "cold cloud" process. <br />