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Physical Properties of Water

Introduces surface tension and vapor pressure of water

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A Chance of Showers

A Chance of Showers


Credit: Bidgee
Source: http://en.wikipedia.org/wiki/File:FoggDam-NT.jpg
License: CC BY-NC 3.0

We wake up in the morning, see a clear sky, and plan a day of outside activities (a picnic, working in the garden, a long hike). However, clear skies in the summer can create problems for farmers. They need rain to water their crops. Fire fighters hope for rain to help put out forests fires. These groups need the water that rain can bring.

Amazing But True

  • Rain occurs when moisture collects around particles in the air. When the system becomes saturated, water droplets will form and precipitate from the sky. Cloud seeding facilitates this process by providing additional nuclei on which the water can condense. Dry ice was first used for this purpose, later to be replaced with silver iodide, whose form is similar to ice crystals. Calcium chloride will often be used in tropical areas.
  • Credit: DooFi
    Source: http://en.wikipedia.org/wiki/File:Cloud_Seeding.svg
    License: CC BY-NC 3.0

    Cloud seeding can be done from the ground or from the air [Figure2]

  • Prior to the development of cloud seeding techniques, many people thought that loud noises would cause rain to fall. A number of attempts (including Napoleon’s firing of artillery cannons into the air) were attempted, but with little or no success. This approach became known as the “Concussion Theory of Rainmaking”.
  • A newer approach to making rain is being studied in deserts in the Middle East. Negative ions are generated when the moisture in the air reaches about 30%. These ions help stimulate the formation of condensation nuclei, allowing water droplets to form and be more stable than usual.
  • Rain water is not pristine, but can contain large amounts of bacteria. The microorganisms form colonies on plants and are then swept up into the sky by wind. The bacteria are then surrounded by ice crystals, followed by water droplets that deposit on the ice. As the clumps get heavier they then fall back down to the ground to start the process all over again. This phenomenon is known as “bio-precipitation”.
  • Watch the video about silver iodide seeding at the link below: https://www.youtube.com/watch?v=qavutImbH7c

Show What You Know

Use the links below to learn more about rain making. Then answer the following questions.
  1. What is the correct term for approaches to controlling the weather?
  2. How is temperature a factor in cloud seeding?
  3. Why do locations near the ocean get a lot of rain?
  4. What are typical silver iodide concentrations in snow that has been produced by seeding?

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    Image Attributions

    1. [1]^ Credit: Bidgee; Source: http://en.wikipedia.org/wiki/File:FoggDam-NT.jpg; License: CC BY-NC 3.0
    2. [2]^ Credit: DooFi; Source: http://en.wikipedia.org/wiki/File:Cloud_Seeding.svg; License: CC BY-NC 3.0

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