The clouds that are shown on a radar map are a satellite picture that has been superimposed onto the map.
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The radar data is from various National Weather Service weather radar stations across the country. The satellite cloud data is provided NASA.
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The radar and cloud pictures are free and available to the public through [Intellicast][1] and the [NWS][2] (National Weather Service) websites. Both of these site will be helpful to you in learning more about both of these features/services.
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Hope this helps! :)





  [1]: http://www.intellicast.com/National/Satellite/Infrared.aspx "Intellicast"
  [2]: http://www.weather.gov/ "NWS"