Images and text on NOAA websites are in the public domain. We just ask that you credit NOAA. In general, there is no set standard for how you do this.

You have permission to use the images in NOAA's photo library (http://www.photolib.noaa.gov), unless they are identified as copyrighted (a few of our tornado images are). Please credit National Oceanic and Atmospheric Administration/Department of Commerce and also the photographer, if identified.

Suggested wording: "Courtesy of the National Oceanic and Atmospheric Administration Central Library Photo Collection"

Thanks.  

For citing web pages see:  http://www.lib.duke.edu/libguide/cite/web_pages.htm