Weather Underground is a commercial weather service that provides real-time weather information via the Internet. Weather Underground provides weather reports for most major cities across the world on its Web site, as well as local weather reports for newspapers and Web sites. Most of its United States information comes from the National Weather Service (NWS), as information from that agency is within the public domain by federal law. The Web site is available in many languages, and an ad-free version of the site with additional features is available for an annual fee.
Jeff Masters, a PhD candidate in meteorology at the University of Michigan, working under the direction of Professor Perry Samson, wrote a menu-based  interface in 1991 that displayed real-time weather information around the world. By 1992, they claim that the two servers they used were the most popular service on the Internet. In 1993 they initiated a project to bring Internet weather into K-12 classrooms.
In 1995 the Weather Underground, Inc. evolved as a commercial entity separate from the university. It has grown to provide weather for print sources, in addition to its online presence. In 2005 the Weather Underground became the weather provider for the Associated Press; WU also provides weather reports for some newspapers (including the San Francisco Chronicle) and the Google search engine.