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    What is the difference between Cooperative and First–Order stations?

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    Cooperative refers to weather stations that are part of the U.S. Cooperative Observing Network. This network consists of several thousand temperature and/or precipitation stations that, in general, are manned by volunteer observers. A subset of about one thousand of the longer-term stations from this network make up the U.S. Historical Climatology Network (U.S. HCN), which is a very important and celebrated network in the climate research community.

    First–Order refers to weather stations that are professionally maintained, primarily through the National Weather Service or Federal Aviation Administration. Modernization of the National Weather Service during the 1990s resulted in the consolidation of many manned weather stations and the introduction of Automated Surface Observing System (ASOS) instrumentation throughout the United States. ASOS instrumentation is now in use at the vast majority of first-order sites, which are primarily located at airports.

    Both cooperative and first–order sites are essential to the 1971-2000 normals. In many cases, cooperative sites offer a well-established, consistent observing site with periods of record that are continuous through the normals period. First-order sites are the professionally manned sites with hourly observations.