Surface land and maritime observing stations report various meteorological variables that describe the current weather. These variables include basic measurements such as temperature, relative humidity, wind, precipitation, etc., as well as various types of weather occurrences such as hail, fog, and thunder.
The MADIS meteorological surface dataset includes reports from many observing networks ... run by different "providers". Through the MADIS API, the user can select only a subset of the total by specifying which providers to include or exclude. Most national-scale networks in North America that report surface conditions are included. Over land, this includes all stations that report standard METARs (ASOS, AWOS, non-automated stations) or SAOs (Canadian stations), as well as modernized (i.e., automated) National Weather Service Cooperative Observer reports. Maritime reports are also available, including the Coastal Marine Automated Network (C-MAN), fixed and drifting buoys, and ship reports. The MADIS meteorological surface dataset also contains a unique collection of thousands of mesonet stations from local, state, and federal agencies, and private firms. One of these mesonets, GPSMET, includes water vapor observations from geo-positioning satellites (GPS) in addition to surface observations.
Some of the mesonets are restricted. Click here for details. http://madis.noaa.gov/
[Summary Extracted from the MADIS home page]