Mesoscale meteorology is closely identified with surface observing systems, perhaps because disruptive weather is intrinsically at the mesoscale, and impacts are most often experienced at or near the surface. The United States has enormous diversity and complexity within its inventory of surface-based observing assets, which are operated by federal, state, and local agencies, numerous segments of the private sector, universities, schools, and hobbyists and other enthusiasts. Surface-based observing systems employ both in-situ sensing as well as active and passive remote sensing technologies. A number of efforts have summarized observational capabilities in the United States. For the last decade and with funding from the Global Energy and Water Cycle Experiment (GEWEX) America’s Prediction Project (GAPP), University Corporation for Atmospheric Research/National Center for Atmospheric Research (UCAR/NCAR) has developed a database that describes and maps what is available (http://www.eol.ucar.edu/projects/hydrometnet). The National Science Foundation (NSF) recently sponsored development of another database to serve the dual purpose of providing users with information about available resources and to identify future observational needs in atmospheric research (see http://www.eol.ucar.edu/fadb/). The National Oceanic and Atmospheric Administration (NOAA) is currently developing an Observing Systems Architecture website with a comprehensive list of NOAA networks at http://www.nosa.noaa.gov (check “Observing System Inventory” on the left side of the page). A summary table based on these websites appears in Appendix B. Other useful websites for such information include http://madis.noaa.gov and http://www.met.utah.edu/cgi-bin/databbase/mnet_no.cgi.

Most commonly, “surface” measurements consist of temperature and relative humidity, wind, precipitation, and air pressure. World Meteorological Organization (WMO) standards prescribe wind measurements at a height of 10 m in open areas, and pressure, temperature, and humidity at about eye level (1.5 m), but many surface measurements deviate from these standards, often for good reason. For example, routine observations are made for applications in transportation, agriculture, the power industry, air quality, and public safety, nearly all of which have specific criteria that differ from WMO standards.

There are many thousands of surface sites gathering weather and related information. Based on the UCAR/NCAR and NSF surveys, approximately 500 surface networks operate in the United States and its coastal waters. Federal and state agencies as well as universities and the private sector take