monitoring of terrestrial ECVs. Networks One of the first tasks of the GCOS programme was to define a subset of the World Weather Watch (WWW) stations appropriate for basic climate monitoring. The subset of roughly 1000 baseline surface stations became the GCOS Surface Network (GSN), while a subset of 150 upper air stations was designated as the GCOS Upper-Air Network (GUAN). These were built on existing WMO classifications and became the initial baseline components of the atmospheric networks. Considerations for selection of GSN included spatial distribution, length and quality of record, long-term commitment, and degree of urbanization. Similar considerations were used for GUAN. Designation of these networks benefited both the GCOS and the National Meteorological and Hydrological Services (NMHS). For NMHSs, designation of a station as part of the global climate network helped sustain support for these sites with long-term records. The networks provided the foundation for the Regional Basic Climatological Network, which provides far greater spatial detail on the variability of climate. Recognizing that a balance has to be struck between standards and completeness of ground-based measurement, the GCOS programme recognized a hierarchy of observational networks and systems, comprising comprehensive, baseline and reference networks based on assumptions of spatial sampling needs. An example of a particularly successful step forward in implementing a global observing system for climate is the initiation of a reference network for upper-air observations - the GCOS Reference Upper-Air Network (GRUAN). The network is the prototype of a hybrid observing system, combining operational upper-air measurement sites with research sites and providing high-quality reference data for atmospheric profiles. GRUAN sites are undertaking high-quality atmospheric profile measurements that will help understand trends in upper-air ECVs, assist in investigating processes in the upper-troposphere and lower stratosphere, and provide data for calibrating satellite measurements and validating independent climate analyses and models. At GRUAN sites, the principles of quality, traceability and complete error characterization have been heeded, for at least part of the observing programme. The network is planned to grow over its initial size of 15 stations in coming years; introducing climate quality standards to a larger number of sites. See also References External links GCOS website GOOS website website of AOPC website of OOPC website of TOPC website of the Global Observing Systems Information Center - GOSIC Global Climate Observing System Earth observation projects