Data Management Center (DMC) allows any scientist (U.S. or international) to download via the Internet the signals recorded at approximately 20 global seismic stations within about 1 hour of an event's occurrence (magnitude greater than 5).18 Similarly convenient Internet access to continuous recordings of many other international stations is possible through the DMC, but with a time lag necessitated by not having real-time or near-real-time data transmission from some of the available stations.
IRIS's DMC has become the international Federation of Digital Seismic Networks' archive for continuous digital data. Global digital seismic data from stations distributed around the globe are available through the DMC. Users can browse electronically to determine what data are available and can place requests for data sets they wish to receive; their requests are filled and the data transferred either electronically via the Internet (for modest-size data sets) or via high-density media such as Exabyte cassettes (for large requests). The DMC also serves as a broker for individuals who wish to obtain data from foreign stations that are not routinely archived at the DMC. This valuable service is accomplished by means of data transfer links to data archives in other countries; users would otherwise have to access and transfer data from these various sources individually. In this way, the DMC operates as a ''virtual" data center from which the user extracts desired data, some of which do not physically reside at the center.
The World Weather Watch is the most formally organized international global observation, communication, processing, and archiving system at this time.19 This distinction stems from the early recognition that scientific understanding and prediction of the atmosphere, even for only a day or two in advance, require observations from very large areas. Beginning more than 100 years ago, the observations were sent by communication systems in near-real time through internationally agreed upon arrangements and procedures. For the last several decades, data have been processed and archived on a global basis through a system of world and regional meteorological centers and world and regional data centers for meteorology and oceanography. During this period the World Weather Watch has developed many of the characteristics required for an effective system for international exchanges of scientific data and therefore can be considered one of the primary models for other such systems.
The development of the World Weather Watch was accelerated in the 1960s as a result of the potential capability of Earth-orbiting satellites to obtain atmospheric and oceanographic data on a global basis, and the advent of computers capable of handling large volumes of diverse data for numerical weather predictions on a global basis. An extensive planning and coordination process was put in place in the World Meteorological Organization (WMO) to expand the global observing component of the World Weather Watch through polar and geostation-