Individual raw datastreams from instrumentation at the Atmospheric Radiation Measurement (ARM) Climate Research Facility fixed and mobile sites are collected and sent to the Data Management Facility (DMF) at Pacific Northwest National Laboratory (PNNL) for processing in near real-time. Raw and processed data are then sent approximately daily to the ARM Archive, where they are made available to users. For each instrument, we calculate the ratio of the actual number of data records received daily at the Archive to the expected number of data records. The results are tabulated by (1) individual datastream, site, and month for the current year and (2) site and fiscal year (FY) dating back to 1998. The U.S. Department of Energy (DOE) requires national user facilities to report time-based operating data. The requirements concern the actual hours of operation (ACTUAL); the estimated maximum operation or uptime goal (OPSMAX), which accounts for planned downtime; and the VARIANCE [1-(ACTUAL/OPSMAX)], which accounts for unplanned downtime. The OPSMAX time for the fourth quarter of FY2010 for the Southern Great Plains (SGP) site is 2097.60 hours (0.95 2208 hours this quarter). The OPSMAX for the North Slope of Alaska (NSA) locale is 1987.20 hours (0.90 2208) and for the Tropical Western Pacific (TWP) locale is 1876.80 hours (0.85 2208). The first ARM Mobile Facility (AMF1) deployment in Graciosa Island, the Azores, Portugal, continues, so the OPSMAX time this quarter is 2097.60 hours (0.95 x 2208). The differences in OPSMAX performance reflect the complexity of local logistics and the frequency of extreme weather events. It is impractical to measure OPSMAX for each instrument or datastream. Data availability reported here refers to the average of the individual, continuous datastreams that have been received by the Archive. Data not at the Archive are caused by downtime (scheduled or unplanned) of the individual instruments. Therefore, data availability is directly related to individual instrument uptime. Thus, the average percentage of data in the Archive represents the average percentage of the time (24 hours per day, 92 days for this quarter) that the instruments were operating this quarter. Table 1 shows the accumulated maximum operation time (planned uptime), actual hours of operation, and variance (unplanned downtime) for the period July 1-September 30, 2010, for the fixed sites. Because the AMF operates episodically, the AMF statistics are reported separately and not included in the aggregate average with the fixed sites. This fourth quarter comprises a total of 2208 possible hours for the fixed and mobile sites. The average of the fixed sites exceeded our goal this quarter. The Site Access Request System is a web-based database used to track visitors to the fixed and mobile sites, all of which have facilities that can be visited. The NSA locale has the Barrow and Atqasuk sites. The SGP site has historically had a Central Facility, 23 extended facilities, 4 boundary facilities, and 3 intermediate facilities. Beginning in the second quarter of FY2010, the SGP began a transition to a smaller footprint (150 km x 150 km) by rearranging the original instrumentation and new instrumentation made available through the American Recovery and Reinvestment Act of 2009 (ARRA). The Central Facility and 4 extended facilities will remain, but there will be up to 12 new surface characterization facilities, 4 radar facilities, and 3 profiler facilities sited in the smaller domain. This new configuration will provide observations at scales more appropriate to current and future climate models. The transition to the smaller footprint is ongoing through this quarter. The TWP locale has the Manus, Nauru, and Darwin sites. These sites will also have expanded measurement capabilities with the addition of new instrumentation made available through ARRA funds. It is anticipated that the new instrumentation at all the fixed sites will be in place by the end of calendar year 2011. AMF1 continues its 20-month deployment in Graciosa Island, the Azores, Portugal, that began on May 1, 2009. The AMF will also have additional observational capabilities by the end of 2011. The second ARM Mobile Facility (AMF2) was deployed this quarter to Steamboat Springs, Colorado, in support of the Storm Peak Lab Cloud Property Validation Experiment (STORMVEX). The first field deployment of the second ARM Mobile Facility will be used to validate ARM-developed algorithms that convert the remote sensing measurements to cloud properties for liquid and mixed phase clouds. Although AMF2 is being set up this quarter, the official start date of the field campaign is not until November 1, 2010. This quarterly report provides the cumulative numbers of scientific user accounts by site for the period October 1, 2009-September 30, 2010.