Commercial aircraft now provide over 130,000 meteorological observations per day, including temperature,winds, and in some cases, humidity, vertical wind gust, or eddy dissipation rate (turbulence). The temperature and wind data are used in most operational numerical weather prediction models at NCEP and at other centers worldwide. At nonsynoptic times, these data are often the primary source of upper-air information over the United States. Even atsynoptic times, these data are critical in depicting the atmosphere along oceanic air routes. A Web site http://acweb.fsl.noaa.gov/ has been developed that gives selected users access to these data. Because the data are proprietary to the airlines, realtime access is restricted to entities such as government agencies and nonprofit research institutions (although sample past data are available to all). Data can be displayed in a variety of ways and can be downloaded for local processing. These data are described here, and examples of how they have been useful in weather forecasting and numerical weather prediction are shown.