The NOAA/NESDIS operational aerosol optical thickness product has provided an exceptional view of the development of the Mt. Pinatubo stratospheric aerosol layer. The product is derived from reflected solar radiation measurements of the Advanced Very High Resolution Radiometer onboard the NOAA/11 polar orbiting environmental satellite. The greater the optical thickness, the greater the amount of reflected solar radiation. Daily and weekly composites of aerosol optical thickness (AOT) at a wavelength of 0.5 micrometers have been analyzed to monitor the spatial and temporal variability of the aerosol layer and its optical thickness since the major eruption of Mt. Pinatubo on June 15, 1991. These analyses show that: the volcanic aerosol layer circled the Earth in 21 days; there are inhomogeneities in the layer that seem to remain after over two months of circling the Earth; using an AOT of 0.1 to define the layer, it covered about 42% of the Earth's surface area after two months, over twice the area covered by the El Chichon aerosol layer two months after its eruption; the layer is confined to the latitude zone 20S to 30N, with occasional patches seen at somewhat higher latitudes; the largest mean optical thickness of the layer was 0.31, occurring on August 23rd; the mass of SO2 required to produce this aerosol optical thickness is 13.6 megatons; and, the globally averaged net radiation at the top of the atmosphere may be reduced by about 2.5 Wm−2 (cooling effect of at least 0.5°C) once the aerosol is distributed globally over the next two to four years.