Upwelling radiation from the earth's surface has been found to be a successful indicator of the rate of rainfall, according to new analyses of data obtained by the Nimbus 7 and Seasat satellites. In a recent report describing the functions of the Scanning Multichannel Microwave Radiometer (SMMR) on these satellites, R. W. Spencer, D. W. Martin, B. B. Hinton, and J . A. Weinman of the Space Science and Engineering Center at the University of Wisconsin demonstrated that microwave emissions at the frequencies 37, 21, 18, 10.7, and 6.6 GHz are sensitive to changes in the rain rate (Nature, July 14, 1983). The results are significant because other satellite methods for the measurement of precipitation, which operate in the visible and infrared frequency ranges, infer precipitation from cloud observations.

The results of Spencer et al. are a comparison of SMMR U.S. data with rain rates derived from local weather radar observations. The comparison was done by digitizing radar plan position indicator photographs which are taken routinely by the U.S. National Weather Service. These rates and microwave brightness temperatures were entered in the University of Wisconsin Man-computer Interactive Data Access System. The output was displayed as television images and the radar brightness rain scale was contrasted with the SMMR images.