FIGURE 2.1 Hurricane Camille as it approaches the Gulf States in 1969, as photographed from the NASA Nimbus III satellite. Image courtesy of NASA/Nimbus III Satellite.

vision) do, most satellite instruments detect the inherent emission of radiation (heat) from the atmosphere and terrestrial surface at wavelengths that reveal details invisible to human eyes. When the atmosphere itself is of interest, opaque wavelengths that do not pass through the atmosphere but are absorbed by it offer more information. Each window and opaque band responds differently to the various properties of the terrestrial surface and atmosphere, allowing those properties to be studied by a simultaneous analysis at multiple frequencies. The accuracy of these studies increases with the number of observed frequencies. The unique ability of passive microwave sensors to “see through” most clouds makes those sensors essential, particularly where clouds are persistent. The sensors are passive in that they do not transmit signals but instead only receive the natural background emission. Scientists can thus extract information from the radio spectrum on environmental properties as varied as atmospheric temperature and humidity, precipitation rate, soil moisture, ocean salinity, and ocean waves (and therefore surface winds and