An example is this spectacular plume from Mt Rinjani on the island of Lombok, Indonesia, from an eruption on July 2nd 1994. However, on conventional infrared and visible imagery, volcanic plumes are difficult to distinguish from normal clouds. The Rinjani eruption above occurred during the 'dry season' when there only a few normal clouds about. When the eruption occurs into an already cloudy sky, the problems become significant.
Modern satellites provide a way to distinguish ash from water.
These satellites take images at several different wavelength bands, including visible light and several infrared bands. By simply subtracting one band from another, we are able to identify areas of volcanic ash, and process the satellite imagery to show where an apparent ash signature exists. Japanese GMS-5 imagery clearly shows, after processing, ash streaming away to the north west. Suspected ash is shown in green. Without the processing, this ash would just show as normal cloud and be virtually undetectable.