Discharges of lightning from a volcanic ash cloud occur only when the cloud' s highest point exceeds an altitude of 5 kilometers  Those discharges become progressively more frequent as the ash cloud moves higher still  Weather radar can measure the altitude of ash clouds, but it is not available in all parts of the world
Hence lightning discharge data can sometimes be our only reliable indicator of the altitude of ash clouds
A volcanic ash cloud whose highest point exceeds an altitude of 5 kilometers is likely to be at least partly beyond the reach of weather radar.