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. Lightning discharges are no more frequent for large volcanic ash clouds than for small volcanic ash clouds. Lightning discharges can be detected in some regions in which weather radar is unavailable. The highest point of any volcanic ash cloud will eventually exceed an altitude of 5 kilometers.
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.
Which one of the following is an assumption required by the argument?