WeatherWhys for July 19,2013

It has been estimated that the weight of a single thunderstorm cloud is roughly 1.1 million tons. So it's easy to understand how a storm of this nature can produce so much rain. Imagine several of these waterlogged giants moving over the same location for hours on end. Needless to say, the flooding is often severe and that is precisely what happened in Johnstown, Pa., on July 20, 1977.