The present article is the third in a series of papers intended to identify and, where possible, to quantify the key sources of the heat inevitably released during the Genesis Flood. In contrast with the first two papers, which are concerned mainly with the thermal history of the oceans, it considers the thermal consequences of vapour canopy models of the pre-Flood earth and of the onset of the Flood. Such models were developed in the last decades of the twentieth century by creationist authors who proposed that the pre-Flood earth possessed an atmosphere characterised by a high-altitude water vapour canopy which would have collapsed at the onset of the Flood, thereby providing a proportion of the water involved in the Flood. Here we find that the vapour canopy idea arose from outside mainstream biblical scholarship, apparently with the long-age catastrophist Isaac Newton Vail, who in turn attributed it to philosopher Immanuel Kant. Other strands of thought seem likely to have contributed, but their origin is not entirely clear. Our appraisal of the biblical and scientific arguments used to support the development of the idea into computationally tractable models of the pre-Flood atmosphere has found that the biblical arguments are not compelling. Furthermore these models fail scientifically, mainly because they predict a pre-Flood environment which would have been too hot for life except where the canopy water content was far too small to contribute significantly to the floodwaters.