There is so little distinctive about this cloud-form that it scarcely appears in folk-lore, though I believe that in Lancashire these flat sheets of condensed vapour are still called "the blanket of the sun." We will therefore pass on to the most striking and important.
Rocky Structure.
In this form the summit of the cloud is always more or less rocky or lumpy, but the varieties are innumerable. Meteorologists call the whole class—cumulus.
Sometimes, as in this illustration from the Brazilian coast, you see small detached clouds, each with its own rocky top above a flat base; while in this beautiful picture of Rio Janeiro you see a mountainous mass of cloud rising out of the gloom below it. The third example I have put on the screen is a rocky cloud in London, simply to show that the form of cloud there is essentially the same as in South America. There is no doubt that some mythical caves and mountains have their origin in rocky clouds, but it is always difficult to separate these legends from the purely folk-lore story of human incident. Sometimes these threatening masses of rainy cloud are associated with low hairy cloud, something like the form of cirrus we have called "goat's-hair." Here is a typical illustration from a thunderstorm in Borneo (Fig. 4), where you see the cloud on the top of the picture
Fig. 4.— Mountainous Cumulus, drawn out into a sort of "Goat's hair" above, over a thunderstorm in Borneo.
