Cumulus clouds are much beloved of landscape painters. Their attraction lies, no doubt, not just in the interesting shapes they assume but also in the fact that cumulus clouds typically occur on bright, airy, sunny afternoons, when the colours of the countryside are shown to best advantage. Since there are gaps between the cumuli, the artist can often capture a landscape pleasingly dappled with the passing shadows of the clouds. By contrast, clouds formed at higher altitudes are generally layered, making the landscape dull and unstimulating from an artist's viewpoint and presenting in themselves a matt, grey sheen less likely to appeal to painters.
John Constable was an expert on such matters. "The landscape painter who does not make his skies a very material part of his compositions," he wrote, "neglects to avail himself of one of his greatest aids." He was meticulous, often making notes for future reference on the back of sketches he had made, and because of this attention to detail, Constable's skies are much revered by meteorologists for the accuracy of their depiction.
Others, alas, are not as conscientious. Some cumulus clouds are depicted in such a way that they would be just as good - or bad - if turned on their sides, or upside down. Yet, of all the features of a real cumulus, the most obvious is that it has a definite top and bottom, the latter being flat and horizontal and the former rounded and multibulbous, like a cauliflower.
Likewise, the painter should bear in mind, as we noted in this column yesterday, that cumulus clouds, more often than not, arrange themselves in parallel lines across the sky. These "streets" of cumuli, as they are called, are orientated parallel to the direction in which the wind is blowing. It should go without saying, therefore, that the smoke from any fire or chimney in a scene should be depicted in a manner consistent with this organised behaviour.
Finally, another pitfall into which the unwary or unobservant painter of the skies may tumble is to portray an abundance of well-developed cumulus clouds cavorting around the heavens while early morning mists and hazes grace the mountain valleys. The coincidence is quite unusual; at any distance from the sea the large cumulus cloud is, by and large, a creature of the afternoon, when morning mists have long since disappeared.
Likewise, the convective updrafts that give rise to cumulus clouds weaken and eventually die as the sun sinks in the evening sky. Ultimately, in the gathering twilight, the cumulus clouds disappear completely or flatten out into a dull layer of grey-pink stratocumulus.