Luke Howard, a chemist by profession, knew well how to value the results of scientific knowledge above traditional folk-knowledge. He saw the superiority of scientifically acquired knowledge in the fact that it was universally communicable, whereas folk-wisdom is bound up with the personality of its bearer, his individual observations and his memory of them. Nevertheless, the increasing mathematizing of science, including his own branch of it, gave him great concern, for he could not regard it as helpful in the true progress of man's understanding of nature. Accordingly, he sought for a method of observation in which the practice of 'the weatherwise mariner and husbandman' could be raised to the level of scientific procedure. To this end he studied the changing phenomena of the sky for many years, until he was able so to read its play of features that it disclosed to him the archetypal forms of cloud-formation underlying all change. To these he gave the now well-known names (in Latin, so that they might be internationally comprehensible): Cirrus: Parallel, flexuous or divergent fibres extensible in any and all directions. Cumulus: Convex or conical heaps, increasing upwards from a horizontal base. Stratus: A widely extended, continuous, horizontal sheet, increasing from below. Nimbus: The rain cloud. Let us, on the background of Howard's brief definitions, try to form a more exact picture of the atmospheric dynamics at work in each of the stages he describes.2 Among the three formations of cirrus, cumulus and stratus, the cumulus has a special place as representing in the most actual sense what is meant by the term 'cloud'. The reason is that both cirrus and stratus have characteristics which in one or the other direction tend away from the pure realm of atmospheric cloud-formation. In the stratus, the atmospheric vapour is gathered into a horizontal, relatively arched layer around the earth, and so anticipates the actual water covering below which extends spherically around the earth's centre. Thus the stratus arranges itself in a direction which is already conditioned by the earth's field of gravity. In the language of physics, the stratus forms an equipotential surface in the gravitational field permeating the earth's atmosphere. As the exact opposite of this we have the cirrus. If in the stratus the form ceases to consist of distinct particulars, because the entire cloud-mass runs together into a single layer, in the cirrus the form begins to vanish before our eyes, because it dissolves into the surrounding atmospheric space. In the cirrus there is present a tendency to expand; in the stratus to contract.