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.
Between the two, the cumulus, even viewed simply as a form-type, represents an exact mean. In how densely mounded a shape does the majestically towering cumulus appear before us, and yet how buoyantly it hovers aloft in the heights! If one ever comes into the midst of a cumulus cloud in the mountains, one sees how its myriads of single particles are in ceaseless movement. And yet the whole remains stationary, on windless days preserving its form unchanged for hours. More recent meteorological research has established that in many cumulus forms the entire mass is in constant rotation, although seen from outside, it appears as a stable, unvarying shape. Nowhere in nature may the supremacy of form over matter be so vividly observed as in the cumulus cloud. And the forms of the cumuli themselves tell us in manifold metamorphoses of a state of equilibrium between expansive and contractive tendencies within the atmosphere.
Our description of the three cloud-types of cirrus, cumulus and stratus, makes it clear that we have to do with a self-contained symmetrical system of forms, within which the two outer, dynamically regarded, represent the extreme tendencies of expansion and contraction, whilst in the middle forms these are held more or less in balance. By adding Howard's nimbus formation to this system, we destroy its symmetry. Actually, in the nimbus we have cloud in such a condition that it ceases to be an atmospheric phenomenon in any real sense of the word; for it now breaks up into single drops of water, each of which, under the pull of gravity, makes its own independent way to the earth. (The symmetry is restored as soon as we realize that the nimbus, as a frontier stage below the stratus, has a counterpart in a corresponding frontier stage above the cirrus. To provide insight into this upper frontier stage, of which neither Howard nor Goethe was at that time in a position to develop a clear enough conception to deal with it scientifically, is one of the aims of this book.)
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In order to understand what prompted Goethe to accept, as he did, Howard's classification and terminology at first glance, and what persuaded him to make himself its eloquent herald, we must note from what point Goethe's labours for a natural understanding of nature had originated.