Abstract
THE object of the paper was to explain a theory with regard to the principles that may have the greatest effect in producing the leading cloud-forms. Neglecting occasional and exceptional influences, the author stated that the causes with which his paper dealt might be classed under three heads: (1) the diminished specific gravity of the air when more or less charged with invisible vapour, (2) the differential horizontal motion of the atmo sphere, (3) the vertical motion in the atmosphere produced by the heat of the sun expanding the lower air. The first of these was universally recognised as the initial cause of the cumulus, or first-born primary cloud. It was produced when there was so much vapour generated in the lower atmosphere that the vapour-laden layer projected up within the limit of condensation. Of course the vapour below this limit would itself become condensed if cooled in the course of its travels. During the formation of the cumulus, calm was supposed to prevail. When the atmosphere was in motion, its differential horizontal movement produced the first important modification. Retarded by friction and other causes, the lower portion of the cumulus moved more slowly than the upper, and the cloud sheared over into a slanting position, and ultimately became the cutnulo-stratus. A young cloud was thus distinguishable from those that had travelled even a short distance. In this climate large well-developed cumuli, though common in summer, were seldom seen in the cold season. The majority of the clouds of the first stage seen here were born in warm latitudes, and, coming as travelled cumuli, showed more or less the condition of the cumulostratus. The invisible vapour was subject to this same shearing motion, and far-travelled water-vapour would, on its rising, as it soon does in this climate, to the height necessary for condensation, at once take the shape of the stratus. In the next stratum above, Mr. Glaisher's investigations in his balloon ascents showed a rather rapid change to a drier atmosphere. Here were found the cirro-cumulus, and cirro-stratus. The differential motion of the atmosphere, though diminished, was still an important agent, and produced results that were not possible in the more bulky and dense clouds of the lowest range. When the sun's heat expanded the lower atmosphere, the upper cloud-stratum would be lifted, flattened, and broken into patches, the result being a mackerel sky. Should, however, the expansion in the lower atmosphere take place very slowly, it was possible that the cloud, though thinned, would remain unbroken. Rapid motion of the atmosphere would elongate the cloud in the direction of motion; and, if accompanied by expansion from below, would rupture the cloud into ribs or bars at right angles to the current. If the mass of the cloud were stationary or moving slowly, prominent parts might be drawn out into “mares'-tails.”
Article PDF
Rights and permissions
About this article
Cite this article
On the Forms of Clouds 1 . Nature 35, 164–165 (1886). https://doi.org/10.1038/035164a0
Issue Date:
DOI: https://doi.org/10.1038/035164a0