The Origin of the Stratus-Cloud, and Some Suggested Changes in the International Methods of Cloud-Measurement

Abstract

IN his “Instructions for Observing Clouds” (London, 1888, p. 12), Hon. Ralph Abercromby defines stratus as “a thin uniform layer of cloud at a very low level,” and as an illustration reproduces a photograph of a low sheet of cloud which he says is exceedingly characteristic of east winds in London. In his book “Weather,” p. 48, he shows by a diagram that the position of the stratus is in the south-west quadrant of the anticyclone. By carefully plotting the observations made at the Blue Hill Meteorological Observatory during the past ten years, I find that this type of cloud has the same position in the anticyclones on the eastern coast of the United States that Abercromby found for England. Moreover the continuous records, made by instruments lifted by kites at the Blue Hill Observatory, furnish a very evident explanation of its origin. In a number of cases the recording instruments were lifted into or through such clouds, and in every case the temperature and humidity rose suddenly as the thermograph entered and passed through the stratus-cloud. This rise of temperature is not shown when the thermograph is lifted into cumulus or nimbus clouds. Hence it is evident that the stratus described by Abercromby is found at the plane of meeting between a cold current and a warmer, damp current overflowing it. The cause of the stratus is undoubtedly the mixture between the two currents and the consequent condensation of moisture in the warmer current.