The Causes of Anticyclones and Cyclones

Abstract

A MEMOIR presented to the Vienna Academy of Sciences on April 17 last by Prof. J. Hann, giving the results of his study of an anticyclone which lay over Central Europe from November 12 to 24, 1889,4 brings to a climax one of those investigations that rank as landmarks in the advance of science; and compels us to modify in some important particulars the views now generally current on some of the leading phenomena in meteorology. Next to the facts of the general circulation of the atmosphere, which, in recent years, have been treated of more particularly by Ferrel, Hann, Siemens, Sprung, Oberbeck, and Pernter, the relations between areas of high and low pressure, or anticyclones and cyclones, have played a chief part in the science of atmospheric movements; and indeed in that large and popular department that deals with the weather and its vicissitudes, they may be said almost to monopolize the field. Hitherto, however, excepting in so far as the movements of the clouds afford us any information of the changes in progress in the higher atmosphere, our experiential knowledge of cyclones and anticyclones has been almost restricted to what can be observed within a small distance of the general land-surface. As a rule, a region of high barometer, especially in the winter, is one of low surface temperature, while cyclones, which originate in regions of low pressure, are fed by warm southerly winds. Interpreting these facts by the light of well-known physical laws, it has become the common teaching of our text-books that the former are due to the low mean temperature and therefore increased density of the superincumbent air column, while the latter are brought about by the opposite conditions. The correctness of these views, in so far, at. least, as regards anticyclones, was challenged by Dr. Hann as long ago as 1875, in a paper published in the Vienna Zeitschrift (vol. x. p. 210), wherein he showed that, as a result both of theory and observation, the cold that prevails in a region of high barometer in winter is really due to terrestrial radiation under the clear skies that are characteristic of such an area, that it is restricted to a stratum of very moderate thickness, and that above this the compression of the sinking atmosphere must induce a high temperature, and consequently greatly reduce its density.