Does Precipitation Influence the Movement of Cyclones?

Abstract

IN Prof. Elias Loomis's first “Contribution to Meteorology,” in the American Journal of Arts and Science, he examined the distribution of rain around 152 storms (cyclones) in the United States, in order to determine whether there exists any relation between the velocity of a storm's progress and the extent of the accompanying rain area. He found that “the average extent of the rain area on the east side of the storm's centre is 500 miles; and when the rain area extends more than 500 miles, the storm advances with a velocity greater than the mean; but when the extent of the rain area is less than 500 miles, the storm advances with a velocity less than the mean.” In his twelfth “Contribution” he examined 39 storms which moved with exceptional velocity (1000 miles or more per day) and found that “the rain area generally extended a great distance in advance of the storm centre, the average distance being 667 miles.” Finally, Loomis examined 29 cases of those abnormal cyclones in the United States which moved toward the west. He says:—“In nearly every case we find a fall of rain or snow in the region toward which the low centre advanced, and in most of the cases the rainfall was unusually great. … It may be inferred from these comparisons that the fall of rain or snow is one of the most important causes which determine the abnormal movements of areas of low pressure” (ninth memoir, p. 44). Ley and Abercromby state that in Great Britain the relation of the weather to the cyclone centre is the same whatever the path of the cyclone; thus when storms advance toward the west the greatest cloud development and rainfall is to the west of the cyclone centre. In the Proceedings of the Royal Meteorological Society, vol. xliii., Abercromby gives a table showing the relation between the intensity of “trough phenomena” and the velocity of cyclones. This table indicates very clearly that the greater the velocity of the cyclone the more marked the “trough phenomena.” Hence, according to Abercromby's definition of “trough phenomena” the heaviest rain and cloud areas are massed toward the front of rapidly advancing cyclones, while immediately after the passage of the line of minimum pressure the sky begins to show signs of clearing. This is especially well marked in cyclones passing off the north-east coast of the United States. When the cyclones are moving with unusual rapidity, not only all the rain, but almost all of the cloud area is confined to the front half of the cyclone.