Dangerous Semicircle

This term, now somewhat archaic, is given to that portion of a tropical cyclone in which the winds and rain are most intense. Primarily used by mariners, the term is derived from the practice of dividing a hurricane, typhoon, or cyclone into dangerous and navigable semicircles, or right and left halves, based upon the system’s forward motion. In the Northern Hemisphere, where tropical cyclones spin in a counterclockwise direction, an observer facing into the winds of an approaching hurricane will find the dangerous semicircle on the right, or eastern side, and the navigable semicircle on the left, or western side. The reverse is true, of course, in the Southern Hemisphere, where the clockwise spinning of a cyclone will yield a dangerous semicircle on the left, or western side, and a navigable semicircle on the right, or eastern half.

While in actuality both halves of a tropical cyclone are dangerous, the half that finds itself strengthened by both the forward speed of the system’s steering current and the storm’s own forward velocity will possess significantly faster winds and higher seas. For this reason, those mariners who have divided an oncoming hurricane, typhoon, or cyclone into dangerous and navigable semicircles have been better able to guide their vessels away from the storm’s most furious aspects, thus greatly improving their odds of surviving it.