A Rotary Wind Current off the Florida Coast

Abstract

C URRENTS in the open sea, particularly those in the areas adjacent to the continental coasts, are exceedingly complex in character. Numerous forces, astronomical and meteorological, are reflected in these ceaseless and ever varying horizontal movements of the water. Tidal currents, into which are woven all the periodic gravitational pulls resulting from the changing relative positions of sun, moon, and earth, combine with wind currents, erratic or continuous, and with the permanent drifts of large masses of water known as ocean currents. Currents also result from the discharge of rivers into the sea, from the melting of ice, unequal precipitation, barometric changes, and other causes. A rotary wind current at St. Johns Lightship, off the mouth of the St. Johns River, Florida, has been revealed through an analysis of current observations, and it is shown that the wind that sets up this current is itself rotary.' The observations on which the evidence of this cyclic wind current is founded were begun in December, 1933, and were continued through the entire calendar year 1934.2 Measurements of the velocity and direction of the current were taken and recorded at the beginning of every hour. The velocity and direction of the current were determined by means of a 15-foot wooden current pole so weighted at one end that it floated vertically with 14 feet of its length submerged. Attached to the pole was a light line so marked that, by allowing the pole to drift for a given period of time, the velocity could be read directly from the line in nautical miles an hour or in knots. To determine the direction of the current, the direction of the ship's head and also the angle that the current line made with the fore-and-aft line of the ship were noted and recorded. These two values properly combined gave the direction of each observed velocity. At the time of each current observation the direction and estimated velocity of the wind were entered in the record.