ELECTRICALLY CHARGED DROPS FROM BUBBLES IN SEA WATER AND THEIR METEOROLOGICAL SIGNIFICANCE

Abstract

A modification of Millikan's classic oil-drop experiment was used to determine the electric charge and radius of drops that were ejected from a bursting bubble at an air-sea water interface. Charge measurements were made of both the natural and the induced charge. Drops of 2 to 20 microns in radius carry natural charges of at least 2 ? 102 to 5 ? 103 elementary units, respectively. The induced charges are considerably higher, reaching 106 elementary units on drops of 50 microns radius. The sign of the natural charge is positive on drops < about 4 microns. For larger drops both the sign and magnitude of the charge appear to be a function of the depth of water through which the bubble rises. The meteorological significance stems from the fact that rain and snow, as well as whitecaps, can produce great numbers of small bubbles in the surface waters of the oceans. Both laboratory and field work suggest that the majority of these bubbles produce positively charged drops that contribute to the atmospheric space-charge. Of special significance is the fact that, for positive induction fields less than about 25 v cm?1, a positive charge is found on the small drops. For fields greater than 25 v cm?1 the induced negative-charge exceeds the natural positive-charge and so the drops carry a net negative-charge. Consequently, small bubbles breaking at the surface of the sea in the presence of the earth's fair-weather positive field of about 1 v cm?1 will produce drops that carry a positive charge. Calculations based on measurements of the bubble spectrum produced by whitecaps indicate that the charge on the drops may, under some conditions, provide a counter-current of the same order of magnitude as the fair-weather conduction current. Thus the sea may be a source as well as a sink for the charge that maintains the earth's positive electric-field.