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The glass-pipe technology used for RAFOS floats is applied to the monitoring of convection in deep mixed layers. The velocity of a vertical current is estimated from the relationship between the drag force exerted on a float by the vertical current and the buoyancy force induced by the float's resultant displacement from hydrostatic equilibrium. Tests conducted in the winters of 1990 and 1991 in the 18°C waters of the northwestern Sargasso Sea reveal definite convective events. Vertical velocities of both upwelling and downwelling plumes are estimated to approach maxima nearing 0.05 m s−1, with durations of up to 2 h. One float that crossed the Gulf Stream and entered the Newfoundland Basin showed evidence of very active vertical currents in the near-surface waters with maximum velocities greater than 0.09 m s−1.