Buoy, floating object, usually anchored but occasionally allowed to float freely or to be dragged by sea anchor. Buoys are widely used as navigation markers to indicate channels, the presence of shoals, etc. In oceanography, buoys function to protect or mark the position of equipment and as platforms to hold instruments.
Instrumented buoys range in size from 10 m diameter, discus-shaped hulls moored to the seafloor and weighing many tens of tonnes to small, 10 cm diameter, free-drifting cylinders weighing a few kilograms. Large hulls, with mast heights of up to 10 m, are used at open-ocean sites such as continental shelves; smaller hulls, of discus, boat, spar or spherical shape, are suitable nearshore or in lakes.
Sensors on buoys monitor environmental parameters such as air and water temperature, wind, ocean currents and ocean wave motion. Information can be recorded on board or telemetered to a shore station, directly or through a geostationary satellite. Data from buoys support research, forecasting, monitoring and emergency measures. For example, buoy networks on the East and West coasts monitor sea state to provide wave-climate information.
Canada has developed open-ocean "drifters" which use a polar orbiting satellite for tracking and communication. They have circumnavigated Antarctica, drifted across the Pacific and followed the circulation pattern around and across the North Atlantic. Similar drifters track spilled oil, sea ice and icebergs. Nearshore drifters can be tracked using a VHF beacon.