 Drifting buoys are smaller than their moored counterparts , measuring 30 centimetres ( 12 in ) to 40 centimetres ( 16 in ) in diameter . They are made of plastic or fiberglass , and tend to be either bi @-@ colored , with white on one half and another color on the other half of the float , or <unk> black or blue . It measures a smaller subset of meteorological variables when compared to its moored counterpart , with a barometer measuring pressure in a tube on its top . They have a <unk> ( metallic <unk> ) on its base , and an underwater <unk> , or sea anchor , located 15 metres ( 49 ft ) below the ocean surface connected with the buoy by a long , thin <unk> . 
