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 solidly 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 thermistor (metallic thermometer) on its base, and an underwater drogue, or sea anchor, located 15 metres (49 ft) below the ocean surface connected with the buoy by a long, thin tether.[18]