During the 1980s and 1990s, a network of buoys in the central and eastern tropical Pacific Ocean helped study the El Niño-Southern Oscillation. Moored weather buoys range from 1.5–12 metres (5–40 ft) in diameter, while drifting buoys are smaller, with diameters of 30–40 centimetres (12–16 in). Drifting buoys are the dominant form of weather buoy in sheer number, with 1250 located worldwide.