The World Ocean: parts, floor and water

The main property of ocean water is salinity: if 1 litre of water holds more than 1 g of salt, it is called salty. The average salinity of the World Ocean is 35 ‰; it is higher in the tropics (36–37 ‰), 34–35 ‰ at the equator, and varies in seas from 2 ‰ (Gulf of Finland) to 42 ‰ (Red Sea). The average surface temperature is about +17.5 °C; in the open ocean it ranges from –2 °C to +29 °C, and below 1 000 m averages 2–3 °C. Ocean water freezes at –2 °C. Ocean currents are long-distance horizontal movements of water; they are mostly caused by wind, are warm or cold, and influence the climate of the continents. The main currents are the North and South Equatorial currents, the Equatorial Countercurrent and the West Wind Drift. Tides come from the pull of the Sun and Moon: twice a day the water rises and twice it falls. Waves are made by wind (the highest recorded was 34 m). A tsunami is caused by an earthquake or volcano: in the open ocean it is under 1 m high and travels 700–800 km/h, but at the coast it can rise tens of metres and cause great destruction.