The
hydrosphere (from
Greek ὕδωρ -
hudōr, "water" and σφαῖρα -
sphaira, "sphere") in
physical geography describes the combined mass of
water found on, under, and over the surface of a
planet.
The total mass of the
Earth's hydrosphere is about 1.4 × 10
18 tonnes, which is about 0.023% of the Earth's total mass. About 20 × 10
12 tonnes of this is in the
Earth's atmosphere (the volume of one tonne of water is approximately 1 cubic metre). Approximately 75% of the Earth's
surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by
ocean. The average
salinity of the Earth's oceans is about 35 grams of
salt per kilogram of sea water (3.5%)