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Most water in Earth's atmosphere and crust comes from saline seawater, while fresh water accounts for nearly 1% of the total. The vast bulk of the water on Earth is saline or salt water, with an average salinity of 35‰ (or 3.5%, roughly equivalent to 34 grams of salts in 1 kg of seawater), though this varies slightly according to the amount of runoff received from surrounding land.
The total mass of Earth's hydrosphere is about 1.4 × 10 18 tonnes, which is about 0.023% of Earth's total mass. At any given time, about 2 × 10 13 tonnes of this is in the form of water vapor in the Earth's atmosphere (for practical purposes, 1 cubic metre of water weighs 1 tonne).
The collective mass of water found on, under, and over the surface of a planet is called the hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1.386 billion cubic kilometres (333 million cubic miles). [24]
The specific weight, also known as the unit weight (symbol γ, the Greek letter gamma), is a volume-specific quantity defined as the weight W divided by the volume V of a material: = / Equivalently, it may also be formulated as the product of density, ρ, and gravity acceleration, g: = Its unit of measurement in the International System of Units (SI) is newton per cubic metre (N/m 3), with ...
As there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use. The SI unit of kilogram per cubic metre (kg/m 3 ) and the cgs unit of gram per cubic centimetre (g/cm 3 ) are probably the most commonly used units for density.
The mass of Earth's oceans is estimated to be 1.37 × 10 21 kg, which is 0.023% of the total mass of Earth, 6.0 × 10 24 kg. An additional 5.0 × 10 20 kg of water is estimated to exist in ice, lakes, rivers, groundwater, and atmospheric water vapor. [20] A significant amount of water is also stored in Earth's crust, mantle, and core.
Usually, the relationship between mass and weight on Earth is highly proportional; objects that are a hundred times more massive than a one-liter bottle of soda almost always weigh a hundred times more—approximately 1,000 newtons, which is the weight one would expect on Earth from an object with a mass slightly greater than 100 kilograms.
the electronvolt (eV), a unit of energy, used to express mass in units of eV/c 2 through mass–energy equivalence; the dalton (Da), equal to 1/12 of the mass of a free carbon-12 atom, approximately 1.66 × 10 −27 kg. [note 2] Outside the SI system, other units of mass include: the slug (sl), an Imperial unit of mass (about 14.6 kg)