Any current in either a liquid or a gas that is kept in motion by the force of gravity acting on small differences in density. A density difference can exist between two fluids or between different parts of the same fluid. Density currents flow along ocean and lake bottoms, because the water entering is colder, saltier, or contains more suspended sediment and thus is denser than the surrounding water. Density currents are a factor in water pollution, as the industrial discharge of large amounts of polluted or heated water can generate density currents that affect neighbouring human or animal communities.
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Sometimes, the above relation is written as
In general relativity, the continuity equation is written as:
where the semi-colon represents a covariant derivative.