If there are more molecules in a given volume of air, the density is greater. If there are fewer molecule, the density decreases. Density=Mass/Volume
water density
mixing with sediment.
Currents in the ocean are primarily driven by a combination of wind, temperature, and density differences. Wind helps generate surface currents, while temperature and density variances, such as those caused by differences in salinity, contribute to deeper ocean currents like thermohaline circulation. These factors work together to create complex patterns of ocean currents that help circulate heat and nutrients around the globe.
water density
deep
mixing with sediment
deep
An object sinks if it is denser than the liquid it is immersed in, causing it to displace an amount of liquid that weighs less than the object itself. Conversely, an object floats if it is less dense than the liquid it is placed in, allowing it to displace a sufficient amount of liquid to support its weight. Archimedes' principle explains this phenomenon.
density currents and Earth's rotation.
Density Current
I sugest you look up "steam tables". Since convection currents are caused solely by diferences in density ... yes, it is assumed that tempreture diferences CAUSED the density gradiant - but that is not always true.
Optical Density. Which often is caused by normal density.