Stratification of Water

The vertical distribution of water density in bodies of fresh or salt water is known as stratification and is characterized by the vertical density gradient. The greater the increase in density with depth and the greater the vertical gradient, the higher the stability of the stratification. When the vertical density gradient is small or the density decreases with depth, the stratification is unstable. Stable stratification causes a decrease in the vertical exchange of heat, mass, and momentum. Unstable stratification leads to intense vertical exchange in the body of water.

In oceans and seas, stratification is governed mainly by variations in water temperature and salinity at the surface and also below the surface, where the variations are due to advection and adiabatic processes.

In bodies of fresh water, the temperature of the water of greatest density is 4°C, and the stratification depends solely on temperature. In this case, two types of stratification are possible: direct and inverse. Direct stratification occurs when the temperature of all the water in the lake is not less than 4°C. The warmest masses of water then lie at the surface; the cooler the other masses, the greater the depth at which they are located. Inverse stratification occurs when the water temperature is less than 4°C. The water at the surface is then cooler than in the lower layers.