Stratification (water)

Standing water usually have a temperature stratification of the water. This is due to the temperature-dependent density differences in the water body.

Causes

Modifying a special role is played by the density anomaly of water, after which the latter has its maximum density of 1.0 g / ml at 3.98 ° C. Both cooler and warmer water has an even lower density, the further value of its temperature of 3.98 ° C.

This creates static buoyancy, which can ascend the specifically lighter than the water over denser. So it is formed from a temperature-induced density stratification, which can preferably be seen as " stratification ". This layering can also be modified by other factors, which affect the water density in a real body of water. These are in particular the dissolved solids in the water and feinsuspendierten and gases.

The temperatures of the water are subject to constant change due to the absorption and release of heat by the waters. The most important factors here are

• Heat absorption of incident global radiation, consisting of sunlight and IR back radiation of the atmosphere
• Heat loss by infrared radiation (heat radiation, depending on the surface temperature )
• Heat loss by evaporation of water
• Loss of heat by direct heat dissipation to the air (" sensible heat " )

This heat exchange is subject to both daily and seasonal cycles. Just so cyclical, it comes also to changes in the temperature stratification. These arise both from a decreasing with depth absorption of heat from seeping light, as well as from a mechanical mixing of water layers that are driven on the one hand by the wind and the other by the convection of cooling mode to surface water.

In the temperate zones have sufficiently deep standing water, for example, to a " dimiktische " water circulation. This means that these waters mix twice a year completely. Shallower waters, however, can repeatedly ponds even every night, are mixed down to the bottom ( polymictic circulation). But there are in the various regions of the world yet another, very different mixing types.

Example: Dimiktische lakes

Dimictic called the waters of the two completely mixed in the course of the year. Typically, these are sufficiently deep lakes in the lowlands of the temperate zones.

In winter, this is more than 4 ° C ' warm ' deep water with even colder, less dense water and optionally superimposed ice stable. This layer is heated in spring to a temperature of 4 ° C uniform because colder water rises by convection to the surface. Then the temperature evenly 4 ° C, ranging from wind -driven turbulence to the bottom where it ensures a sufficiently high oxygen content and a uniform distribution of nutrients (spring circulation).

With further increasing temperature, the density differences increase again, mixing by wind is rare and always reaches shallower depth. The most recent complete mixing determines the temperature of the hypolimnion, the uniformly cold deep layer. It is in summer at least, in winter at most 3.98 ° C.

Nocturnal cooling of the surface builds only in an upper layer, the epilimnion, the gradient and ensures that they are regularly mixing (Part circulation).

Between the epilimnion and hypolimnion of the metalimnion, called the thermocline, with a strong temperature gradient even at night. The metalimnion typically has a fine structure in which the traces of larger wind effects and stronger cooling periods are emerging as occasionally sharp centimeter gradations of alternating homogeneous and falling temperatures. Internal homogenization between the layers also include compensatory currents concluded that occur during the frequent internal waves ( seiche ), because " stagnant waters " are internally never really quiet.

In lakes with lower depth can wind mix even in summer the lake to the bottom, below is the formation of a hypolimnion. In shallow lakes and ponds even the metalimnion is omitted, so that the water consists only of the epilimnion, where although daily cyclic temperature stratification forms, but which is often mixed to the bottom.

The stability of the temperature stratification in the winter is lower, in not too deep lakes the autumn circulation goes on in the spring circulation, if not a layer of ice on the lake surface the drive completely off by wind. Shallow ponds can freeze to the bottom.