Temperature stratification
Waters such as standing water , but also areas of the world's oceans with little current , usually show a temperature stratification of the water : This is due to the temperature-dependent density differences in the water body. There is z. B. an interaction between the temperature of the sea water , its density and its salinity , and thus influence on the Thermohaline circulation , the "engine of meteorology " worldwide.
In the earth's atmosphere , too , different temperatures of different air layers are associated with certain phenomena : Fata Morgana , inversion weather situation , stratification stability of the earth's atmosphere , vertical profile (meteorology) .
causes
The density anomaly of the water plays a special modifying role , according to which it has its maximum density of 1.0 g / ml at 3.98 ° C. Both colder and warmer water have a lower density the further its temperature deviates from 3.98 ° C.
This creates static buoyancy , which allows the specifically lighter water to rise compared to the denser one. A temperature-dependent density stratification is thus formed, which can preferably be recognized as a "temperature stratification". This stratification can, however, be modified by other factors that influence the water density in a real body of water. These are in particular the solids and gases dissolved and finely suspended in water .
The temperatures of the water are subject to constant change due to the absorption and release of heat by the water. The most important sizes are
- Heat absorption from incident global radiation , consisting of sunlight and IR counter-radiation from the atmosphere
- Heat loss through IR radiation (heat radiation, depending on the surface temperature)
- Loss of heat through evaporation of water
- Heat loss through direct heat dissipation to the air ("sensible heat")
This heat exchange is subject to both daily and seasonal cycles. Changes in the temperature stratification therefore occur just as cyclically. These result from the absorption of heat from penetrating light, which decreases with depth, as well as from a mechanical mixing of water layers, which are driven on the one hand by the wind and on the other hand by the convection currents of cooling surface water.
In the temperate latitudes, for example, sufficiently deep still waters show a “dimictic” water circulation . This means that these waters mix completely twice a year. Shallower waters, on the other hand, can be mixed to the bottom several times, ponds even every night (polymictic circulation). But there are other, very different types of mixing in the various regions of the world.
Example: Dimiktische Seen
Dimiktisch is the name given to bodies of water that are completely mixed twice during the year. Typically, these are sufficiently deep lakes in the flatlands of the temperate zones.
In winter , the deep water, which is no more than 4 ° C, is stably overlaid with even colder, less dense water and possibly ice . This layer heats up evenly in spring up to a temperature of 4 ° C, since colder water rises to the surface through convection . If the temperature is then evenly 4 ° C, wind-induced turbulence extends to the bottom, where it ensures a sufficiently high oxygen content and an even distribution of nutrients ( spring circulation ).
As the temperature continues to rise, the differences in density increase again, mixing by wind becomes less frequent and reaches ever shallower depths. The last complete mixing determines the temperature of the hypolimnion , the evenly cold deep layer. In summer it is at least 3.98 ° C in winter.
Nocturnal cooling of the surface only breaks down the gradient in an upper layer, the epilimnion , and ensures that it is regularly mixed (partial circulation).
Between the epi- and hypolimnion there is the metalimnion , the so-called thermocline, with a temperature gradient that is also pronounced at night . As a rule, the metalimnion has a fine structure in which the traces of greater wind effects and stronger cooling periods emerge as occasional centimeter-sharp gradations of alternating homogeneous and falling temperatures. Internal homogenizations between the layers also come about through balancing currents that occur during the frequent internal waves ( seiche ), because "standing water" is never really calm internally.
In lakes with shallower depths, wind can mix the lake down to the bottom even in summer, without the formation of a hypolimnion. In shallow lakes and ponds, even the metalimnion is absent, so that the body of water consists only of epilimnion, in which a temperature stratification develops during the day, but which is often mixed down to the bottom.
The stability of the temperature stratification in winter is lower, in not too deep lakes the autumn circulation changes into the spring circulation, unless a layer of ice on the lake surface completely switches off the wind propulsion. Shallow ponds can freeze to the bottom.
See also
Individual evidence
- ↑ Sunke Schmidtko, Lothar Stramma, Martin Visbeck: Decline in global oceanic oxygen content during the past five Decades. In: Nature. 542, 2017, p. 335, doi : 10.1038 / nature21399 . From: spiegel.de , Wissenschaft , February 16, 2017: Less oxygen in the oceans: the fish stay out of breath (February 17, 2017).