AskDefine | Define thermocline

User Contributed Dictionary



  1. A layer within a body of water or air where the temperature changes rapidly with depth.


layer in water

Extensive Definition

The thermocline (sometimes metalimnion) is a layer within a body of water or air where the temperature changes rapidly with depth.


Since water is not a perfectly transparent medium, almost all sunlight is absorbed in the surface layer, which heats up. Wind and waves mix the water in the surface layer and distribute heat within it somewhat, such that the temperature may be relatively uniform for up to 100 m (300 ft). Below this mixed layer, however, the temperature drops very rapidly—perhaps as much as 20 degrees Celsius with an additional 150 m (500 ft) of depth. This area of rapid transition is the thermocline; below it, the temperature continues to drop with depth, but far more gradually. In the Earth's oceans, 90% of the water is below the thermocline. This deep ocean consists of layers of equal density, being poorly mixed, and may be as cold as 0 to 3 °C.
The thermocline varies with latitude and season: it is permanent in the tropics, variable in the temperate climates (strongest during the summer), and weak to nonexistent in the polar regions, where the water column is cold from the surface to the bottom.
In the open ocean, it is characterized by a negative sound speed gradient, making the thermocline important in submarine warfare, because it can reflect active sonar.
When scuba diving, a thermocline of a few degrees Celsius can often be seen between two bodies of water, for example a colder upwelling or current running into a surface layer of warmer water. It gives the water an appearance of the wrinkled glass that is often used to obscure bathroom windows, and is caused by the altered refractive index of the cold or warm water column; these same schlieren can be observed when hot air rises off the tarmac at airports or desert roads and is the cause of mirages.

Other water bodies

Thermoclines can also be observed in relatively shallow lakes. In colder climates, this leads to a phenomenon called turnover. During the summer, warm water, which is less dense, will sit on top of colder, denser water that sinks to the bottom; with a thermocline separating them. Because the warm water is also exposed to the sun during the day, a stable system exists, and very little mixing of warm water and cold water occurs. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface. As winter approaches, the temperature of the surface water will also drop until it approaches 4 °C (39 °F), which is the temperature at which water is densest. 4 °C is, generally speaking, the temperature of the water below a thermocline. When the entire body of water is at or close to 4 °C, 'fall (autumn) turnover' begins - the thermocline disappears, (or, to say a different way, it reaches the surface) and the water from the bottom of the lake can mix freely with the water from the top. This process is aided by wind or any other process that agitates the water. This effect also occurs in Arctic and Antarctic waters, enriching surface nutrients and producing blooms of phytoplankton, making these very rich and diverse ecosystems.
As the temperature continues to drop, in those locations where it does, the water on the surface begins to get cold enough to freeze and the lake begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C.
Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single point to oscillate (usually as a form of seiche). Alternately the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.


Formation of a verical profile thermocline is a very common phenomenon, particularly in the lower atmosphere, due to the diurnal effects of the solar heating/cooling cycle. For example, in midday conditions of intense solar insolation, the temperature at the earth's surface is typically higher than conditions aloft. This circumstance leads to an effect of stratified verical temperature of the air, such that higher air temperatures exist near the earth's surface. Conversely, at night cooler, denser air typically settles near the earth, surface, leading to the opposite style of thermocline. This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways and noise barriers.
thermocline in Danish: Springlag
thermocline in German: Metalimnion
thermocline in Spanish: Termoclina
thermocline in French: Thermocline
thermocline in Dutch: Thermocline
thermocline in Japanese: サーモクライン
thermocline in Norwegian: Termoklin
thermocline in Norwegian Nynorsk: Termoklin
thermocline in Polish: Metalimnion
thermocline in Portuguese: Termoclina
thermocline in Swedish: Termoklin
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