Thermal bar
Encyclopedia
A thermal bar is a hydrodynamic feature that forms around the edges of holomictic
lakes during the seasonal transition to stratified
conditions, due to the shorter amount of time required for shallow areas of the lake to stratify.
, shallow areas generally become stratified before deeper areas. In large lakes this condition may persist for weeks, during which a temperature front
known as a thermal bar forms between the stratified and unstratified areas of the lake. The thermal bar generally forms parallel to shore and moves toward the lake center as deeper areas of the lake stratify. While thermal bars can form in both fall and spring
, most studies of the thermal bar have investigated aspects of the feature in the spring, when the lake is warming up and the summer thermocline
is beginning to form.
At the lake surface, the thermal bar may be visible as a foam line between the stratified water shoreward of the thermal bar and unstratified water on the offshore side. At this convergence, waters mix and sink when they reach the temperature of maximum density
, roughly 4 degrees Celsius for freshwater, a process known as cabbeling.
The downwelling
of dense water at the thermal bar acts as a barrier to horizontal mixing. In spring, this concentrates warm water and suspended materials in the near shore waters around the edge of the lake. Satellite imagery
has been used to identify thermal bars using their thermal characteristics as well as the concentration of suspended materials on their shoreward side, typically due to surface runoff
to the lake.
Isotherms on the stratified side of the thermal bar slope away from the bar, producing a pressure gradient force
that when balanced by the Coriolis force produces a cyclonic coastal geostrophic current that transports water and suspended matter along the shore.
The thermal bar phenomena was first described by François-Alphonse Forel
in his study of Lac Leman. Additional studies have been carried out in Lake Ladoga
, Lake Baikal
and the Laurentian Great Lakes.
by restricting mixing between coastal and offshore waters. This role is particularly evident during the spring runoff period when the retention of coastal waters may benefit aquatic organisms by providing warmer water temperatures and elevated nutrient concentrations, or may threaten coastal environments by retaining pollutants in coastal waters .
Holomictic
Holomictic lakes are lakes, which, at some time during the year, have a uniform temperature and density from top to bottom, allowing the lake waters to completely mix. Holomictic lakes are non-meromictic lakes....
lakes during the seasonal transition to stratified
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
conditions, due to the shorter amount of time required for shallow areas of the lake to stratify.
Description
During the process of lake stratificationLake stratification
Lake stratification is the separation of lakes into three layers:# Epilimnion - top of the lake.# Metalimnion - middle layer that may change depth throughout the day.# Hypolimnion - the bottom layer....
, shallow areas generally become stratified before deeper areas. In large lakes this condition may persist for weeks, during which a temperature front
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...
known as a thermal bar forms between the stratified and unstratified areas of the lake. The thermal bar generally forms parallel to shore and moves toward the lake center as deeper areas of the lake stratify. While thermal bars can form in both fall and spring
Spring (season)
Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...
, most studies of the thermal bar have investigated aspects of the feature in the spring, when the lake is warming up and the summer thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
is beginning to form.
At the lake surface, the thermal bar may be visible as a foam line between the stratified water shoreward of the thermal bar and unstratified water on the offshore side. At this convergence, waters mix and sink when they reach the temperature of maximum density
Maximum density
The maximum density of a substance is the highest attainable density of the substance under given conditions.-Attaining maximum density:Almost all known substances undergo thermal expansion in response to heating, meaning that a given mass of substance contracts to a low volume at low temperatures,...
, roughly 4 degrees Celsius for freshwater, a process known as cabbeling.
The downwelling
Downwelling
Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together...
of dense water at the thermal bar acts as a barrier to horizontal mixing. In spring, this concentrates warm water and suspended materials in the near shore waters around the edge of the lake. Satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
has been used to identify thermal bars using their thermal characteristics as well as the concentration of suspended materials on their shoreward side, typically due to surface runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
to the lake.
Isotherms on the stratified side of the thermal bar slope away from the bar, producing a pressure gradient force
Pressure gradient force
The pressure gradient force is not actually a 'force' but the acceleration of air due to pressure difference . It is usually responsible for accelerating a parcel of air from a high atmospheric pressure region to a low pressure region, resulting in wind...
that when balanced by the Coriolis force produces a cyclonic coastal geostrophic current that transports water and suspended matter along the shore.
The thermal bar phenomena was first described by François-Alphonse Forel
François-Alphonse Forel
François-Alphonse Forel was a Swiss scientist who pioneered the study of lakes, and is thus considered the founder of limnology....
in his study of Lac Leman. Additional studies have been carried out in Lake Ladoga
Lake Ladoga
Lake Ladoga is a freshwater lake located in the Republic of Karelia and Leningrad Oblast in northwestern Russia, not far from Saint Petersburg. It is the largest lake in Europe, and the 14th largest lake by area in the world.-Geography:...
, Lake Baikal
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...
and the Laurentian Great Lakes.
Ecological significance
Although a temporary seasonal feature, the thermal bar plays an important role in lake ecologyEcology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
by restricting mixing between coastal and offshore waters. This role is particularly evident during the spring runoff period when the retention of coastal waters may benefit aquatic organisms by providing warmer water temperatures and elevated nutrient concentrations, or may threaten coastal environments by retaining pollutants in coastal waters .
See also
- Water (molecule)#Density of water and ice
- LimnologyLimnologyLimnology , also called freshwater science, is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters...
- WaterWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
- Aquatic ecology
Additional references
- Paul R. Holland and Anthony Kay, “A review of the physics and ecological implications of the thermal bar circulation,” Limnologica - Ecology and Management of Inland Waters 33, no. 3 (September 2003): 153-162.
- C. H. Mortimer, “Lake hydrodynamics." Mitt. Int, Ver. Limnol 20 (1974): 124–197.
- F. Peeters and R. Kipfer, “Currents in Stratified Water Bodies 1: Density-Driven Flows,” in Encyclopedia of Inland Waters (Oxford: Academic Press, 2009), 530-538.