319
thermocline
tion and turbulence, is more important. In the sea, if convection and turbulence are suppressed by stable stratification thermal properties are conserved
over long periods and distances.
thermal expansion the increase in specific volume (-7density) with increasing
temperature. In seawater the rate of increase, given by the thermal expansion coefficient, depends on the salinity. For freshwater, below 4 °C this coefficient is negative, but for normal seawater it is always positive.
thermal pollution effects due to man-made heat input in natural systems, e.g.,
due to input of hot cooling water. -7pollution.
thermal spring hot-water spring, typically occurring on land, rather than on the
deep-sea floor. The springs on Iceland are a prime example, Iceland being
the surface expression of the Mid-Atlantic Ridge. -7hydrothermal vents.
thermal wind relation originally in meteorology, also used in oceanography for
baroclinic flow, denoting that the rate of change of the horizontal current with
depth is related to the horizontal gradient of the density field.
thermistor a temperature-sensitive conductor. The electrical resistance of a conductor is always to a certain extent dependent on temperature. Some materials have a very high temperature coefficient, mostly negative (NTC resistors), sometimes positive (PTCs). Such materials which strongly change
their conductive properties in response to temperature changes can be used
as temperature sensors. By measuring the electrical resistance with a
Wheatstone or Scheering bridge, one obtains a signal that is (nonlinearly)
related to temperature. The bridge (balance) method permits an extremely
high sensitivity. The disadvantage that a thermistor produces heat (electric
current is passing through a resistor), can be minimized by measuring
pulsewise. Thermistors can also be used to measure tiny water movements,
as their actual temperature results from the heat they produce and the rate
at which this heat is dissipated to the medium, while dissipation is larger
when the medium flows faster along the thermistor.
thermocline layer at some distance below the surface where the temperature
rapidly decreases with depth (-7stratification). During the summer a seasonal
thermocline often develops at a depth between 10 and 100 m which disappears in winter. Large ocean areas also have a permanent thermocline that
is less sharp and is often defined as reaching down to the more homogeneous deep water masses. This permanent thermocline may cover a depth
range from 100 m down to over 1000 m, depending on water structure and
location. On quiet days a diurnal thermocline may also develop on top of the
seasonal thermocline. (Figure see p. 320)
thermocline
tion and turbulence, is more important. In the sea, if convection and turbulence are suppressed by stable stratification thermal properties are conserved
over long periods and distances.
thermal expansion the increase in specific volume (-7density) with increasing
temperature. In seawater the rate of increase, given by the thermal expansion coefficient, depends on the salinity. For freshwater, below 4 °C this coefficient is negative, but for normal seawater it is always positive.
thermal pollution effects due to man-made heat input in natural systems, e.g.,
due to input of hot cooling water. -7pollution.
thermal spring hot-water spring, typically occurring on land, rather than on the
deep-sea floor. The springs on Iceland are a prime example, Iceland being
the surface expression of the Mid-Atlantic Ridge. -7hydrothermal vents.
thermal wind relation originally in meteorology, also used in oceanography for
baroclinic flow, denoting that the rate of change of the horizontal current with
depth is related to the horizontal gradient of the density field.
thermistor a temperature-sensitive conductor. The electrical resistance of a conductor is always to a certain extent dependent on temperature. Some materials have a very high temperature coefficient, mostly negative (NTC resistors), sometimes positive (PTCs). Such materials which strongly change
their conductive properties in response to temperature changes can be used
as temperature sensors. By measuring the electrical resistance with a
Wheatstone or Scheering bridge, one obtains a signal that is (nonlinearly)
related to temperature. The bridge (balance) method permits an extremely
high sensitivity. The disadvantage that a thermistor produces heat (electric
current is passing through a resistor), can be minimized by measuring
pulsewise. Thermistors can also be used to measure tiny water movements,
as their actual temperature results from the heat they produce and the rate
at which this heat is dissipated to the medium, while dissipation is larger
when the medium flows faster along the thermistor.
thermocline layer at some distance below the surface where the temperature
rapidly decreases with depth (-7stratification). During the summer a seasonal
thermocline often develops at a depth between 10 and 100 m which disappears in winter. Large ocean areas also have a permanent thermocline that
is less sharp and is often defined as reaching down to the more homogeneous deep water masses. This permanent thermocline may cover a depth
range from 100 m down to over 1000 m, depending on water structure and
location. On quiet days a diurnal thermocline may also develop on top of the
seasonal thermocline. (Figure see p. 320)