Temperature can therefore be determined through direct measurements of these properties.īut how do we define a numerical scale for temperature, as measured by our thermometer? The simplest way is to define two reference points, plus a method that can be used to interpolate between them. These physical properties include the volume-to-mass ratio of a solid, liquid, or gas, or the electrical resistance of a metal or a semiconductor. In turn, the temperature of the thermometer can be related to its physical properties. This gives us both a way of talking about energy and a way of measuring temperature using a known reference. Conversely, if water and a thermometer within it are at different temperatures, then there must be a flow of energy (heat) between the water and the thermometer. Thus, if we know the temperature of one object (call it a thermometer), and it is in thermal equilibrium with water around it, which will occur after enough time has passed, we also know the temperature of the water. " There exists a scalar quantity called temperature, which is a property of all thermodynamic systems (in equilibrium states) such that temperature equality is a necessary and sufficient condition for thermal equilibrium."Īs a corollary to this definition, objects in contact with one another will tend toward thermal equilibrium by an exchange of heat between them.
A textbook definition (the Zeroth Law of Thermodynamics) says: What is temperature? This is more difficult to define than it first appears. That is, the pressure at a depth of 1000 meters is about 1000 dbars. Pressure values expressed in this way are numerically similar (within 2%) to depth in meters below the surface. Pressures in the ocean are usually measured in dbar (decibars), with values offset to read zero at the surface. Ocean state variables, their typical ranges and mean values in the ocean, and the accuracy to which they are measured (or estimated) in the deep ocean. Standards thus play an important role in ocean science. In turn, standards like EOS-80 or TEOS-10 rely on other international standards that precisely define the state variables of temperature and salinity.
This new standard is called the Thermodynamic Equation of Seawater (2010), or TEOS-10. However, a new international standard for seawater density, as well as all other thermodynamic properties, has recently been developed. For many years the internationally accepted standard for seawater densities has been the 1980 International Equation of State, known by the acronym EOS-80. Density is usually calculated using a mathematical function of temperature, salinity, and pressure, sometimes called an equation of state.
In addition to controlling physical properties, the variation in space and time of temperature and salinity are also important water mass tracers that can be used to map the ocean circulation. Important state variables measured for parcels of water in the ocean are therefore temperature, which is related to the heat content, salinity, which is related to the amount of dissolved matter, and the pressure (Table 1). Physical properties vary with the amount of heat and the amount of dissolved matter contained in the water, as well as the ambient pressure. Such a table or formula, when formally defined in a published document and endorsed by a scientific authority, is known as a standard. The table or formula that is used is usually derived from careful laboratory measurements.
However, direct measurements can be complicated to carry out, especially in the field, and in many cases it is more convenient to measure a few important 'state variables' on which the properties depend, and then look up the desired property as a function of the measured state in a table, or calculate it using a mathematical formula. Physical properties can be measured directly. Density in particular is an important property in ocean science because small spatial changes in density result in spatial variations in pressure at a given depth, which in turn drive the ocean circulation. The physical properties of seawater include both 'thermodynamic properties' like density and freezing point, as well as 'transport properties' like the electrical conductivity and viscosity.