What is temperature?
Temperature is the physical quantity which determines the degree of hotness or coldness of a body. More precisely, temperature is that property which governs net heat transfer between two systems. When two systems that are at different temperatures are brought into contact with each other there is a flow of heat from the hotter to the colder system until thermal equilibrium is established. This heat transfer may occur by conduction, convection or radiation.
On a microscopic scale, thermal energy is related to the kinetic energy of atoms and molecules that make up a system (a body or a fluid). The thermal agitation of its constituent molecules is manifested both in linear and vibrational motion. It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy.
The basic unit of temperature in the International System of Units is the kelvin (K). Two other units are currently used: the degree Celsius (°C) and the degree Fahrenheit (°F).
The current and official scale is the International Temperature Scale ITS-90.
It is defined from fixed points (freezing points, triple points or boiling points of single elements) and interpolating equations between fixed points.
A good and simple overview of temperature and heat can be found at the Wikipedia website, which can be read in several languages. There is brief discussion of temperature measurement, some elementary theory, ITS-90, temperature scales and a temperature conversion calculator. There are also links to other sites.
Why is temperature important?
What is a temperature scale?
The history of temperature scale development is long and far from straightforward. A sketch of the main development from 1878 to 1990 can be found at the BIPM site.
The International Temperature Scale ITS-90 is the current and official scale. It is based on fixed points (phase transitions of pure materials) and interpolating equations. It is the scale which accredited labs refer to in calibration certificates.
There is another scale for low temperatures: The Provisional Low Temperature Scale of 2000 (see below) is mainly for temperatures below 0.65 K, the lower limit of the ITS-90. The PLTS-2000 was recommended by the CIPM in 2000 to extend the ITS-90 to temperatures in the temperature range 0.9 mK to 1 K. T2000 is defined in terms of the melting pressure of 3He.
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