Temperature is a physical quantity that is a measure of hotness and coldness on a numerical scale. It is a measure of the local thermal energy of matter or radiation; it is measured by a thermometer, which may be calibrated in any of various temperature scales, Celsius, Fahrenheit, Kelvin, etc. Empirically it is found that an isolated system, one that exchanges no energy or material with its environment, tends to a spatially uniform temperature as time passes. When a path permeable only to heat is open between two bodies, energy always flows spontaneously as heat from a hotter body to a colder one. The flow rate increases with the temperature difference and with a property of the path called the thermal conductance. Between two bodies with the same temperature, connected by a path permeable only to heat, no energy will flow. These bodies are said to be "in the relation of thermal equilibrium". In a body in its own internal thermal equilibrium, the temperature is spatially uniform. In kinetic theory and in statistical mechanics, temperature is explained by the motions and interactions of microscopic constituents such as molecules and photons. In theory, there is a coldest possible temperature, which is called absolute zero, though it cannot be reached by any actual physical device. It is denoted by 0 K on the Kelvin scale, −273.15°C on the Celsius scale. In matter at the absolute zero of temperature, the motions of microscopic constituents are minimal; moreover their energies are also minimal. For the microscopic constituent particles of a body as a whole, there is no maximal energy. For a body in internal thermal equilibrium as a whole, the ordinary thermodynamic temperature is always positive. Besides this, there is also a special concept of 'negative thermodynamic temperature', requiring continual energetic maintenance, for certain component particles of a body, such as have available to them constrained maximal energies. The rest of the body is very cold, and the body as a whole is not in internal thermodynamic equilibrium, so that it does not have a temperature as ordinarily and properly defined. The component particles of the body with 'negative temperature' tend by fluctuations to give up energy to the other parts of the body, and in that sense, the parts with 'negative temperature' are hotter than the other parts. Temperature is important in all fields of natural science, including physics, geology, chemistry, atmospheric sciences and biology.
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