in physics, property of systems that determines whether they are in thermal equilibrium (see  Thermodynamics). The concept of temperature stems from the idea of measuring relative hotness and coldness and from the observation that the addition of heat to a body leads to an increase in temperature as long as no melting or boiling occurs. In the case of two bodies at different temperatures, heat will flow from the hotter to the colder until their temperatures are identical and thermal equilibrium is reached . Thus, temperatures and heat, although interrelated, refer to different concepts, temperature being a property of a body and heat being an energy flow to or from a body by virtue of a temperature difference. 

Temperature changes have to be measured in terms of other property changes of a substance. Thus, the conventional mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change. If heat is added to an ideal gas contained in a constant-volume vessel, the pressure increases, and the temperature change can be determined from the pressure change by Gay-Lussac's law (see  Gases), provided the temperature is expressed on the absolute scale.


II. Temperature Scales



One of the earliest temperature scales was that devised by the German physicist Gabriel Daniel Fahrenheit. According to this scale, at standard atmospheric pressure, the freezing point (and melting point of ice) is 32° F, and the boiling point is 212° F. The centigrade, or Celsius scale, invented by the Swedish astronomer Anders Celsius, and used throughout most of the world, assigns a value of 0° C to the freezing point and 100° C to the boiling point. In scientific work, the absolute or Kelvin scale, invented by the British mathematician and physicist William Thomson, 1st Baron Kelvin, is most widely used. In this scale, absolute zero is at -273.16° C, which is zero K, and the degree intervals are identical to those measured on the Celsius scale . The corresponding "absolute Fahrenheit" or Rankine scale, devised by the British engineer and physicist William J. M. Rankine, places absolute zero at -459.69° F, which is 0° R, and the freezing point at 491.69° R. A more consistent scientific temperature scale, based on the Kelvin scale, was adopted in 1933.

III. Effects of Temperature


Temperature plays an important part in determining the conditions in which living matter can exist. Thus, birds and mammals demand a very narrow range of body temperatures for survival and must be protected against extreme heat or cold . Aquatic species can exist only within a narrow temperature range of the water, which differs for various species. Thus, for example, the increase in temperature of river water by only a few degrees as a result of heat discharged from power plants may kill most of the native fish.

The properties of all materials are also markedly affected by temperature changes. At arctic temperatures, for example, steel becomes very brittle and breaks easily, and liquids either solidify or become very viscous, offering high frictional resistance to flow . At temperatures near absolute zero, many materials exhibit strikingly different characteristics . At high temperatures, solid materials liquefy or become gaseous; chemical compounds may break up into their constituents.

The temperature of the atmosphere is greatly influenced by both the land and the sea areas. In January, for example, the great landmasses of the northern hemisphere are much colder than the oceans at the same latitude, and in July the situation is reversed. At low elevations the air temperature is also determined largely by the surface temperature of the earth. The periodic temperature changes are due mainly to the sun's radiant heating of the land areas of the earth, which in turn convect heat to the overlying air. As a result of this phenomenon, the temperature decreases with altitude, from a standard reference value of 15.5° C (60° F) at sea level (in temperate latitudes), to about -55° C (about -67° F) at about 11,000 m (about 36,000 ft). Above this altitude, the temperature remains nearly constant up to about 33,500 m (about 110,000 ft). For the temperature-humidity index.