A convenient operational definition of temperature is that it is a measure of the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/temper.html</ref>
Temperature is expressed in degrees. There are two units of temperature in common use - Celsius (°C) and Fahrenheit (°F). Both make reference to the freezing and boiling points of pure water at normal atmospheric pressure.
Celsius (sometimes known as centigrade in the past) sets the freezing point at 0 degrees (0 °C) and the boiling point at 100 degrees (100 °C).<br> Fahrenheit sets the freezing point at 32 degrees (32 °F) and the boiling point at 212 degrees (212 °F).<br> Conversion of Fahrenheit to Celsius: °C = ( °F - 32 ) x 5 ÷ 9<br> Conversion of Celsius to Fahrenheit: °F = ( °C x 9 ÷ 5 ) + 32
Universally Celsius is the predominant system but Fahrenheit still holds sway in the USA.
An alternative scale, the Kelvin scale (after the famous Irish-born physicist), is used in scientific and engineering circles. It is a Absolute temperature scale scale: it sets 0 K at the absolute zero: the coldest possible temperature in the universe, where an object has no thermal energy. The unit of the Kelvin scale (K) is equal to the unit of the Celsius scale (°C). Thus the freezing point of water at normal atmospheric pressure occurs at 273.15 K.
The Kelvin is one of the fundamental seven SI units. Today the Kelvin scale is defined by two points: the absolute zero and the triple point of water (273.16 K). No degree symbol (°) is used with the Kelvin symbol (K).
In US engineering fields another scale, the Rankine scale, is still used. As the Kelvin it is a thermodynamic (absolute) temperature scale, but one Rankine unit (°R) equals one Fahrenheit unit (°F). Thus, the freezing point of water at normal atmospheric pressure occurs at 459.67 °R.
- Snippet from Wikipedia: Temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making up a substance.
Thermometers are calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. The most common scales are the Celsius scale with the unit symbol °C (formerly called centigrade), the Fahrenheit scale (°F), and the Kelvin scale (K), the latter being used predominantly for scientific purposes. The kelvin is one of the seven base units in the International System of Units (SI).
Absolute zero, i.e., zero kelvin or −273.15 °C, is the lowest point in the thermodynamic temperature scale. Experimentally, it can be approached very closely but not actually reached, as recognized in the third law of thermodynamics. It would be impossible to extract energy as heat from a body at that temperature.
Temperature is important in all fields of natural science, including physics, chemistry, Earth science, astronomy, medicine, biology, ecology, material science, metallurgy, mechanical engineering and geography as well as most aspects of daily life.