degree Celsius
Temperatureexact by definition
The degree Celsius is the everyday metric temperature unit, defined exactly as the kelvin shifted down by 273.15, so t/°C = T/K − 273.15. A degree Celsius and a kelvin are the same size, which is why the conversion factor is 1 and only the offset differs. Both the factor and the 273.15 offset are exact by definition, not measured.
Watch out: 0 °C and 100 °C are no longer definitions. They were fixed points on the old scale, but since the kelvin was tied to the triple point and later to the Boltzmann constant, the freezing and boiling points of water at one atmosphere are measured values that land at about 0.0000 °C and 99.974 °C. The boiling point is the one that visibly misses.
| 1 °C | 1 °C (offset scale) |
Anders Celsius proposed his hundred-step scale in 1742 with the numbers running the other way: 0 marked the boiling point of water and 100 the freezing point. The scale was inverted to its modern sense shortly after his death, and the unit was called the centigrade degree until 1948, when it was renamed for Celsius to avoid a clash with the centigrade angle used by French surveyors.
About the degree celsius
The Celsius scale is a shifted kelvin scale and nothing more. Add 273.15 and you have kelvin, subtract it and you are back. The size of the step is identical, which is the whole reason scientists can quote a specific heat in J/(kg·K) and then use it in a calculation where the temperatures are in Celsius: the kelvins in the denominator are temperature differences, and a difference of one kelvin is a difference of one Celsius degree exactly.
What is worth knowing is that the two famous anchors are no longer part of the definition. Celsius originally fixed his scale on the boiling point and the melting point of ice, and for two centuries the freezing and boiling points of water were what the scale meant. Today the scale is defined off the kelvin, and the kelvin is defined off the Boltzmann constant, so water's phase changes have become experimental results. Ice melts at 0 °C to within a fraction of a millikelvin, but water at standard atmospheric pressure boils at about 99.974 °C, not 100. Nobody notices in a kitchen, and everybody notices in a calibration laboratory.
A practical note for anyone reading a thermometer: 0 °C is 32 °F, 100 °C is 212 °F, and the two scales cross at −40, where \(-40\)°C and \(-40\)°F are the same temperature. The mental shortcut for weather is to double the Celsius number and add 30, which gets you within a couple of degrees over the range people actually live in.