Celsius, formerly known as centigrade, is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval, a difference between two temperatures or an uncertainty. The unit was known until 1948 as "centigrade" from the Latin centum translated as 100 and gradus translated as "steps".
From 1744 until 1954, 0 °C was defined as the freezing point of water and 100 °C was defined as the boiling point of water, both at a pressure of one standard atmosphere with mercury being the working material. Although these defining correlations are commonly taught in schools today, by international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero, and the triple point of VSMOW (specially-purified water). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible at which matter reaches minimum entropy, is defined as being precisely 0 K and −273.15 °C. The temperature of the triple point of water is defined as precisely 273.16 K and 0.01 °C.
This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 (approximately 0.00366) of the difference between absolute zero and the triple point of water. Thus, it sets the magnitude of one degree Celsius and that of one kelvin as exactly the same. Additionally, it establishes the difference between the two scales' null points as being precisely 273.15 degrees Celsius (and).
In 1742, Swedish astronomer Anders Celsius (1701–1744) created a temperature scale which was the reverse of the scale now known by the name "Celsius": 0 represented the boiling point of water, while 100 represented the freezing point of water. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that the melting point of ice is essentially unaffected by pressure. He also determined with remarkable precision how the boiling point of water varied as a function of atmospheric pressure. He proposed that the zero point of his temperature scale, being the boiling point, would be calibrated at the mean barometric pressure at mean sea level. This pressure is known as one standard atmosphere. The BIPM's 10th General Conference on Weights and Measures (CGPM) later defined one standard atmosphere to equal precisely per square centimeter (101.325 kPa).
In 1744, coincident with the death of Anders Celsius, the Swedish botanist Carolus Linnaeus (1707–1778) reversed Celsius's scale upon receipt of his first thermometer featuring a scale where zero represented the melting point of ice and 100 represented the boiling point. His custom-made "linnaeus-thermometer", for use in his greenhouses, was made by Daniel Ekström, Sweden's leading maker of scientific instruments at the time and whose workshop was located in the basement of the Stockholm observatory. As often happened in this age before modern communications, numerous physicists, scientists, and instrument makers are credited with having independently developed this same scale; among them were Pehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop) and with whom Linnaeus had been corresponding; Christian of Lyons; Daniel Ekström, the instrument maker; and Mårten Strömer (1707–1770) who had studied astronomy under Anders Celsius.
The first known document reporting temperatures in this modern "forward" Celsius scale is the paper Hortus Upsaliensis dated 16 December 1745 that Linnaeus wrote to a student of his, Samuel Nauclér. In it, Linnaeus recounted the temperatures inside the orangery at the Botanical Garden of Uppsala University:
Since the 19th century, the scientific and thermometry communities worldwide referred to this scale as the centigrade scale. Temperatures on the centigrade scale were often reported simply as degrees or, when greater specificity was desired, as degrees centigrade. The symbol for temperature values on this scale is °C.
Because the term centigrade was also the Spanish and French language name for a unit of angular measurement (1/10,000 of a right angle) and had a similar connotation in other languages, the term centesimal degree was used when very precise, unambiguous language was required by international standards bodies such as the BIPM. The 9th CGPM and the CIPM (Comité international des poids et mesures) formally adopted "degree Celsius" (symbol: °C) in 1948. 
For scientific use, "Celsius" is the term usually used with "centigrade" otherwise continuing to be in common use.
Some key temperatures relating the Celsius scale to other temperature scales are shown in the table below.
(precisely, by definition)
|0 K||−273.15 °C||−459.67 °F|
|Boiling point of liquid nitrogen||77.4 K||−195.8 °C||−320.3 °F|
|Sublimation point of dry ice.||195.1 K||−78 °C||−108.4 °F|
|Intersection of Celsius and Fahrenheit scales.||233.15 K||−40 °C||−40 °F|
|Melting point of H2O (purified ice)||273.15 K||−0.0001 °C||32 °F|
|Water's triple point|
(precisely, by definition)
|273.16 K||0.01 °C||32.018 °F|
|Normal human body temperature (approximate average)||310. K||37.0 °C||98.6 °F|
|Water's boiling point at 1 atm (101.325 kPa)|
(approximate: see Boiling point)
|373.1339 K||99.9839 °C||211.971 °F|
The "degree Celsius" has been the only SI unit whose full unit name contains an uppercase letter since the SI base unit for temperature, the kelvin, became the proper name in 1967 replacing the term degree Kelvin. The plural form is degrees Celsius.
The general rule of the International Bureau of Weights and Measures (BIPM) is that the numerical value always precedes the unit, and a space is always used to separate the unit from the number, (not "" or ""). Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle (°, ′, and ″, respectively), for which no space is left between the numerical value and the unit symbol. Other languages, and various publishing houses, may follow different typographical rules.
Unicode provides a compatibility character for the degree Celsius at U+2103 (decimal 8451), for compatibility with CJK encodings that provide such a character (as such, in most fonts the width is the same as for fullwidth characters). Its appearance is similar to the one synthesized by individually typing its two components (°) and (C). Shown below is the degree Celsius character followed immediately by the two-component version:℃ °C
When viewed on computers that properly support Unicode, the above line may be similar to the image in the line below (enlarged for clarity):
The canonical decomposition is simply an ordinary degree sign and "C", so some browsers may simply display "°C" in its place due to Unicode normalization.
The degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures. The degree Celsius is also subject to the same rules as the kelvin with regard to the use of its unit name and symbol. Thus, besides expressing specific temperatures along its scale (e.g. "Gallium melts at 29.7646 °C" and "The temperature outside is 23 degrees Celsius"), the degree Celsius is also suitable for expressing temperature intervals: differences between temperatures or their uncertainties (e.g. "The output of the heat exchanger is hotter by 40 degrees Celsius", and "Our standard uncertainty is ±3 °C"). Because of this dual usage, one must not rely upon the unit name or its symbol to denote that a quantity is a temperature interval; it must be unambiguous through context or explicit statement that the quantity is an interval. This is sometimes solved by using the symbol °C (pronounced "degrees Celsius") for a temperature, and C° (pronounced "Celsius degrees") for a temperature interval, although this usage is non-standard.
What is often confusing about the Celsius measurement is that it follows an interval system but not a ratio system; that it follows a relative scale not an absolute scale. This is put simply by illustrating that while 10 °C and 20 °C have the same interval difference as 20 °C and 30 °C the temperature 20 °C is not twice the air heat energy as 10 °C. As this example shows, degrees Celsius is a useful interval measurement but does not possess the characteristics of ratio measures like weight or distance.
In science and in engineering, the Celsius scale and the Kelvin scale are often used in combination in close contexts, e.g., "...a measured value was 0.01023 °C with an uncertainty of 70 µK..."). This practice is permissible because the magnitude of the degree Celsius is equal to that of the kelvin.
Notwithstanding the official endorsement provided by decision
This practice should be avoided for literature directed to lower-level technical fields and in non-technical articles intended for the general public where both the kelvin and its symbol, K, are not well recognized and could be confusing.
One effect of defining the Celsius scale at the triple point of Vienna Standard Mean Ocean Water (VSMOW, 273.16 K and 0.01 °C), and at absolute zero (0 K and −273.15 °C), is that neither the melting nor boiling point of water under one standard atmosphere (101.325 kPa) remain defining points for the Celsius scale. In 1948 when the 9th General Conference on Weights and Measures (CGPM) in Resolution 3 first considered using the triple point of water as a defining point, the triple point was so close to being 0.01 °C greater than water's known melting point, it was simply defined as precisely 0.01 °C. However, current measurements show that the triple and melting points of VSMOW are actually very slightly (<0.001 °C) greater than 0.01 °C apart. Thus, the actual melting point of ice is very slightly (less than a thousandth of a degree) below 0 °C. Also, defining water's triple point at 273.16 K precisely defined the magnitude of each 1 °C increment in terms of the absolute thermodynamic temperature scale (referencing absolute zero). Now decoupled from the actual boiling point of water, the value "100 °C" is hotter than 0 °C—in absolute terms—by a factor of precisely
This boiling-point difference of 16.1 millikelvin between the Celsius scale's original definition and the current one (based on absolute zero and the triple point) has little practical meaning in common daily applications because water's boiling point is very sensitive to variations in barometric pressure. For example, an altitude change of only 28 cm (11 in) causes the boiling point to change by one millikelvin.
Throughout the world, except in the United States, Belize and a few other countries, the Celsius temperature scale is used for practically all purposes. The only exceptions are some specialist fields (e.g., low-temperature physics, astrophysics, light temperature in photography) where the closely related Kelvin scale dominates instead.
Even in the U.S., almost the entire scientific field and many engineering fields use the Celsius scale, and the metric system in general. However, most Americans remain more accustomed to the Fahrenheit scale, which is the scale that U.S. broadcasters use in weather forecasting. It is also commonly used in the U.S. for measurement of body temperature, and household use such as cooking, and is the scale commonly seen on ovens and in recipes. In Canada, kitchen devices, literature, and packaging include both Fahrenheit and Celsius quotations.
The United Kingdom has gradually increased use of the Celsius scale since the 1970s and is now the predominent temperature scale used, but it is widely called centigrade. Most broadcasters and publications still quote Fahrenheit air temperatures alongside Celsius in weather forecasts, and air-temperature thermometers sold show both scales.
1. All common temperature scales would have their units named after someone closely associated with them; namely, Kelvin, Celsius, Fahrenheit, Réaumur and Rankine.
2. Notwithstanding the important contribution of Linnaeus who gave the Celsius scale its modern form, Celsius' name was the obvious choice because it began with the letter C. Thus, the symbol °C that for centuries had been used in association with the name centigrade could continue to be used and would simultaneously inherit an intuitive association with the new name.
3. The new name eliminated the ambiguity of the term "centigrade", freeing it to refer exclusively to the French-language name for the unit of angular measurement.