Celsius (or centigrade) is a temperature scale named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed such a system two years before his death. The term degrees Celsius (symbol: °C) refers to a specific temperature on the Celsius temperature scale. The degree Celsius is also a unit increment of temperature for use in indicating a temperature interval (a difference between two temperatures).
The Celsius scale has been adopted as a standard for regular temperature measurements by most countries of the world and by the entire scientific community. In the United States, however, the Celsius scale is used mainly by scientists and many engineers (especially in high-tech fields), while the Fahrenheit scale is commonly used by the lay public and by people in government, industry, and meteorology.
|Fahrenheit||Celsius||°F = (°C × 1.8) + 32|
|Celsius||Fahrenheit||°C = (°F − 32) ÷ 1.8|
|kelvin||Celsius||K = °C + 273.15|
|Celsius||kelvin||°C = K − 273.15|
|For temperature intervals rather than specific temperatures,
1 °C = 1 kelvin
1 °C = 1.8 °F
Comparisons among various temperature scales
Conversion calculator for units of temperature
Defining the scale
Until 1954, 0 °C on the Celsius scale was defined as the melting point of ice and 100 °C was the boiling point of water under a pressure of one standard atmosphere; this simplified definition is still commonly taught in schools. However, the unit “degree Celsius” and the Celsius scale are now, by international agreement, defined by two points: absolute zero and the triple point of specially prepared water (Vienna Standard Mean Ocean Water, or VSMOW).
Absolute zero—the temperature at which nothing could be colder and no heat energy remains in a substance—is defined as being precisely 0 K and −273.15 °C. The temperature of the triple point of water is defined as being precisely 273.16 K and 0.01 °C.
This definition fixes the magnitude of both the degree Celsius and degree Kelvin as being precisely 1/273.16 the difference between absolute zero and the triple point of water. Thus, it sets the magnitude of one degree Celsius and one Kelvin to be exactly equivalent. In addition, it establishes the difference between the two scales’ null points as being precisely 273.15 degrees Celsius (−273.15 °C = 0 K and 0.01 °C = 273.16 K).
Some key temperatures relating the Celsius scale to other temperature scales are shown in the table below.
(precise, by definition)
|0 K||−273.15 °C||−459.67 °F|
|Melting point of ice||273.15 K||0 °C||32 °F|
|Water’s triple point
(precise, by definition)
|273.16 K||0.01 °C||32.018 °F|
|Water’s boiling point A||373.1339 K||99.9839 °C||211.9710 °F|
A For Vienna Standard Mean Ocean Water at a pressure of one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. Older definitions of the Celsius scale once defined the boiling point of water under one standard atmosphere as being precisely 100 °C. However, the current definition results in a boiling point that is actually 16.1 mK less. For more about the actual boiling point of water, see The melting and boiling points of water below.
In 1742, Anders Celsius created a “backwards” version of the modern Celsius temperature scale, using zero to represent the boiling point of water and 100 to represent the melting point of ice. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that ice’s melting point was effectively unaffected by pressure. He also determined with remarkable precision how water’s boiling point varied as a function of atmospheric pressure. He proposed that zero on his temperature scale (water’s boiling point) would be calibrated at the mean barometric pressure at mean sea level. This pressure is known as one standard atmosphere. In 1954, Resolution 4 of the 10th CGPM (the General Conference on Weights and Measures) established internationally that one standard atmosphere was a pressure equivalent to 1,013,250 dynes per cm2 (101.325 kPa).
In 1744, coincident with the death of Anders Celsius, the famous botanist Carolus Linnaeus (1707 – 1778) effectively reversed  Celsius’s scale upon receipt of his first thermometer featuring a scale where zero represented the melting point of ice and 100 represented water’s 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. 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 Swedish 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 his student, Samuel Nauclér. In it, Linnaeus recounted the temperatures inside the orangery at the Botanical Garden of Uppsala University:
- “…since the caldarium (the hot part of the greenhouse) by the angle
- of the windows, merely from the rays of the sun, obtains such heat
- that the thermometer often reaches 30 degrees, although the keen
- gardener usually takes care not to let it rise to more than 20 to 25
- degrees, and in winter not under 15 degrees…”
For the next 204 years, 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, “degrees centigrade.” The symbol for temperature values on this scale was °C (in several formats over the years). Because the term “centigrade” was also the French-language name for a unit of angular measurement (one-hundredth 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 Bureau international des poids et mesures (BIPM). The 9th CGPM (Conférence générale des poids et mesures) and the CIPM (Comité international des poids et mesures) formally adopted “degree Celsius” (symbol: °C) in 1948. For lay-people worldwide—including school textbooks—the full transition from centigrade to Celsius required nearly two decades after this formal adoption.
Temperatures and intervals
The term "degrees Celsius" can be used in a couple of different ways: (a) to express temperature measurements, and (b) to express temperature intervals, that is, differences between temperatures or uncertainties in temperature measurements. Examples of the first case would be: “Gallium melts at 29.7646 °C”; or, “The temperature outside is 23 degrees Celsius.” Examples of the second case would be: “This heat exchanger has an output that is hotter by 40 degrees Celsius”; or, “The standard uncertainty in the measurement of this temperature is ±3 °C.”
Given 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.
The temperature interval of one degree Celsius is the same as that of one kelvin. For this reason, in science (especially) and engineering, the Celsius and Kelvin scales are often used simultaneously in the same article (for example: “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”). Notwithstanding the official endorsements of Resolution 3 of the 13th CGPM (1967/68)] and Resolution 7 of the 9th CGPM (1948), the practice of simultaneously using both “°C” and “K” remains widespread throughout the technical world, as the use of SI prefixed forms such as “µ°C” or “millidegrees Celsius” to express a temperature interval has not been well-adopted.
The melting and boiling points of water
The effect of defining the Celsius scale at the triple point of VSMOW water (273.16 kelvins and 0.01 °C), and at absolute zero (zero kelvin and −273.15 °C), is that the melting and boiling points of water under a pressure of one standard atmosphere (1013.25 mbar) are no longer the 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 water 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 373.15/273.15 (approximately 36.61% thermodynamically hotter). When adhering strictly to the two-point definition for calibration, the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C). When calibrated to ITS-90 (a calibration standard comprising many definition points and commonly used for high-precision instrumentation), the boiling point of VSMOW water is slightly less, about 99.974 °C.
This boiling–point difference of 16.1 millikelvins (thousandths of a degree Celsius) between the Celsius scale’s original definition and the current one (based on absolute zero and the triple point) has little practical meaning in real life, because the boiling point of water is extremely sensitive to variations in barometric pressure. For example, an altitude change of only 28 cm (11 inches) alters this boiling point by one millikelvin.
The “degree Celsius” is the only SI unit that has an uppercase letter in its full unit name in English.
The word “degree” may be abbreviated as “deg.” Accordingly, the following are permissible ways to express degree Celsius: singular / (plural)
- degree Celsius / (degrees Celsius)
- deg Celsius / (same)
- degree C / (degrees C)
- deg C / (same)
- °C / (same)
As with most other unit symbols and all the temperature symbols, a space is placed between the numeric value and the °C symbol; e.g., “23 °C” (not “23°C” or “23° C”). Only the unit symbols for angles are placed immediately after the numeric value without an intervening space; e.g., “a 90° turn”.
The special Unicode °C character
Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special “°C” character at U+2103. One types
℃ when encoding this special character in a Web page. Its appearance is similar to that obtained by typing its two components (° and C) one after the other. To better see the difference between the two, below in brown text is the degree Celsius character followed immediately by the two-component version:
When viewed on computers that properly support and map Unicode, the above line may be similar to the line below (size may vary):
Depending on the operating system, Web browser, and the default font, the “C” in the Unicode character may be narrower and slightly taller than a plain uppercase C; precisely the opposite may be true on other platforms. However, there is usually a discernible difference between the two.
- ↑ Vienna Standard Mean Ocean Water (VSMOW) is an isotopic water standard defined in 1968 by the International Atomic Energy Agency. It serves as a reference standard for comparing hydrogen and oxygen isotope ratios, mostly in water samples. Very pure, distilled VSMOW water is also used for making high accuracy measurement of water’s physical properties and for defining laboratory standards because it is considered to be representative of “average ocean water.”
- ↑ In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. The single combination of pressure and temperature at which pure water, pure ice, and pure water vapor can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373057 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making infinitesimally small changes in pressure and temperature.
- ↑ Citations: Thermodynamics-information.net, A Brief History of Temperature Measurement and; Uppsala University (Sweden), Linnaeus’ thermometer
- ↑ Citation for Daniel Ekström, Mårten Strömer, Christian of Lyons: The Physics Hypertextbook, Temperature; citation for Christian of Lyons: Le Moyne College, Glossary, (Celsius scale); citation for Linnaeus’ connection with Pehr Elvius and Daniel Ekström: Uppsala University (Sweden), Linnaeus’ thermometer; general citation: The Uppsala Astronomical Observatory, History of the Celsius temperature scale
- ↑ Citations: University of Wisconsin–Madison, Linnæus & his Garden and; Uppsala University, Linnaeus’ thermometer
- ↑ According to The Oxford English Dictionary (OED), the term “Celsius’s thermometer” had been used at least as early as 1797. Further, the term “The Celsius or Centigrade thermometer” was again used in reference to a particular type of thermometer at least as early as 1850. The OED also cites this 1928 reporting of a temperature: “My altitude was about 5,800 meters, the temperature was 28° Celsius.” However, dictionaries seek to find the earliest use of a word or term and are not a useful resource as regards the terminology used throughout the history of science. According to several writings of Dr. Terry Quinn CBE FRS, Director of the BIPM (1988 – 2004), including Temperature Scales from the early days of thermometry to the 21st century (148 kB PDF, here) as well as Temperature (2nd Edition / 1990 / Academic Press / 0125696817), the term Celsius in connection with the centigrade scale was not used whatsoever by the scientific or thermometry communities until after the CIPM and CGPM adopted the term in 1948. The BIPM was not even aware that “degree Celsius” was in sporadic, nonscientific use before that time. It is also noteworthy that the twelve-volume, 1933 edition of OED didn’t even have a listing for the word Celsius (but did have listings for both centigrade and centesimal in the context of temperature measurement). The 1948 adoption of Celsius accomplished three objectives:
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’s 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.
- ↑ In 1948, Resolution 7 of the 9th CGPM stated, “To indicate a temperature interval or difference, rather than a temperature, the word ‘degree’ in full, or the abbreviation ‘deg’ must be used.” This resolution was abrogated in 1967/1968 by Resolution 3 of the 13th CGPM which stated that [“The names "degree Kelvin" and "degree," the symbols "°K" and "deg" and the rules for their use given in Resolution 7 of the 9th CGPM (1948),] …and the designation of the unit to express an interval or a difference of temperatures are abrogated, but the usages which derive from these decisions remain permissible for the time being.” Consequently, there is now wide freedom in usage regarding how to indicate a temperature interval. The most important thing is that one’s intention must be clear and the basic rule of the SI must be followed; namely that the unit name or its symbol must not be relied upon to indicate the nature of the quantity. Thus, if a temperature interval is, say, 10 K or 10 °C (which may be written 10 kelvins or 10 degrees Celsius), it must be unambiguous through obvious context or explicit statement that the quantity is an interval. Rules governing the expression of temperatures and intervals are covered in the BIPM’s SI Brochure, 8th edition (1.4 MB PDF, here).
- ↑ Citation: London South Bank University, Water Structure and Behavior, notes c1 and c2
- ↑ For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST as well as the BIPM’s SI brochure: Subsection 5.3.3, Formatting the value of a quantity.
- Childs, Peter R.N. (2001). Practical Temperature Measurement. Oxford, UK; Woburn, MA: Butterworth-Heinemann. ISBN 075065080X
- Kuhn, Karl F. (1996). Basic Physics: A Self-Teaching Guide, 2nd edition. Hoboken, NJ: John Wiley & Sons. ISBN 0471134473
- Beckman, Olof (2001). "History of the Celsius temperature scale". The Uppsala Astronomical Observatory, Uppsala University, Sweden. Retrieved December 30, 2006.
- Sizes.com (2003). "Centigrade and Celsius temperature scales". Sizes, Inc. Retrieved December 30, 2006.
- Anne's and Johan's Web site (10 May 1997). "The Origin of the Celsius Temperature Scale". Retrieved December 30, 2006.
All links retrieved April 27, 2013.
- NIST, Basic unit definitions: Kelvin
- London South Bank University, Water, scientific data
- BIPM, SI brochure, section 18.104.22.168, Unit of thermodynamic temperature
- Celsius, Fahrenheit, Kelvin, Réaumur, and Rankine Temperature Conversion CSGNetwork.com.
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