Celsius

A domestic wall thermometer

Celsius (known until 1948 as centigrade) is a temperature scale that 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).

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. 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 points: absolute zero, and the triple point of VSMOW (specially prepared water). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature (symbol: K). Absolute zero, the hypothetical but unattainable temperature at which matter exhibits zero entropy, is defined as being precisely 0 K and −273.15 °C. The temperature value of the triple point of water is defined as being precisely 273.16 K and 0.01 °C.^[1]

This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 parts, 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 (−273.15 °C = 0 K and 0 °C = 273.15 K).^[2]

An illustration of Anders Celsius's original thermometer. Note the reversed scale, where 0 is the boiling point of water and 100 is its freezing point.

History

In 1742 Swedish astronomer Anders Celsius (1701–1744) originally created a "reversed" version of the modern Celsius temperature scale whereby zero represented the boiling point of water and one hundred represented the freezing point of water. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that ice's melting point was essentially 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. (The BIPM's 10th CGPM later defined one standard atmosphere to equal precisely 1,013,250 dynes per square centimeter (101.325 kPa))

In 1744, coincident with the death of Anders Celsius, the Swedish botanist Carolus Linnaeus (1707–1778) effectively reversed^[3] 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 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;^[4] 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^[5] 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 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 ..."

Centigrade and Celsius

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 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 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.^[6][7] Some people still use the old term.

Common temperatures

Some key temperatures relating the Celsius scale to other temperature scales are shown in the table below.

Formatting

The "degree Celsius" has been the only SI unit whose full unit name contains an uppercase letter since its SI base unit, the kelvin, became the proper name in 1967 for the obsolete term, the "degree Kelvin". The correct plural form is "degrees Celsius".

The general rule is that the numerical value always precedes the unit, and a space is always used to separate the unit from the number, e.g., "23 °C" (not "23°C" or "23° C"). 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.^[11]

The special Unicode degree Celsius character

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.

Temperatures and intervals

The degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures.^[12] 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").^[13] 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.^[14] What is often confused about the Celsius measurement is that it follows an interval system but not a ratio system or 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.^[15]

Why technical articles use a mix of kelvin and Celsius scales

In science (especially) and in engineering, the Celsius scale and the kelvin are often used simultaneously in the same article (e.g. "…its measured value was 0.01023 °C with an uncertainty of 70 µK…"). This practice is permissible because:

1. the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures, and
2. the magnitude of the degree Celsius is precisely equal to that of the kelvin.

Notwithstanding the official endorsement provided by decision #3 of Resolution 3 of the 13th CGPM, which stated "a temperature interval may also be expressed in degrees Celsius," the practice of simultaneously using both "°C" and "K" remains widespread throughout the scientific world as the use of SI prefixed forms of the degree Celsius (such as "µ°C" or "microdegrees Celsius") to express a temperature interval has not been well-adopted.

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 recognised and could be confusing.

The melting point of ice and the boiling point of water

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 and 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.^[16] 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 (approximately 36.61% thermodynamically hotter). When adhering strictly to the two-point definition for calibration, the boiling point of VSMOW 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 is slightly less, about 99.974 °C.^[17]

This boiling-point difference of 16.1 millikelvin (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 water's boiling point is extremely sensitive to variations in barometric pressure. For example, an altitude change of only 28 cm (11 in) causes water's boiling point to change by one millikelvin.

Worldwide adoption

Throughout the world, except in the United States and a few other countries (for example, Belize^[18]), 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, especially high-tech ones, 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. The Fahrenheit scale is also commonly used in the U.S. for body temperatures. The Fahrenheit system is used in the U.S. for cooking, and is the scale commonly seen on ovens and in recipes. In Canada, due to its close relationship with the U.S., ovens include both Fahrenheit and Celsius. This practice extends to recipes and packaging where typically both temperatures are listed.

The United Kingdom has almost exclusively used the Celsius scale since the 1970s (but it is sometimes called centigrade). A notable exception is that some broadcasters and publications still quote Fahrenheit air temperatures alongside Celsius in weather forecasts (especially during summer), for the benefit of generations born before 1960, and air-temperature thermometers sold still show both scales for the same reason. Schools in the United Kingdom teach the Celsius scale exclusively.

See also

• Absolute zero
• ITS-90
• Temperature
• Temperature conversion
• Thermodynamic temperature

Notes

External links

• NIST, Basic unit definitions: Kelvin
• The Uppsala Astronomical Observatory, History of the Celsius temperature scale
• London South Bank University, Water, scientific data
• BIPM, SI brochure, section 2.1.1.5, Unit of thermodynamic temperature
• TAMPILE, Comparison of temperature scales

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