TheFahrenheit scale (/ˈfærənhaɪt,ˈfɑːr-/) is atemperature scale based on one proposed in 1724 by the European physicistDaniel Gabriel Fahrenheit (1686–1736).[1] It uses thedegree Fahrenheit (symbol:°F) as the unit. Several accounts of how he originally defined his scale exist, but the original paper suggests the lower defining point, 0 °F, was established as the freezingtemperature of a solution ofbrine made from a mixture of water,ice, andammonium chloride (asalt).[2][3] The other limit established was his best estimate of the averagehuman body temperature, originally set at 90 °F, then 96 °F (about 2.6 °F less than the modern value due to a later redefinition of the scale).[2]
For much of the 20th century, the Fahrenheit scale was defined by two fixed points with a 180 °F separation: the temperature at which pure water freezes was defined as 32 °F and the boiling point of water was defined to be 212 °F, both atsea level and understandard atmospheric pressure. It is now formally defined using theKelvin scale.[4][5]
Historically, on the Fahrenheit scale thefreezing point of water was 32 °F, and theboiling point was 212 °F (atstandard atmospheric pressure). This put the boiling and freezing points of water 180 degrees apart.[8] Therefore, a degree on the Fahrenheit scale was1⁄180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water were originally defined to be 100 degrees apart. A temperature interval of 1 °F was equal to an interval of5⁄9 degrees Celsius. With the Fahrenheit and Celsius scales now both defined by thekelvin, this relationship was preserved, a temperature interval of 1 °F being equal to an interval of5⁄9 K and of5⁄9 °C. The Fahrenheit and Celsius scales intersect numerically at −40 in the respective unit (i.e., −40 °F corresponds to −40 °C).
Absolute zero is 0 K, −273.15 °C, or −459.67 °F. TheRankine temperature scale uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0 °R – the same way that theKelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K.[8]
The combination ofdegree symbol (°) followed by an uppercase letter F is the conventional symbol for the Fahrenheit temperature scale. A number followed by this symbol (and separated from it with a space) denotes a specific temperature point (e.g., "Gallium melts at 85.5763 °F"). A difference between temperatures or an uncertainty in temperature is also conventionally written the same way as well, e.g., "The output of the heat exchanger experiences an increase of 72 °F" or "Our standard uncertainty is ±5 °F". However, some authors instead use the notation "An increase of 50 F°" (reversing the symbol order) to indicate temperature differences. Similar conventions exist for the Celsius scale, seeCelsius § Temperatures and intervals.[9][10]
For an exact conversion between degrees Fahrenheit and Celsius, and kelvins ofa specific temperature point, the following formulas can be applied. Here,f is the value in degrees Fahrenheit,c the value in degrees Celsius, andk the value in kelvins:
f °F toc °C:c =f − 32/1.8
c °C tof °F:f =c × 1.8 + 32
f °F tok K:k =f + 459.67/1.8
k K tof °F:f =k × 1.8 − 459.67
There is also an exact conversion between Celsius and Fahrenheit scales making use of the correspondence −40 °F ≘ −40 °C. Again,f is the numeric value in degrees Fahrenheit, andc the numeric value in degrees Celsius:
When converting atemperature interval between the Fahrenheit and Celsius scales, only the ratio is used, without any constant (in this case, the interval has the same numeric value in kelvins as in degrees Celsius):
Fahrenheit proposed his temperature scale in 1724, basing it on two reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point was determined by placing thethermometer in "a mixture ofice, water, andsalis Armoniaci[note 1] [transl.ammonium chloride] or even sea salt".[11] This combination forms aeutectic system, which stabilizes its temperature automatically: 0 °F was defined to be that stable temperature. A second point, 96 degrees, was approximately the human body's temperature.[11] A third point, 32 degrees, was marked as being the temperature of ice and water "without the aforementioned salts".[11]
According to a German story, Fahrenheit actually chose the lowest air temperature measured in his hometownDanzig (Gdańsk,Poland) in winter 1708–09 as 0 °F, and only later had the need to be able to make this value reproducible using brine.[12][failed verification]
According to a letter Fahrenheit wrote to his friendHerman Boerhaave,[13] his scale was built on the work ofOle Rømer, whom he had met earlier. InRømer scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by 4 in order to eliminate fractions and make the scale morefine-grained. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, and body temperature 96 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval 6 times (since 64 = 26).[14][15]
Fahrenheit soon after observed that water boils at about 212 degrees using this scale.[16] The use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work ofAnders Celsius, and these fixed points were adopted by a committee of theRoyal Society led byHenry Cavendish in 1776–77.[17][18] Under this system, the Fahrenheit scale is redefined slightly so that the freezing point of water was exactly 32 °F, and the boiling point was exactly 212 °F, or 180 degrees higher. It is for this reason thatnormal human body temperature is approximately 98.6 °F (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).[19]
In the present-day Fahrenheit scale, 0 °F no longer corresponds to the eutectic temperature of ammonium chloride brine as described above. Instead, that eutectic is at approximately 4 °F on the final Fahrenheit scale.[note 2]
The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in Anglophone countries until the 1960s. In the late 1960s and 1970s, the Celsius scale replaced Fahrenheit in almost all of those countries—with the notable exception of the United States.
Fahrenheit is used in the United States, its territories and associated states (all serviced by the U.S.National Weather Service), as well as the (British)Cayman Islands andLiberia for everyday applications. The Fahrenheit scale is in use in U.S. for all temperature measurements including weather forecasts, cooking, and food freezing temperatures, however for scientific research the scale is Celsius and Kelvin.[20]
Early in the 20th century,Halsey and Dale suggested that reasons for resistance to use the centigrade (now Celsius) system in the U.S. included the larger size of each degree Celsius and the lower zero point in the Fahrenheit system; and claimed the Fahrenheit scale is more intuitive than Celsius for describing outdoor temperatures in temperate latitudes, with 100 °F being a hot summer day and 0 °F a cold winter day.[21]
Canadahas passed legislation favoring theInternational System of Units, while also maintaining legal definitions for traditional Canadian imperial units.[22] Canadian weather reports are conveyed using degrees Celsius with occasional reference to Fahrenheit especially forcross-border broadcasts. Fahrenheit is still used on virtually all Canadian ovens.[23] Thermometers, both digital and analog, sold in Canada usually employ both the Celsius and Fahrenheit scales.[24][25][26]
In the European Union, it is mandatory to use Kelvins or degrees Celsius when quoting temperature for "economic, public health, public safety and administrative" purposes, though degrees Fahrenheit may be used alongside degrees Celsius as a supplementary unit.[27]
Most British people use Celsius.[28] However, the use of Fahrenheit still may appear at times alongside degrees Celsius in the print media with no standard convention for when the measurement is included.
For example,The Times has an all-metric daily weather page but includes a Celsius-to-Fahrenheit conversion table.[29] Some UK tabloids have adopted a tendency of using Fahrenheit for mid to high temperatures.[30] It has been suggested that the rationale to keep using Fahrenheit was one of emphasis for high temperatures: "−6 °C" sounds colder than "21 °F", and "94 °F" sounds more sensational than "34 °C".[31]
Unicode provides the Fahrenheit symbol at code pointU+2109℉DEGREE FAHRENHEIT. However, this is acompatibility character encoded forroundtrip compatibility with legacy encodings. The Unicode standard explicitly discourages the use of this character: "The sequenceU+00B0°DEGREE SIGN +U+0046FLATIN CAPITAL LETTER F is preferred overU+2109℉DEGREE FAHRENHEIT, and those two sequences should be treated as identical for searching."[32]
^"Sal Armoniac" was an impure form of ammonium chloride. The French chemistNicolas Lémery (1645–1715) discussed it in his bookCours de Chymie (A Course of Chemistry, 1675), describing where it occurs naturally and how it can be prepared artificially. It occurs naturally in the deserts of northern Africa, where it forms from puddles of animal urine. It can be prepared artificially by boiling 5 parts of urine, 1 part of sea salt, and ½ part of chimney soot until the mixture has dried. The mixture is then heated in a sublimation pot until it sublimates; the sublimated crystals aresal Armoniac. See:
Nicolas Lémery,Cours de chymie […], 7th ed. (Paris, France: Estienne Michallet, 1688),Chapitre XVII : du Sel Armoniac,pp. 338–339.
English translation: Nicolas Lémery with James Keill, trans.,A Course of Chymistry […], 3rd ed. (London, England: Walter Kettilby, 1698), Chap. XVII: of Sal Armoniack, p. 383, available on-line atHeinrich Heine University (Düsseldorf, Germany).
^Eutectic temperature of ammonium chloride and water is listed as −15.9 °C (3.38 °F) and as −15.4 °C (4.28 °F) in (respectively)
^Celsius, Anders (1742) "Observationer om twänne beständiga grader på en thermometer" (Observations about two stable degrees on a thermometer), Kungliga Svenska Vetenskapsakademiens Handlingar (Proceedings of the Royal Swedish Academy of Sciences), 3: 171–180 and Fig. 1.
^Frautschi, Steven C.; Richard P. Olenick; Tom M. Apostol; David L. Goodstein (14 January 2008).The mechanical universe: mechanics and heat. Cambridge University Press. p. 502.ISBN978-0-521-71590-4.
^Halsey, Frederick A.; Dale, Samuel S. (1919).The metric fallacy (2 ed.). The American Institute of Weights and Measures. pp. 165–166,176–177. Retrieved19 May 2009.
^Statutory Instrument 2009/3046 – Weights and Measures – The Units of Measurement Regulations 2009(PDF), archived fromthe original(PDF) on 1 January 2017,"The Secretary of State, being a Minister designated(a) for the purposes of section 2(2) of the European Communities Act 1972(b) in relation to units of measurement to be used for economic, health, safety, or administrative purposes, in exercise of the powers conferred by that subsection, makes the following Regulations:
"SI Units - Temperature".NIST. National Institute of Standards and Technology (US Department of Commerce). 15 November 2019. Retrieved25 February 2020.
