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The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...
Daniel Gabriel Fahrenheit (1686–1736). It uses the degree 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 freezing temperature of a solution of
brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for b ...
made from a mixture of water,
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
, and ammonium chloride (a salt). The other limit established was his best estimate of the average human 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). 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 at sea level and under standard atmospheric pressure. It is now formally defined using the Kelvin scale and hence ultimately by the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
, the Planck constant, and the unperturbed ground-state
hyperfine transition In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the ...
frequency of the
caesium Caesium ( IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
-133 atom. It continues to be officially used in the United States (including its unincorporated territories), its
freely associated states The Compact of Free Association (COFA) is an international agreement establishing and governing the relationships of free association between the United States and the three Pacific Island sovereign states of the Federated States of Micronesia (F ...
in the Western Pacific ( Palau, the Federated States of Micronesia and the Marshall Islands), the
Cayman Islands The Cayman Islands () is a self-governing British Overseas Territory—the largest by population in the western Caribbean Sea. The territory comprises the three islands of Grand Cayman, Cayman Brac and Little Cayman, which are located to the s ...
, and the former American colony of Liberia. Fahrenheit is used alongside the
Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...
scale in
Antigua and Barbuda Antigua and Barbuda (, ) is a sovereign country in the West Indies. It lies at the juncture of the Caribbean Sea and the Atlantic Ocean in the Leeward Islands part of the Lesser Antilles, at 17°N latitude. The country consists of two majo ...
and other countries which use the same
meteorological Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
service, such as Saint Kitts and Nevis, the
Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to 88% of the archi ...
, and Belize. A handful of
British Overseas Territories The British Overseas Territories (BOTs), also known as the United Kingdom Overseas Territories (UKOTs), are fourteen territories with a constitutional and historical link with the United Kingdom. They are the last remnants of the former Bri ...
, including the Virgin Islands,
Montserrat Montserrat ( ) is a British Overseas Territory in the Caribbean. It is part of the Leeward Islands, the northern portion of the Lesser Antilles chain of the West Indies. Montserrat is about long and wide, with roughly of coastline. It is n ...
,
Anguilla Anguilla ( ) is a British Overseas Territory in the Caribbean. It is one of the most northerly of the Leeward Islands in the Lesser Antilles, lying east of Puerto Rico and the Virgin Islands and directly north of Saint Martin. The territor ...
, and Bermuda, still use both scales. All other countries now use
Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...
("centigrade" until 1948), a scale formalized about 20 years after the Fahrenheit scale. The United Kingdom started to change from Fahrenheit to Celsius in 1962, and many people remain aware of Fahrenheit temperatures; degrees Fahrenheit are sometimes used in newspaper headlines to sensationalize heatwaves.


Definition and conversion

Historically, on the Fahrenheit scale the
freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
of water was 32 °F, and the
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...
was 212 °F (at standard atmospheric pressure). This put the boiling and freezing points of water 180 degrees apart. Therefore, a degree on the Fahrenheit scale was 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 of  degrees Celsius. With the Fahrenheit and Celsius scales now both defined by the kelvin, this relationship was preserved, a temperature interval of 1 °F being equal to an interval of  K and of  °C. The Fahrenheit and Celsius scales intersect numerically at −40 in the respective unit (i.e. −40 °F ≘ −40 °C).
Absolute zero Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibratio ...
is 0 K, −273.15 °C, or −459.67 °F. The Rankine 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 the Kelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K. The combination of degree 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 Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (alumini ...
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, see .


Conversion (specific temperature point)

For an exact conversion between degrees Fahrenheit and Celsius, and kelvins of ''a specific temperature point'', the following formulas can be applied. Here, is the value in degrees Fahrenheit, the value in degrees Celsius, and the value in kelvins: *  °F to  °C: = *  °C to  °F: = × 1.8 + 32 *  °F to  K: = *  K to  °F: = × 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, is the numeric value in degrees Fahrenheit, and the numeric value in degrees Celsius: *  °F to  °C: = − 40 *  °C to  °F: = ( + 40) × 1.8 − 40


Conversion (temperature difference or interval)

When converting a ''temperature 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): *  °F to  °C or  K: = = *  °C or  K to  °F: = × 1.8 = × 1.8


History

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 the thermometer in "a mixture of
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
, water, and ''salis Armoniaci'' ammonium_chloride.html" ;"title="ransl. ammonium chloride">ransl. ammonium chlorideor even sea salt". This combination forms a
eutectic system A eutectic system or eutectic mixture ( ) is a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutectic temp ...
, 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.Fahrenheit, Daniele Gabr. (1724
Experimenta & observationes de congelatione aquæ in vacuo factæ a D. G. Fahrenheit, R. S. S
(Experiments and observations on water freezing in the void by D. G. Fahrenheit, R. S. S.), ''Philosophical Transactions of the Royal Society of London'', vol. 33, no. 382, page 78 (March–April 1724). Cited and translated in http://www.sizes.com:80/units/temperature_Fahrenheit.htm.
A third point, 32 degrees, was marked as being the temperature of ice and water "without the aforementioned salts". According to a German story, Fahrenheit actually chose the lowest air temperature measured in his hometown Danzig (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. According to a letter Fahrenheit wrote to his friend
Herman Boerhaave Herman Boerhaave (, 31 December 1668 – 23 September 1738Underwood, E. Ashworth. "Boerhaave After Three Hundred Years." ''The British Medical Journal'' 4, no. 5634 (1968): 820–25. https://www.jstor.org/stable/20395297.) was a Dutch botanist, ...
, his scale was built on the work of Ole Rømer, whom he had met earlier. In
Rømer scale The Rømer scale (; notated as °Rø), also known as Romer or Roemer, is a temperature scale named after the Danish astronomer Ole Christensen Rømer, who proposed it in 1701. It is based on the freezing point of pure water being 7.5 degrees a ...
, 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 more fine-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). Fahrenheit soon after observed that water boils at about 212 degrees using this scale. The use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work of
Anders Celsius Anders Celsius (; 27 November 170125 April 1744) was a Swedish astronomer, physicist and mathematician. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Ger ...
, and these fixed points were adopted by a committee of the Royal Society led by Henry Cavendish in 1776–77. 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 that normal 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). 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. The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibratio ...
instead.


Usage


General

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in English-speaking 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 and in certain cases, the United Kingdom—typically during their general
metrication Metrication or metrification is the act or process of converting to the metric system of measurement. All over the world, countries have transitioned from local and traditional units of measurement to the metric system. This process began in F ...
process. Fahrenheit is used in the United States, its territories and associated states (all served by the U.S.
National Weather Service The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
), as well as the
Cayman Islands The Cayman Islands () is a self-governing British Overseas Territory—the largest by population in the western Caribbean Sea. The territory comprises the three islands of Grand Cayman, Cayman Brac and Little Cayman, which are located to the s ...
and Liberia for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. Scientists, including meteorologists, use degrees Celsius or kelvin in all countries.


United States

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; put another way, 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.


Canada

Canada has passed legislation favoring the International System of Units, while also maintaining legal definitions for traditional Canadian imperial units. Canadian weather reports are conveyed using degrees Celsius with occasional reference to Fahrenheit especially for cross-border broadcasts. Fahrenheit is still used on virtually all Canadian ovens. Thermometers, both digital and analog, sold in Canada usually employ both the Celsius and Fahrenheit scales.


European Union

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. For example, the laundry symbols used in the United Kingdom follow the recommendations of ISO 3758:2005 showing the temperature of the washing machine water in degrees Celsius only. The equivalent label in North America uses one to six dots to denote temperature with an optional temperature in degrees Celsius.


United Kingdom

Although Fahrenheit was the most popular scale in the United Kingdom, for many years Celsius has been the primary scale used, and it has been taught in schools since the 1970s. Within unregulated sectors, such as journalism, the use of Fahrenheit in the United Kingdom follows no fixed pattern, with degrees Fahrenheit sometimes appearing alongside degrees Celsius. '' The Daily Telegraph'' does not mention Fahrenheit on its daily weather page, while '' The Times'' also has an all-metric daily weather page but has a Celsius-to-Fahrenheit conversion table. When publishing news stories, some UK tabloids have adopted a tendency of using degrees Celsius in headlines and discussion relating to low temperatures and Fahrenheit for mid to high temperatures. In February 2006, the writer of an article in '' The Times'' suggested that the rationale was one of emphasis: "−6 °C" sounds colder than "21 °F", and "94 °F" sounds more impressive than "34 °C".


Unicode representation of symbol

Unicode provides the Fahrenheit symbol at code point . However, this is a compatibility character encoded for roundtrip compatibility with legacy encodings. The Unicode standard explicitly discourages the use of this character: "The sequence + is preferred over , and those two sequences should be treated as identical for searching."


See also

*
Comparison of temperature scales This is a collection of temperature conversion formulas and comparisons among eight different temperature scales, several of which have long been obsolete. Temperatures on scales that either do not share a numeric zero or are nonlinearly related c ...
*
Degree of frost A degree of frost is a non-standard unit of measure for air temperature meaning degrees below melting point (also known as "freezing point") of water (0 degrees Celsius or 32 degrees Fahrenheit). "Degree" in this case can refer to degree Celsius ...


Notes


References


External links

* *
Daniel Gabriel Fahrenheit (Polish-born Dutch physicist) – Encyclopædia Britannica

"At Auction , One of Only Three Original Fahrenheit Thermometers"
''Enfilade'' page for 2012 ''
Christie's Christie's is a British auction house founded in 1766 by James Christie (auctioneer), James Christie. Its main premises are on King Street, St James's in London, at Rockefeller Center in New York City and at Alexandra House in Hong Kong. It is ...
'' sale of a Fahrenheit mercury thermometer
''Christie's'' press release
* {{United States Customary Units Customary units of measurement in the United States Imperial units Scales of temperature 1724 introductions Scales in meteorology