### Fahrenheit

Fahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit (1686–1736). Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees. The temperature scale was replaced by the Celsius scale in most countries during the mid to late 20th century, but it remains the official scale of the United States, Cayman Islands and Belize. The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.

## Definition and conversions

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (at standard atmospheric pressure). This puts the boiling and freezing points of water exactly 180 degrees apart. Therefore, a degree on the Fahrenheit scale is 1180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 degree Fahrenheit is equal to an interval of 59 degrees Celsius. The Fahrenheit and Celsius scales intersect at −40 °F (−40 °F and −40 °C represent the same temperature).

Absolute zero is defined as −273.15 °C or −459.67 °F. The Rankine temperature scale was created to use degree intervals the same size as those of the Fahrenheit scale, such that a temperature difference of one degree Rankine (1 °R) is equal to a difference of 1 °F, 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 Fahrenheit scale uses (in the same manner as the later Celsius scale) the symbol ° to denote a point on the temperature scale and the letter F to indicate the Fahrenheit scale is being used (e.g. "Gallium melts at 85.5763 °F"), as well as to denote a difference between temperatures or an uncertainty in temperature (e.g. "The output of the heat exchanger experiences an increase of 72 °F" and "Our standard uncertainty is ±5 °F").

## History

According to an article Fahrenheit wrote in 1724, he based his scale on three reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat".

According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer’s 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 four in order to eliminate fractions and increase the granularity of the scale. 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 six times (since 64 is 2 to the sixth power).

Fahrenheit observed, somewhat inaccurately, that water boils at about 212 degrees using this scale. Later, other scientists decided to redefine the degree slightly to make the freezing point exactly 32°F, and the boiling point exactly 212 °F or 180 degrees higher. It is for this reason that normal human body temperature is approximately 98° (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).

## Usage

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 (known until 1948 as centigrade) replaced Fahrenheit in many countries, typically during their metrication process.

Fahrenheit is still used in the United States, Belize, and the United States territories of Puerto Rico, Guam and the U.S. Virgin Islands for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. Scientists, such as meteorologists, use Celsius or Kelvin in all countries. In some states, both measures are quoted.
Various reasons are given for the resistance to the Celsius system in the U.S., including the larger size of each degree Celsius (resulting in the need for decimals where integer Fahrenheit degrees were adequate for much non-technical work). Another reason is the lower zero point in the Fahrenheit system which reduces the number of negative signs when measurements such as weather data were averaged.

In some countries, Fahrenheit may still be used in daily life alongside Celsius. For example, Canada has passed legislation favouring the International System of Units, while also maintaining legal definitions for traditional Canadian imperial units. This means that Canadian weather reports are often conveyed using degrees Celsius yet virtually all Canadian ovens still make legal use of the Fahrenheit scale. Thermometers, both digital and analogue, sold in Canada usually employ both the Celsius and Fahrenheit scales.

Fahrenheit is also still utilized in the United Kingdom alongside Celsius. Weather reports displayed in newspapers and online quote temperatures in both Celsius and Fahrenheit (or have an option to enable this), especially during record breaking weather. However, TV weather reports are conveyed using degrees Celsius. Cooking instructions on most packaged food also lists dual temperature scales along with digital/analogue thermometers.

## Unicode representation of symbol

The Fahrenheit symbol has its own Unicode character: "℉"(U+2109). This is a compatibility character encoded for roundtrip compatibility with legacy CJK encodings (which included it to conform to layout in square ideographic character cells) and vertical layout. Use of compatibility characters is discouraged by the Unicode Consortium. The ordinary degree sign (U+00B0) followed by the Latin letter F ("°F") is thus the preferred way of recording the symbol for degree Fahrenheit.