In this scale, the freezing point of water is 32 degrees (this is written "32 °F"), and the boiling point is 212 degrees Fahrenheit, placing the boiling and melting points of water 180 degrees apart. Thus the unit of this scale, a degree Fahrenheit, is 5/9ths of a kelvin (which is a degree Celsius), and minus 40 degrees Fahrenheit is equal to minus 40 degrees Celsius.
There are several competing versions of the story of how Fahrenheit came to devise his temperature scale. One states that Fahrenheit established the zero (0 °F) and 100 °F points on his scale by recording the lowest outdoor temperatures he could measure, and his own body temperature. He took as his zero point the lowest temperature he measured in the harsh winter of 1708 through 1709 in his home town of Gdańsk (Danzig) (-17.8 °C). (He was later able to reach this temperature under laboratory conditions using a mixture of ice, ammonium chloride and water.) Fahrenheit wanted to avoid the negative temperatures which Ole Rømer's scale had produced in everyday use. Fahrenheit fixed his own body temperature as 100 °F (normal body temperature is closer to 98.6 °F, suggesting that Fahrenheit was suffering a fever when he conducted his experiments or that his thermometer was not very accurate), and divided his original scale into twelve divisions; later dividing each of these into 8 equal subdivisions produced a scale of 96 degrees. Fahrenheit noted that his scale placed the freezing point of water at 32 °F and the boiling point at 212 °F, a neat 180 degrees apart.
Another holds that Fahrenheit established the zero of his scale (0 °F) as the temperature at which an equal mixture of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and ninety-six degrees as the temperature of blood (he initially used horse blood to calibrate his scale). Initially, his scale only contained 12 equal subdivisions, but later he subdivided each division into 8 equal degrees ending up with 96. He then observed that plain water would freeze at 32 degrees and boil at 212 degrees.
A third well-known version of the story, as described in the popular physics television series The Mechanical Universe, holds that Fahrenheit simply adopted Rømer's scale, at which water freezes at 7.5 degrees, and multiplied each value by 4 in order to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he took to be 96 degrees); the freezing point of water was adjusted to 32 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).
His measurements were not entirely accurate, though; by his original scale, the actual freezing and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact freezing and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why the body temperature once taken as 96 or 100 °F by Fahrenheit is today taken by many as 98.6 °F (it is a direct conversion of 37 °C, a case of excess precision), although giving the value as 98 °F would be more accurate.
A fourth, not so well-known version of the origin of the Fahrenheit scale depends on Fahrenheit himself being a Freemason (of which there is no definitive evidence). In Freemasonry, there are 32 degrees of enlightenment, 32 being the highest. The use of the 'degree' as well is said to have been derived from the degrees of masonry. This may well be coincidence, but there is no conclusive evidence to the contrary, so the thought persists.
The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in most English-speaking countries until the 1960s. In the late 1960s and 1970s the Celsius (formerly centigrade) scale was phased-in by governments as part of the standardizing process of metrication.
Fahrenheit supporters claim this is due to Fahrenheit's user-friendliness. The unit of measure, being only 5/9 the size of the Celsius degree, permits more precise communication of measurements without resorting to fractional degrees. Also, the ambient air temperature in most inhabited regions of the world tends not to go far beyond the range of 0 °F to 100 °F: therefore, the Fahrenheit scale would reflect the perceived ambient temperatures, following 10-degree bands that emerge in the Fahrenheit system:
- 10s Deep Frost.
- 20s Light Frost.
- 30s Cold. Close to freezing.
- 40s Cold. Heavy clothing needed.
- 50s Very cool. Moderate Clothing required.
- 60s Cool. Light clothing.
- 70s Comfortable. Summer clothing.
- 80s Warm. Bearable. Minimal clothing.
- 90s Hot.
- 100s Very hot. Take precautions against overheating.
However, such a correlation is largely the result of habit: in the same way, Celsius supporters might indicate that 0–10 °C indicates cold, 10–20 °C mild, 20–30 °C warm and 30–40 °C hot, with the minus sign indicating frost.
Some people also feel that Fahrenheit scale also allows for different levels of accuracy, although it is more of a matter of how a common set of units for something like temperature has entered the language. For example, it is common for American weather forecasts to predict a temperature of the "low 40s" for three days down the road and the "50s" for next week. Expressing this on the Celsius scale would require different wording.
In Jamaica and the United States, where metrication has encountered resistance from industry and consumers, the Fahrenheit system continues to be very widely used. In most parts of the United Kingdom Celsius has been adopted, although Fahrenheit is still occasionally used by older generations for everyday measurement of higher temperatures, while lower temperatures are more often measured in degrees Celsius. Younger generations in the UK and most other countries have adopted Celsius as the primary scale in use.
Template:TemperatureScalesbg:Фаренхайт ca:Fahrenheit cs:Stupeň Fahrenheita da:Fahrenheit de:Grad Fahrenheit es:Grado Fahrenheit fr:Fahrenheit gl:Fahrenheit ko:화씨 it:Fahrenheit hu:Fahrenheit nl:Fahrenheit ja:華氏 no:Fahrenheit pl:Skala Fahrenheita pt:Fahrenheit ru:Градус Фаренгейта simple:Fahrenheit sk:Stupeň Fahrenheita sl:Fahrenheitova temperaturna lestvica fi:Fahrenheit-asteikko sv:Grad Fahrenheit zh:华氏温标