Fahrenheit

Fahrenheit
Thermometer with Fahrenheit (marked on outer bezel) and Celsius (marked on inner dial) degree units.
General information
Unit system	Imperial/US customary
Unit of	Temperature
Symbol	°F
Named after	Daniel Gabriel Fahrenheit
Conversions
x °F in ...	... corresponds to ...
SI base units	5/9(x +459.67) K
SI derived units	5/9(x − 32) °C
Imperial/US absolute scale	x + 459.67 °Ra

The Fahrenheit scale (/ˈfærənˌhaɪt, ˈfɑːr-/) is a temperature scale based on one proposed in 1724 by the European physicist Daniel Gabriel Fahrenheit (1686–1736).^[1] 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 made from a mixture of water, ice, and ammonium chloride (a salt).^[2][3] 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).^[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 at sea level and under standard atmospheric pressure. It is now formally defined using the Kelvin scale.^[4][5]

It continues to be officially used in the United States (including its unincorporated territories), its freely associated states in the Western Pacific (Palau, the Federated States of Micronesia and the Marshall Islands), the Cayman Islands, and Liberia.

Fahrenheit is commonly still used alongside the Celsius scale in other countries that use the U.S. metrological service, such as Antigua and Barbuda, Saint Kitts and Nevis, the Bahamas, and Belize. A handful of British Overseas Territories, including the Virgin Islands, Montserrat, Anguilla, and Bermuda, also still use both scales.^[6] All other countries now use Celsius ("centigrade" until 1948), which was invented 18 years after the Fahrenheit scale.^[7]

Though the United Kingdom changed from Fahrenheit to Celsius in metrology, Fahrenheit is sometimes still used in newspaper headlines to sensationalise heatwaves.^[8]