The first of 160 consecutive days of 100° Fahrenheit at Marble Bar, Western Australia.
The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist 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 made from a mixture of water, ice, 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). However, he noted a middle point of 32 °F, to be set to the temperature of ice water.
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 Planck constant, and the unperturbed ground state hyperfine transition frequency of the caesium 133 atom.In the first decades of the 18th century in the Dutch Republic, Fahrenheit made two revolutionary breakthroughs in the history of thermometry. He invented the mercury-in-glass thermometer (first widely used, accurate, practical thermometer) and the Fahrenheit scale. Fahrenheit was the first standardized temperature scale to be widely used, although its use is now limited. It is the official temperature scale 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 the former American colony of Liberia. Fahrenheit is used alongside the Celsius scale in Antigua and Barbuda and other countries which use the same meteorological service, such as Saint Kitts and Nevis, the Bahamas, and Belize. A handful of British Overseas Territories still use both scales, including the British Virgin Islands, Montserrat, Anguilla, and Bermuda. In the United Kingdom, degrees Fahrenheit figures are sometimes used in newspaper headlines to sensationalize heatwaves. All other countries now use Celsius (also known as centigrade), a scale formalized about 20 years after the Fahrenheit scale. However, the name Celsius was given to the centigrade scale much later, in 1948, in honor of the Swedish astronomer Anders Celsius.