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Fahrenheit is a temperature scale named after the physicist Daniel Gabriel Fahrenheit (1686–1736), who proposed it in 1724. Today, the scale has largely been replaced by the Celsius scale; it is still in use for non-scientific purposes in the United States and a few other countries and is sometimes given in weather forecasts and used by older people in the United Kingdom.

In the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the normal boiling point is 212 °F, placing the boiling and freezing points of water exactly 180 degrees apart. Absolute zero in the Fahrenheit scale is -459.67 °F. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart and absolute zero is -273.15 °C.

A temperature interval of 1 degree Fahrenheit is equal to an interval of 5/9 degrees Celsius. The Fahrenheit and Celsius scales converge at −40 degrees (i.e. −40 °F and −40 °C represent the same temperature).

The Rankine scale

The Rankine scale is a temperature base named after the Scottish physicist and engineer, Willam John Macquorn Rankine, who proposed it in 1839. It is based on one degree Rankine being equal to one degree Fahrenheit and, as with the Kelvin scale, zero in the Rankine scale is absolute zero. A temperature of -459.67 °F is exactly 0 °R. T

he freezing point of water in the Rankine scale is 491.67 °R and the normal boiling point of water is 671.67 °R (i.e., 459.67 + 212).

Comparisons and conversions

Reference points	Kelvin	Celsius	Fahrenheit	Rankine
Absolute zero	0 K	-273.15 °C	−459.67 °F	0 °R
Freezing point of water	273.15 K	0 °C	32 °F	491.67 °R
Normal boiling point of water	373.1339 K	99.9839 °C	211.9710 °F	671.641 °R

Usage

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 1960's and 1970's, the Celsius (formerly centigrade) scale was adopted by most of these countries as part of the standardizing process called metrication.

Only in the United States and a few other countries (such as Belize) does the Fahrenheit system continue to be used, and only for non-scientific use. Most other countries have adopted Celsius as the primary scale in all use, although Fahrenheit continues to be the scale of preference for a minority of people in the UK, particularly when referring to summer temperatures. Many British people are conversant with both Celsius and Fahrenheit.