Fahrenheit (°F)

The degree Fahrenheit is a unit of temperature on a scale where water freezes at 32 and boils at 212 under standard atmospheric pressure. Its symbol is the degree sign followed by F. It is the everyday temperature unit of the United States and a handful of small territories, and it is used almost nowhere else.

Daniel Gabriel Fahrenheit devised the scale in 1724 using three reference points. Zero was the temperature of a freezing mixture of ice, water and ammonium chloride, the coldest he could reliably reproduce. Thirty-two was the freezing point of plain water, and ninety-six was intended as human body temperature, though later measurement put normal body temperature nearer 98.6. The odd-looking numbers are the residue of that construction rather than arbitrary choices.

The degree is smaller than the Celsius degree, five Fahrenheit degrees to every nine Celsius, which gives the scale finer resolution in whole numbers. Defenders argue this suits weather reporting, where the range from zero to a hundred covers most of the inhabited world's climate. Critics point out that the same resolution is available in Celsius by using one decimal place, and that the scale's reference points relate to nothing anyone encounters.

American usage is nearly total in daily life: forecasts, thermostats, ovens and body temperature are all in Fahrenheit. American science, medicine and engineering are not, which produces the routine domestic conversion of a laboratory result in Celsius into a clinical note in Fahrenheit. The two scales cross at minus forty, where both read the same number.

Its finer graduation is the one technical argument in its favour. Because a Fahrenheit degree is five ninths of a Celsius degree, ordinary weather can be described in whole numbers across a wider range without fractions, and the span from a cold night to a hot afternoon covers roughly a hundred units rather than forty. Defenders of the scale add that it is calibrated to human comfort rather than to water: zero and a hundred bracket the range a person is likely to meet outdoors, whereas nought and a hundred Celsius bracket the range a kettle meets. Neither argument has moved any country to adopt it, but both explain why American resistance to changing has been durable.

Thirty-two degrees Fahrenheit equals 0 degrees Celsius, and a change of one degree Fahrenheit equals a change of five ninths of a degree Celsius.

Fahrenheit conversion table