The three temperature scales
| Scale | Symbol | Reference point |
|---|---|---|
| Celsius | °C | 0 °C = water freezes |
| Fahrenheit | °F | 32 °F = water freezes |
| Kelvin | K | 0 K = absolute zero (−273.15 °C) |
Frequently asked questions
Why does Kelvin have no degree symbol?
The kelvin is an absolute temperature unit, not a relative scale: its zero is absolute zero, the lowest possible temperature. You write 300 K, not 300 °K. A difference of one kelvin equals a difference of one degree Celsius.
At what temperature do Celsius and Fahrenheit coincide?
At exactly −40 degrees: −40 °C equals −40 °F. It is the only point where both scales show the same number.
Which countries still use Fahrenheit?
The United States and its territories, Puerto Rico, Guam, the US Virgin Islands, American Samoa and the Northern Mariana Islands, plus a handful of Caribbean and Pacific states: the Bahamas, Belize, the Cayman Islands, Palau, the Marshall Islands and Micronesia. Everywhere else the weather is announced in Celsius. Canada switched in 1975, yet ovens, pool thermometers and thermostats sold there often still show Fahrenheit, which is why a Canadian recipe can mix both scales on the same page.
Can a feels-like temperature be converted?
Only as a number, and that is the trap. Wind chill and heat index are not measurements but formulas fed by air temperature plus wind speed or humidity. The North American wind chill index, adopted jointly by Canada and the United States in November 2001, assumes wind measured at 10 metres and a walking pace of 4.8 km/h. A thermometer in the shade never shows the felt value, and recomputing the index in the other scale needs the original inputs, not the converted result.
What is the Reaumur scale?
A fourth scale, with water freezing at 0 °Re and boiling at 80 °Re, so one degree Reaumur equals 1.25 °C. It was the standard across much of continental Europe until the late nineteenth century, which is why French and Russian novels of that period quote temperatures no modern reader can place. It survives in a few dairy and sugar-boiling texts in Italy and Switzerland: to read them, multiply by 1.25.
Three scales, two fixed points and an absolute zero
Celsius and Fahrenheit cross at −40 degrees, the only place where both scales give the same number. They differ in origin and in step size: one Fahrenheit degree is only five ninths of a Celsius degree, so a ten-degree swing does not mean the same thing on each. That is what makes an American forecast sound more dramatic than it is.
The kelvin starts at absolute zero, −273.15 °C, the temperature at which thermal motion reaches its minimum and below which nothing goes. It rises at the same rate as the Celsius degree, which is why we write 300 K without the word degree: the kelvin is not one. A fourth scale, the Rankine, does the same from Fahrenheit and survives in a few American engineering calculations.