Three temperature scales are in common use in science and industry. Two of those scales are metric:
The Celsius scale was devised by dividing the range of temperature between the freezing and boiling temperatures of pure water at standard atmospheric conditions (sea level pressure) into 100 equal parts. Temperatures on this scale are called degrees Celsius (°C). They were at one time known as degrees centigrade; however, it is no longer correct to use that terminology. [The official name was changed from “degree centigrade” to “degree Celsius” by the 9th General Conference on Weights and Measures (CGPM) in 1948.]
The Kelvin scale is an extension of the Celsius scale down to absolute zero, a hypothetical temperature characterized by a complete absence of heat energy. Temperatures on this scale are called kelvins (not degrees Kelvin), kelvin is not capitalized, and the symbol (uppercase K) stands alone with no degree symbol. [The official name was changed to “kelvin” and symbol “K” by the 13th General Conference on Weights and Measures (CGPM) in 1967.]
The non-metric Fahrenheit scale was devised and evolved over time so that the freezing and boiling temperatures of water are whole numbers, but not round numbers as in the Celsius temperature scale. Temperatures on this scale are called degrees Fahrenheit (°F).
Baseline temperatures
| Temperature | Kelvins (K) | Degrees Celsius (°C) | Degrees Fahrenheit (°F) |
|---|---|---|---|
| Boiling point of water | 373.15 | 100 | 212 |
| Melting point of ice | 273.15 | 0 | 32 |
| Absolute zero | 0 | -273.15 | -459.67 |
Common temperature comparisons
| Temperature | Degrees Celsius (°C) | Degrees Fahrenheit (°F) |
|---|---|---|
| Boiling point of water | 100 | 212 |
| Average human body temperature | 37 | 98.6 |
| Average room temperature | 20 to 25 | 68 to 77 |
| Melting point of ice | 0 | 32 |
Temperature conversions
The following are formulae for exact conversions among the temperature scales.
Kelvin / Celsius
- Kelvins = degrees Celsius + 273.15
- Degrees Celsius = kelvins – 273.15
Fahrenheit / Celsius
- Degrees Fahrenheit = degrees Celsius × 1.8 + 32
- Degrees Celsius = (degrees Fahrenheit – 32) / 1.8
Celsius mnemonics
There are several memory aids that can be used to help the novice understand the Celsius temperature scale. One such mnemonic is:
When it’s zero it’s freezing,
when it’s 10 it’s not,
when it’s 20 it’s warm,
when it’s 30 it’s hot!
Or, another one to remember:
Thirty is hot
Twenty is nice
Ten is cool
Zero is ice
Quotes about Fahrenheit
The Fahrenheit temperature scale is “antiquated, clumsy, and still in use in only one industrialized nation in the world today—the United States.”
—Walter A. Lyons, PhD, The Handy Weather Answer Book, page 41.
The Fahrenheit temperature scale is “plainly silly and its cultural value [is] slim.”
—Neville Holmes, “The Numerical Dysfunction”, Mathematical Intelligencer, 22(1), 2000-winter, page 8.
Notes
The US is the only nation that continues to use the Fahrenheit scale for shelter-level (surface) weather observations. However, as of all surface temperature observations in National Weather Service METAR/TAF reports are now transmitted in degrees Celsius.
All upper-air (non-surface) temperatures have always been measured and reported in degrees Celsius by all countries.
Wind chill temperatures in metric units
| Temperature | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 10 °C | 5 °C | 0 °C | -5 °C | -10 °C | -15 °C | -20 °C | -25 °C | -30 °C | -35 °C | -40 °C | -45 °C | -50 °C | ||
| Wind speed | 10 km/h | 8.6 | 2.7 | -3.3 | -9.3 | -15.3 | -21.1 | -27.2 | -33.2 | -39.2 | -45.1 | -51.1 | -57.1 | -63.0 |
| 15 km/h | 7.9 | 1.7 | -4.4 | -10.6 | -16.7 | -22.9 | -29.1 | -35.2 | -41.4 | -47.6 | -53.7 | -59.9 | -66.1 | |
| 20 km/h | 7.4 | 1.1 | -5.2 | -11.6 | -17.9 | -24.2 | -30.5 | -36.8 | -43.1 | -49.4 | -55.7 | -62.0 | -68.3 | |
| 25 km/h | 6.9 | 0.5 | -5.9 | -12.3 | -18.8 | -25.2 | -31.6 | -38.0 | -44.5 | -50.9 | -57.3 | -63.7 | -70.2 | |
| 30 km/h | 6.6 | 0.1 | -6.5 | -13.0 | -19.5 | -26.0 | -32.6 | -39.1 | -45.6 | -52.1 | -58.7 | -65.2 | -71.7 | |
| 35 km/h | 6.3 | -0.4 | -7.0 | -13.6 | -20.2 | -26.8 | -33.4 | -40.0 | -46.6 | -53.2 | -59.8 | -66.4 | -73.1 | |
| 40 km/h | 6.0 | -0.7 | -7.4 | -14.1 | -20.8 | -27.4 | -34.1 | -40.8 | -47.5 | -54.2 | -60.9 | -67.6 | -74.2 | |
| 45 km/h | 5.7 | -1.0 | -7.8 | -14.5 | -21.3 | -28.0 | -34.8 | -41.5 | -48.3 | -55.1 | -61.8 | -68.6 | -75.3 | |
| 50 km/h | 5.5 | -1.3 | -8.1 | -15.0 | -21.8 | -28.6 | -35.4 | -42.2 | -49.0 | -55.8 | -62.7 | -69.5 | -76.3 | |
| 55 km/h | 5.3 | -1.6 | -8.5 | -15.3 | -22.2 | -29.1 | -36.0 | -42.8 | -49.7 | -56.6 | -63.4 | -70.3 | -77.2 | |
| 60 km/h | 5.1 | -1.8 | -8.8 | -15.7 | -22.6 | -29.5 | -36.5 | -43.4 | -50.3 | -57.2 | -64.2 | -71.1 | -78.0 | |
For more on the new wind chill temperatures, see the OFCM report. (See in particular the chart and formula in chapter 3, page 3-8 (page 42 of the entire PDF.)
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