Celsius

The degree Celsius (symbol: °C) is the SI unit of temperature.

Definition

1 °C = 1 K

SI multiples

Multiple	Name	Symbol		Multiple	Name	Symbol
100	degree Celsius	°C
101	decadegree Celsius	da°C		10–1	decidegree Celsius	d°C
102	hectodegree Celsius	h°C		10–2	centidegree Celsius	c°C
103	kilodegree Celsius	k°C		10–3	millidegree Celsius	m°C
106	megadegree Celsius	M°C		10–6	microdegree Celsius	µ°C
109	gigadegree Celsius	G°C		10–9	nanodegree Celsius	n°C
1012	teradegree Celsius	T°C		10–12	picodegree Celsius	p°C
1015	petadegree Celsius	P°C		10–15	femtodegree Celsius	f°C
1018	exadegree Celsius	E°C		10–18	attodegree Celsius	a°C
1021	zettadegree Celsius	Z°C		10–21	zeptodegree Celsius	z°C
1024	yottadegree Celsius	Y°C		10–24	yoctodegree Celsius	y°C

Origin

It is named after the Swedish astronomer Anders Celsius (1701 to 1744), who first proposed a similar system in 1742.

Conversions

Temperature conversion:

• °F = °C × 1.8 + 32
• °C = (°F – 32) / 1.8
• K = °C + 273.15
• °C = K – 273.15

Explanation

The temperature scale invented by Anders Celsius was originally designed so that the freezing point of water is 100 degrees, and its boiling point is 0 degrees at standard atmospheric pressure. This was reversed to its modern order some time after his death, in part at the instigation of Daniel Ekström, the manufacturer of most of the thermometers used by Celsius. Several other people, including Elvius from Sweden (1710) and Christian of Lyons (1743), independently invented the same temperature scale. The oft-quoted claim that the botanist Carolus Linnaeus (1740) is amongst those is unsubstantiated. The Delisle scale was another temperature scale that ran "downward".

In 1948 there were three names for the same unit:

• centigrade
• centessimal degree
• degree Celsius

From that date, the official name was chosen to be degree Celsius.

Since there are 100 graduations between these two reference points, the original term for this system was centigrade (100 parts) or centessimal. In 1948 the system's name was officially changed to Celsius (a third name which had also been in use before then) by the 9th General Conference on Weights and Measures (CR 64), both in recognition of Celsius himself and to eliminate confusion caused by conflict with the use of the SI centi- prefix. While the values for freezing and boiling of water remain approximately correct, the original definition is unsuitable as a formal standard: it depends on the definition of standard atmospheric pressure which in turn depends on the definition of temperature. The current official definition of the Celsius sets 0.01 ℃ to be at the triple point of water and a degree to be 1/273.16 of the difference in temperature between the triple point of water and absolute zero. This definition was adopted in 1954 at the 10th General Conference on Weights and Measures, the very same definition given for the kelvin.

The degree Celsius is the only SI unit whose full unit name ("degree Celsius", not "Celsius") in English includes an upper case letter. That is a quirk of English, because it is a proper adjective rather than a noun (before the name was changed from "degree Kelvin" to "kelvin" in 1967, that was another SI unit containing a capital letter in English). SI prefixes are applied normally, so you can have, for example, a measurement of « 12 m℃ ».

The Celsius scale is used throughout most of the world for day-to-day purposes. In broadcast media it was still frequently referred to as centigrade until the late 1980s or early 1990s, particularly by weather forecasters on European networks such as the BBC, ITV, and RTÉ. In the United States and Jamaica, Fahrenheit remains the dominant scale for everyday temperature measurement, although degree Celsius and kelvin are used for aeronautical and scientific applications.

In the United Kingdom, Celsius is the official scale used by the government and the media. It is also the only scale used in British cooking and temperature controllers (for example, room thermostats). Some of the British media, provide Fahrenheit equivalents for temperatures well above room temperature. Temperatures below room temperature are almost exclusively quoted in degrees Celsius in the UK.