Celsius Temperature Scale Definition

What Is the Celsius Scale?

On the Celsius temperature scale, zero degrees is the freezing point of water, while 100 degrees is its boiling point.
On the Celsius temperature scale, zero degrees is the freezing point of water, while 100 degrees is its boiling point. Danita Delimont, Getty Images

Celsius Temperature Scale Definition

The Celsius temperature scale is a common System Internationale (SI) temperature scale (the official scale is Kelvin). The Celsius scale is based on a derived unit defined by assigning the temperatures of 0°C and 100°C to the freezing and boiling points of water, respectively, at 1 atm pressure. More precisely, the Celsius scale is defined by absolute zero and the triple point of pure water.

This definition allows easy conversion between the Celsius and Kelvin temperature scales, such that absolute zero is defined to be precisely 0 K and −273.15 °C. The triple point of water is defined to be 273.16 K (0.01 °C; 32.02 °F). The interval between one degree Celsius and one Kelvin are exactly the same. Note the degree is not used in the Kelvin scale because it is an absolute scale.

The Celsius scale is named in honor of Anders Celsius, a Swedish astronomer who devised a similar temperature scale. Before 1948, when the scale was re-named Celsius, it was known as the centrigrade scale. However, the terms Celsius and centrigrade don't mean precisely the same thing. A centrigrade scale is one which has 100 steps, such as the degree units between freezing and boiling of water. The Celsius scale is thus an example of a centrigrade scale. The Kelvin scale is another centrigrade scale.

Also Known As: Celsius scale, centrigrade scale

Common Misspellings: Celcius scale

Interval Versus Ratio Temperature Scales

Celsius temperatures follow a relative scale or interval system rather than an absolute scale or ratio system. Examples of ratio scales include those used to measure distance or mass. If you double the value of mass (e.g., 10 kg to 20 kg), you know the doubled quantity contains twice the amount of matter and that the change in the amount of matter from 10 to 20 kg is the same as from 50 to 60 kg.

The Celsius scale does not work this way with heat energy. The difference between 10 °C and 20 °C and that between 20 °C and 30 °C is 10 degrees, but a 20 °C temperature does not have twice the heat energy of a 10 °C temperature.

Reversing the Scale

One interesting fact about the Celsius scale is that Anders Celsius' original scale was set to run in the opposite direction. Originally the scale was devised so that water boiled at 0 degrees and ice melted at 100 degrees! Jean-Pierre Christin proposed the change.

Proper Format for Recording a Celsius Measurement

The International Bureau of Weights and Measures (BIPM) states that a Celsius measurement should be recorded in the following manner: The number is placed before the degree symbol and unit. There should be a space between the number and the degree symbol. For example, 50.2 °C is correct, while 50.2°C or 50.2° C are incorrect.

Melting, Boiling, and Triple Point

Technically, the modern Celsius scale is based on the triple point of Vienna Standard Mean Ocean Water and on absolute zero, meaning neither the melting point nor boiling point of water define the scale. However, the difference between the formal definition and the common one is so small as to be insignificant in practical settings.

There is only a 16.1 millikelvin difference between the boiling point of water, comparing the original and modern scales. To put this into perspective, moving 11 inches (28 cm) in altitude changes the boiling point of water one millikelvin.