Celsius to variations in barometric pressure. For example, an altitude change of only 28 cm (11 in) causes the boiling point to change by one millikelvin. Celsius The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States, the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale
International Temperature Scale of 1990 measure the very same temperature with ease due to the advantages of a comprehensive international calibration standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures. Although “International Temperature Scale of 1990” has the word “scale” in its title, this is a misnomer that can be misleading. ITS–90 is not a scale; it is an "equipment calibration standard". Temperatures measured with equipment calibrated per ITS–90 may be expressed using any temperature scale such as Celsius, Kelvin, Fahrenheit, or Rankine. For example, a temperature can be measured using equipment calibrated to the kelvin-based ITS–90 standard, and that