Celsius (abbreviation: °C) is a temperature scale with the freezing point of water and the triple point of water being defined as 0 degrees Celsius (or °C), and the boiling point of water being defined as 100 degrees Celsius (or °C). This scale is widely used in science, particularly in countries where the metric system is widely used, although it is officially limited to temperatures between −50° and 100°. It is also the basis of a number of other scales, including the APH (Atmospheric Pressure-based Humidity) scale and the ANSI standard for reporting weather temperatures. Celsius is named after the Swedish astronomer Anders Celsius. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval, a difference between two temperatures or an uncertainty. For example, it may be used to describe the temperature at which water boils, or that at which the pressure of a given gas is equal to 1 atmosphere (atm).

The rankine temperature unit, also known as the absolute thermodynamic temperature scale, is a non-SI metric unit of measurement for temperature. It is used in a variety of engineering contexts and usually expressed in kelvins (K). The Rankine scale was invented by the Scottish engineer William John Macquorn Rankine. Rankine temperature is a heat energy in British and American units of measurement. It is used extensively in the steam power industry, as it is suitable for working with saturated steam. The Rankine temperature scale was created by the Scottish engineer and physicist William John Macquorn Rankine (1820-1872). He set absolute zero at -459.67 °F (-273.15 °C) and the boiling point of water at 707.2 °F (374.6 °C). This means that a 1:1 mixture of water and ice has a temperature of 32°F (0°C), while a 1:1 mixture of steam and water has a temperature of 212°F (100°C).