# Explanation of Temperature and temperature units

## What is temperature?

A temperature is a quantitative number that describes the energy state of things. Atoms and molecules in all materials are constantly moving, vibrating, or rotating. As a result, an object's temperature can be defined by the average kinetic energy of its atoms and molecules.

## What is Kelvin (K)?

The SI system's base temperature unit is Kelvin (International System of Units). K is the acronym for the Kelvin unit (no degree or degree sign). In 1848, William Thomson (Lord Kelvin) presented Kelvin's unit for the first time. With the redefinition of the SI system in 2019, the kelvin scale was defined by establishing the numerical value of the Boltzmann constant to 1.380649×10−23 J⋅K−1 .

## What is Celsius (°C)?

The SI system now uses Celsius as a derived unit for temperature, with Kelvin serving as the base unit. Anders Celsius, a Swede, invented the unit and the true Celsius scale in 1742. The freezing point of water (or melting point of ice) was set at 0 °C, while the boiling point of water was set at 100 °C. These two values served as the Celsius scale's two primary reference points.

## What is Fahrenheit (°F)?

Dutchman Gabriel Fahrenheit invented the fahrenheit scale in 1724. The freezing point of water, which is 32°F, and the body's temperature, which is 96°F, serve as the scale's two primary reference points. The melting point of ice is currently 32 degrees Fahrenheit, and the boiling point of water is currently 212 degrees Fahrenheit. According to the updated scale, the average body temperature is therefore 98 °F.

## What is Rankine (°R, °Ra)?

William Rankine, a Scot, introduced the Rankine scale in 1859. Like the Kelvin scale, the Rankine scale's reference point is the absolute zero point, which is 0 °Ra. The dimensions of one Rankine degree and one Fahrenheit degree are the same. Water's freezing point is, therefore 491.67 °Ra.

## What is Réaumur (°Ré, °Re)?

In 1730, Réne de Réaumur invented the Réaumur scales. It uses 0 °Re for the freezing point of water and 80 °Re for the boiling point as reference values.