Temperature is a measure of how easily an object gives up or receives heat. Heat will always flow naturally from an object with a higher temperature to an object with a lower temperature.
Thermometers measure temperature in various units. A metric unit known as the degree Celsius (°C) is in general use throughout the world. Another metric unit, the kelvin (K), is used in some scientific work. In the United States, people use the nonmetric degree Fahrenheit (°F) for most commercial and everyday purposes.
The unit 1 °F equals 5/9 of the unit 1 °C, and a temperature measurement of 32 °F equals a measurement of 0 °C. So you can use the following equation to convert a temperature measured in degrees Fahrenheit to one measured in degrees Celsius: °C = (°F – 32) X (5/9).
The unit 1 K is exactly the same as the unit 1 °C, and a measurement of 273.15 K equals a measurement of 0 °C. Thus, you can use the following equation to convert a temperature measured in kelvins to one measured in degrees Celsius: °C = K – 273.15. The International System of Units (SI), the modern metric system, uses this equation to define the Celsius scale in terms of the Kelvin scale.
SI defined the Kelvin scale in terms of two points: (1) absolute zero and (2) the triple point temperature of water. Absolute zero is the lowest possible temperature. It is assigned a value of 0 K. The triple point of a substance is the one temperature and pressure at which its solid, liquid, and gas phases can coexist in equilibrium. This means that, at water’s triple point, a quantity of water can be partly ice, partly liquid, and partly vapor; and the proportions of ice, liquid, and vapor will remain the same. The triple point of water occurs at 0.06 atmosphere of pressure and at a temperature that is defined as 0.01 °C or 273.16 K. One atmosphere equals 14.696 pounds per square inch (101.325 kilopascals).
In 2018, the International Bureau of Weights and Measures (BIPM) approved a new definition of the SI unit K based on a universal value known as the Boltzmann constant. This constant describes the amount of thermodynamic energy in a substance in relation to its temperature. Although the value of the SI unit K remains the same, it is not based on the properties of water.
Temperature is different from heat, or thermal energy. Heat is the total amount of kinetic energy (energy of motion) associated with the random movement of molecules in an object. Thus, a cold lake, with many slowly moving molecules, has more thermal energy than a hot cup of tea, with fewer, faster molecules.
