Temperature is a measure of the average kinetic energy of a substance. There are three commonly used scales for measuring the temperature of an object. Fahrenheit (named after the German instrument maker G. D. Fahrenheit) is commonly used in the United States and is used to report temperatures near the surface. At standard pressure, water freezes at 32F and boils at 212F. The Celsius (or centigrade) temperature scale (named after the Swedish astronomer A. Celsius) is based on the freezing and boiling points of water. This scientific temperature scale is used to report upper air temperature, and is the common temperature scale used in Europe and Canada. The formula to convert from Farenheit to Celsius is C=5/9(F-32).
The Celsisus temperature scale is not an absolute scale in that 0C is not the lowest possible temperature. The Kelvin scale (named after Lord Kelvin) is an absolute scale in that 0K is the lowest possible temperature. As the volume of an ideal gas, such as air, is proportional to its temperature, a gas with a temperature of absolute zero has no volume. Negative Kelvin temperatures are impossible. As temperature is a measure of the random motions of molecules and atoms, 0K physically means that the molecules and atoms are not moving. The Kelvin scale is used in thermodynamics and radiation. The freezing and boiling points of substances are a function of pressure. At standard temperature (0C) and sea-level pressure (1013 mb) water freezes at 0C (273.15K) and boils at 100C (373.15K).