The Celsius, Kelvin, and Fahrenheit temperature scales are shown in relation to the phase change temperatures of water. The Kelvin scale is called absolute temperature and the Kelvin is the SI unit for temperature.
The Fahrenheit Temperature Scale
The Fahrenheit temperature scale sets the freezing point of water at 32 degrees Fahrenheit. Water boils at 212 degrees in this scale. Hence a Fahrenheit degree is 1/180 the temperature difference between the boiling and freezing points of water.
The Centigrade or Celsius Temperature Scale
In the Celsius temperature scale the freezing point of water is defined as 0 degrees Celsius. Water boils at 100 degrees Celsius. Hence a Celsius degree is 1/100 the difference between the boiling and freezing point of water. A Centigrade degree is 100/180 or 5/9 times the size of a Fahrenheit degree. That is why the factors 9/5 or 5/9 appear in the formulas to convert Celsius to Fahrenheit or Fahrenheit to Celsius.
The Kelvin Temperature Scale
A Kelvin degree is the same size as a Centigrade degree. This temperature scale however uses absolute zero, rather than the freezing point of water, as the zero point. In this temperature scale water freezes at 273.15 Kelvins and boils at 373.15 Kelvins.
When an object's temperature increases its internal heat energy causes the random motions of individual atoms and molecules to increase. Random molecular motions are faster at higher temperatures. As the temperature decreases, the random atomic and molecular motions decrease. The lowest possible temperature is when these random atomic and molecular motions are at the minimum possible energy. This temperature is absolute zero. It is zero in the Kelvin temperature scale. Absolute zero is -273.15 degrees Celsius and -459.67 degrees Fahrenheit.