ChemFact: Why Kelvin has no degree sign.

The Kelvin temperature scale is sometimes called the absolute temperature scale, especially in older books. It was developed by William Thomson, also known as Lord Kelvin, in 1848. 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. The Kelvin temperature scale should be used in thermodynamic calculations. Its principle difference is that kelvin measurements, written as K have a much lower starting point: 0K or 0 Kelvin (note the absence of the degree symbol °).

It bears additional mention that kelvins are not measured by degrees. They were considered so until 1968, when the 13th General Conference on Weights and Measures determined to drop the degree reference. This decision was made because Thomson’s measurement referred to an absolute and specific temperature (where no heat energy exists). Celsius, conversely, uses the point of reference of water freezing at the bottom of its scale, and this doesn’t accurately account for heat energy left in the water at this point (273.15 K). Instead, these temperature units are thought of as kelvins. When you measure something by Celsius, for example the boiling point of water, you are measuring in degrees (approximately 100° C). The boiling point of water on Thomson’s scale is approximately 373 kelvins or written as 373 K.
There are some important marking points for Thomson’s scale. Absolute zero is 0 K, and the triple point of water, where water can exist as gas, liquid and solid is 273.16 K (.01° C or 32.018° F). The melting point of ice, 0° C or 32°F, is 273.15 K. The boiling point of water, approximately 100° C or 212° F, is exactly 373.1339 K.
The scientific community often uses kelvin and Celsius measurements interchangeably or at the same time. You may see data on temperature given both a C degrees measurement and a kelvin measurement. This is especially the case when discussing heat energy units between the melting point of ice and absolute zero.