[research]

may be this question seems funny to you  but I will be thankful if you answer me:
when you want to write a Celsius degree you should use this symbol
" ⁰C " but when you want to mention temperature with kelvin you don't need that superscript, what is the reason?

[FreeTheBee]

Apparently, a degree symbol was used with Kelvin in the past, but this was changed in 1967/1968 by the SI people.
Their reason, for the change, "the unit of thermodynamic temperature and the unit of temperature interval are one and the same unit, which ought to be denoted by a single name and a single symbol"

As found on wikipedia
http://en.wikipedia.org/wiki/Kelvin
http://www.bipm.org/en/CGPM/db/13/3/