Why is polar a common noun?

The Word 'polar' is not a noun, it's an adjective, a Word that describes a noun. The noun form for polar is 'pole', a common noun.

Any common noun can become a proper noun if it is used for the name of someone or something, such as the movie 'Beyond the Pole' (2009) or Reginald Pole, Archbishop of Canterbury (1556-1558). Even the adjective polar is a proper noun in the movie title 'Polar Express' (2004); the name of a book, play, or movie is a proper noun.