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For weather purposes the approximation seems accurate enough. Here’s a table listing the differences: http://tomgermeau.com/tools/FC/

This is not an approximation. The conversion formula is:

C = 5/9 * (F - 32)

So GP's table is exact.

The article refers to C = 2 * (F - 30) as an approximation

Ah, I had mistaken your reply, and assumed it was to /u/u801e and not the linked article.