# Why do percentage-based forecast error measures assume a meaningful zero?

I've seen this comment made in various textbooks and papers. For example, the online textbook by Rob J Hyndman and George Athanasopoulos states at http://otexts.com/fpp/2/5/ that

Another prob­lem with per­cent­age errors that is often over­looked is that they assume a mean­ing­ful zero. For exam­ple, a per­cent­age error makes no sense when mea­sur­ing the accu­racy of tem­per­a­ture fore­casts on the Fahren­heit or Cel­sius scales.

Why is this the case?