Is It Autumn?

The American in me is always reluctant to call it autumn (or really, Fall) until the end of September. "Acting" like it's fall at the beginning of the month always seemed a bit ridiculous stateside when it was boiling hot and there was no noticeable change from August to September, but I've found that here people are calling it autumn. I usually say it with a bit of a disclaimer, "I know it's not autumn, but like, have you felt the temperature?!" But others just call it autumn and it has thrown me for a loop...could it possibly BE autumn, as in officially and we are allowed to say that? Which lead me to realize there are two main schools of thought on the subject of...