Is there a difference between temporada and estacao?
I wonder as well!
Yes, temporada is season for something, like hunting season etc. estação is season like summer, autumn, winter or spring. I at least think.
No one has properly answered this question in 4 years. I guess that is on the moderators.
Shouldn't it be "O outono é um estação" with an O at the beginning?
They both work. But as you dont have any extra information, it sounds better to omit the article (Outono não é uma estação muito chuvosa). But no problem to use that.
Autumn is a season
Is it unusual for words ending in ão to be feminine? Or is it fairly common?
Yes, it is pretty common, as well as words ended in ão that are masculine =)