Thanks again, guys, you made it very clear.
I'm just stumped by this phrase, though. Can you break it down?
What's your understanding of the sentences? I can't think of any other meaning.夏天平均光照12小时 vs. 夏季平均光照1080小时
*你看今年夏天奧運會了吗？* vs. 你看今年夏季奧運會了吗?
To me, it is not about register (formal vs. informal). They simply do not mean the same.
又下雨啊。 - 没办法，现在是雨季嘛。 vs. *又下雨啊。 - 没办法，现在是雨天嘛*